1911 Encyclopædia Britannica/Flower
FLOWER (Lat. flos, floris; Fr. fleur), a term popularly used for the bloom or blossom of a plant, and so by analogy for the fairest, choicest or finest part or aspect of anything, and in various technical senses. Here we shall deal only with its botanical interest. It is impossible to give a rigid botanical definition of the term “flower.” The flower is a characteristic feature of the highest group of the plant kingdom—the flowering plants (Phanerogams)—and is the name given to the association of organs, more or less leaf-like in form, which are concerned with the production of the fruit or seed. In modern botanical works the group is often known as the seed-plants (Spermatophyta). As the seed develops from the ovule which has been fertilized by the pollen, the essential structures for seed-production are two, viz. the pollen-bearer or stamen and the ovule-bearer or carpel. These are with few exceptions foliar structures, known in comparative morphology as sporophylls, because they bear the spores, namely, the microspores or pollen-grains which are developed in the microsporangia or pollen-sacs, and the megaspore, which is contained in the ovule or megasporangium.
In Gymnosperms (q.v.), which represent the more primitive type of seed-plants, the micro- or macro-sporophylls are generally associated, often in large numbers, in separate cones, to which the term “flower” has been applied. But there is considerable difference of opinion as to the relation between these cones and the more definite and elaborate structure known as the flower in the higher group of seed-plants—the Angiosperms (q.v.)—and it is to this more definite structure that we generally refer in using the term “flower.”
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| Fig. 1.—Proliferous Rose. | |
| s, | Sepals transformed into leaves. |
| p, | Petals multiplied at the expense of the stamens, which are reduced in number. |
| c, | Coloured leaves representing abortive carpels. |
| a, | Axis prolonged, bearing an imperfect flower at its apex. |
Flowers are produced from flower-buds, just as leaf-shoots arise from leaf-buds. These two kinds of buds have a resemblance to each other as regards the arrangement and the development of their parts; and it sometimes happens, from injury and other causes, that the part of the axis which, in ordinary cases, would produce a leaf-bud, gives origin to a flower-bud. A flower-bud has not in ordinary circumstances any power of extension by the continuous development of its apex. In this respect it differs from a leaf-bud. In some cases, however, of monstrosity, especially seen in the rose (fig. 1), the central part is prolonged, and bears leaves or flowers. In such cases the flowers, so far as their functional capabilities are concerned, are usually abortive. This phenomenon is known as proliferation of the floral axis.
Flower-buds, like leaf-buds, are produced in the axil of leaves, which are called bracts.
The term bract is properly applied to the leaf from which the primary floral axis, whether simple or branched, arises, while the leaves which arise on the axis between the bract and the outer envelope of the flower are bracteoles or bractlets. Bracts sometimes do not differ from the ordinary leaves, as in Veronica Bracts. hederifolia, Vinca, Anagallis and Ajuga. In general as regards their form and appearance they differ from ordinary leaves, the difference being greater in the upper than in the lower branches of an inflorescence. They are distinguished by their position at the base of the flower or flower-stalk. Their arrangement is similar to that of the leaves. When the flower is sessile the bracts are often applied closely to the calyx, and may thus be confounded with it, as in the order Malvaceae and species of Dianthus and winter aconite (Eranthis), where they have received the name of epicalyx or calyculus. In some Rosaceous plants an epicalyx is present, due to the formation of stipulary structures by the sepals. In many cases bracts act as protective organs, within or beneath which the young flowers are concealed in their earliest stage of growth.
When bracts become coloured, as in Amherstia nobilis, Euphorbia splendens, Erica elegans and Salvia splendens, they may be mistaken for parts of the corolla. They are sometimes mere scales or threads, and at other times are undeveloped, giving rise to the ebracteate inflorescence of Cruciferae and some Boraginaceae. Sometimes they are empty, no flower-buds being produced in their axil. A series of empty coloured bracts terminates the inflorescence of Salvia Horminum. The smaller bracts or bracteoles, which occur among the subdivisions of a branching inflorescence, often produce no flower-buds, and thus anomalies occur in the floral arrangements. Bracts are occasionally persistent, remaining long attached to the base of the peduncles, but more usually they are deciduous, falling off early by an articulation. In some instances they form part of the fruit, becoming incorporated with other organs. Thus, the cones of firs and the stroboli of the hop are composed of a series of spirally arranged bracts covering fertile flowers; and the scales on the fruit of the pine-apple are of the same nature. At the base of the general umbel in umbelliferous plants a whorl of bracts often exists, called a general involucre, and at the base of the smaller umbels or umbellules there is a similar leafy whorl called an involucel or partial involucre. In some instances, as in fool’s-parsley, there is no general involucre, but simply an involucel; while in other cases, as in fennel or dill (fig. 15), neither involucre nor involucel is developed. In Compositae the name involucre is applied to the bracts surrounding the head of flowers (fig. 2, i), as in marigold, dandelion, daisy, artichoke. This involucre is frequently composed of several rows of leaflets, which are either of the same or of different forms and lengths, and often lie over each other in an imbricated manner. The leaves of the involucre are spiny in thistles and in teazel (Dipsacus), and hooked in burdock. Such whorled or verticillate bracts generally remain separate (polyphyllous), but may be united by cohesion (gamophyllous), as in many species of Bupleurum and in Lavatera. In Compositae besides the involucre there are frequently chaffy and setose bracts at the base of each flower, and in Dipsacaceae a membranous tube surrounds each flower. These structures are of the nature of an epicalyx. In the acorn the cupule or cup (fig. 3) is formed by a growing upwards of the flower-stalk immediately beneath the flower, upon which scaly or spiny protuberances appear; it is of the nature of bracts. Bracts also compose the husky covering of the hazel-nut.
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Fig. 2.—Head (capitulum) of Marigold (Calendula), showing a congeries of flowers, enclosed by rows of bracts, i, at the base, which are collectively called an involucre. |
From Strasburger’s Lehrbuch der Botanik,
by permission of Gustav Fischer. | |
| Fig. 3.—Cupule of Quercus Aegilops. cp, Cupule; gl, fruit. (After Duchartre.) |
When bracts become united, and overlie each other in several rows, it often happens that the outer ones do not produce flowers, that is, are empty or sterile. In the artichoke the outer imbricated scales or bracts are in this condition, and it is from the membranous white scales or bracts (paleae) forming the choke attached to the edible receptacle that the flowers are produced. The sterile bracts of the daisy occasionally produce capitula, and give rise to the hen-and-chickens daisy. In place of developing flower-buds, bracts may, in certain circumstances, as in proliferous or viviparous plants, produce leaf-buds.
A sheathing bract enclosing one or several flowers is called a spathe. It is common among Monocotyledons, as Narcissus (fig. 4), snow-flake, Arum and palms. In some palms it is 20 ft. long, and encloses 200,000 flowers. It is often associated with that form of inflorescence termed the spadix, and may be coloured, as in Anthurium, or white, as in arum lily (Richardia aethiopica). When the spadix is compound or branching, as in palms, there are smaller spathes, surrounding separate parts of the inflorescence. The spathe protects the flowers in their young state, and often falls off after they are developed, or hangs down in a withered form, as in some palms, Typha and Pothos. In grasses the outer scales or glumes of the spikelets are sterile bracts (fig. 5, gl); and in Cyperaceae bracts enclose the organs of reproduction. Bracts are frequently changed into complete leaves. This change is called phyllody of bracts, and is seen in species of Plantago, especially in the variety of Plantago media, called the rose-plantain in gardens, where the bracts become leafy and form a rosette round the flowering axis. Similar changes occur in Plantago major, P. lanceolata, Ajuga reptans, dandelion, daisy, dahlia and in umbelliferous plants. The conversion of bracts into stamens (staminody of bracts) has been observed in the case of Abies excelsa. A lengthening of the axis of the female strobilus of Coniferae is not of infrequent occurrence in Cryptomeria japonica, larch (Larix europaea), &c., and this is usually associated with a leaf-like condition of the bracts, and sometimes even with the development of leaf-bearing shoots in place of the scales.
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| Fig. 6.—Peduncle of Fig (Ficus Carica), ending in a hollow receptacle, enclosing numerous male and female flowers. |
The arrangement of the flowers on the axis, or the ramification of the floral axis, is called the inflorescence. The primary axis of the inflorescence is sometimes called the rachis; its branches, whether terminal or lateral, which form the stalks supporting flowers or clusters of flowers, are peduncles, and if small branches are given off by it, they are called pedicels. A flower having a stalk is called pedunculate or pedicellate; one having no stalk is sessile. In describing a branching inflorescence, it is common to speak of the rachis as the primary floral axis, its branches as the secondary floral axes, their divisions as the tertiary floral axes, and so on; thus avoiding any confusion that might arise from the use of the terms rachis, peduncle and pedicel.
The peduncle is simple, bearing a single flower, as in primrose; or branched, as in London-pride. It is sometimes succulent, as in the cashew, in which it forms the large coloured expansion supporting the nut; spiral, as in Cyclamen and Vallisneria; or spiny, as in Alyssum spinosum. When the peduncle proceeds from radical leaves, that is, from an axis which is so shortened as to bring the leaves close together in the form of a cluster, as in the primrose, auricula or hyacinth, it is termed a scape. The floral axis may be shortened, assuming a flattened, convex or concave form, and bearing numerous flowers, as in the artichoke, daisy and fig (fig. 6). The floral axis sometimes appears as if formed by several peduncles united together, constituting a fasciated axis, as in the cockscomb, in which the flowers form a peculiar crest at the apex of the flattened peduncles. Adhesions occasionally take place between the peduncle and the bracts or leaves of the plant, as in the lime-tree (fig. 7). The adhesion of the peduncles to the stem accounts for the extra-axillary position of flowers, as in many Solanaceae. When this union extends for a considerable length along the stem, several leaves may be interposed between the part where the peduncle becomes free and the leaf whence it originated, and it may be difficult to trace the connexion. The peduncle occasionally becomes abortive, and in place of bearing a flower, is transformed into a tendril; at other times it is hollowed at the apex, so as apparently to form the lower part of the outer whorl of floral leaves as in Eschscholtzia. The termination of the peduncle, or the part on which the whorls of the flower are arranged, is called the thalamus, torus or receptacle.
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Fig. 9.—Head of flowers (capitulum) of Scabiosa atropurpurea. The inflorescence is simple and indeterminate, and the expansion of the flowers centripetal, those at the circumference opening first. |
There are two distinct types of inflorescence—one in which the flowers arise as lateral shoots from a primary axis, which goes on elongating, and the lateral shoots never exceed in their development the length of the primary axis beyond their point of origin. The flowers are thus always axillary. Inflorescence. Exceptions, such as in cruciferous plants, are due to the non-appearance of the bracts. In the other type the primary axis terminates in a single flower, but lateral axes are given off from the axils of the bracts, which again repeat the primary axis; the development of each lateral axis is stronger than that of the primary axis beyond its point of origin. The flowers produced in this inflorescence are thus terminal. The first kind of inflorescence is indeterminate, indefinite or axillary. Here the axis is either elongated, producing flower-buds as it grows, the lower expanding first (fig. 8), or it is shortened and depressed, and the outer flowers expand first (fig. 9). The expansion of the flowers is thus centripetal, that is, from base to apex, or from circumference to centre.
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Fig. 10.—Plant of Ranunculus bulbosus, showing determinate inflorescence. |
The second kind of inflorescence is determinate, definite or terminal. In this the axis is either elongated and ends in a solitary flower, which thus terminates the axis, and if other flowers are produced, they belong to secondary axes farther from the centre; or the axis is shortened and flattened, producing a number of separate floral axes, the central one expanding first, while the others are developed in succession farther from the centre. The expansion of the flowers is in this case centrifugal, that is, from apex to base, or from centre to circumference. It is illustrated in fig. 10, Ranunculus bulbosus; a′ is the primary axis swollen at the base in a bulb-like manner b, and with roots proceeding from it. From the leaves which are radical proceeds the axis ending in a solitary terminal flower f′. About the middle of this axis there is a leaf or bract, from which a secondary floral axis a″ is produced, ending in a single flower f″, less advanced than the flower f′. This secondary axis bears a leaf also, from which a tertiary floral axis a″′ is produced, bearing an unexpanded solitary flower f″′. From this tertiary axis a fourth is in progress of formation. Here f′ is the termination of the primary axis, and this flower expands first, while the other flowers are developed centrifugally on separate axes.
A third series of inflorescences, termed mixed, may be recognized. In them the primary axis has an arrangement belonging to the opposite type from that of the branches, or vice versa. According to the mode and degree of development of the lateral shoots and also of the bracts, various forms of both inflorescences result.
Amongst indefinite forms the simplest occurs when a lateral shoot produced in the axil of a large single foliage leaf of the plant ends in a single flower, the axis of the plant elongating beyond, as in Veronica hederifolia, Vinca minor and Lysimachia nemorum. The flower in this case is solitary, and the ordinary leaves become bracts by producing flower-buds in place of leaf-buds; their number, like that of the leaves of this main axis, is indefinite, varying with the vigour of the plant. Usually, however, the floral axis, arising from a more or less altered leaf or bract, instead of ending in a solitary flower, is prolonged, and bears numerous bracteoles, from which smaller peduncles are produced, and those again in their turn may be branched in a similar way. Thus the flowers are arranged in groups, and frequently very complicated forms of inflorescence result. When the primary peduncle or floral axis, as in fig. 8, is elongated, and gives off pedicels, ending in single flowers, a raceme is produced, as in currant, hyacinth and barberry. If the secondary floral axes give rise to tertiary ones, the raceme is branching, and forms a panicle, as in Yucca gloriosa. If in a raceme the lower flower-stalks are developed more strongly than the upper, and thus all the flowers are nearly on a level, a corymb is formed, which may be simple, as in fig. 11, where the primary axis a′ gives off secondary axes a″, a″, which end in single flowers; or branching, where the secondary axes again subdivide. If the pedicels are very short or wanting, so that the flowers are sessile, a spike is produced, as in Plantago and vervain (Verbena officinalis) (fig. 12). If the spike bears unisexual flowers, as in willow or hazel (fig. 13), it is an amentum or catkin, hence such trees are called amentiferous; at other times it becomes succulent, bearing numerous flowers, surrounded by a sheathing bract or spathe, and then it constitutes a spadix, which may be simple, as in Arum maculatum (fig. 14), or branching as in palms. A spike bearing female flowers only, and covered with scales, is a strobilus, as in the hop. In grasses there are usually numerous sessile flowers arranged in small spikes, called locustae or spikelets, which are either set closely along a central axis, or produced on secondary axes formed by the branching of the central one; to the latter form the term panicle is applied.
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Fig. 11.—Corymb of Cerasus Mahaleb, terminating an abortive branch, at the base of which are modified leaves in the form of scales, e. a′, Primary axis; a″, secondary axes bearing flowers; b, bract in the axils of which the secondary axes arise. | ||
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Fig. 12.—Spike of Vervain (Verbena officinalis), showing sessile flowers on a common rachis. The flowers at the lower part of the spike have passed into fruit, those towards the middle are in full bloom, and those at the top are only in bud. | ||
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Fig. 13.—Amentum or catkin of Hazel (Corylus Avellana), consisting of an axis or rachis covered with bracts in the form of scales, each of which covers a male flower, the stamens of which are seen projecting beyond the scale. The catkin falls off in a mass, separating from the branch by an articulation. | ||
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| (From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer.) | |
| Fig. 14.—Spadix of Arum maculatum. (After Wossidlo.) a, Female flowers; b, male flowers; c, hairs representing sterile flowers. | Fig. 15.—Compound umbel of Common Dill (Anethum graveolens), having a primary umbel a, and secondary umbels b, without either involucre or involucel. |
If the primary axis, in place of being elongated, is contracted, it gives rise to other forms of indefinite inflorescence. When the axis is so shortened that the secondary axes arise from a common point, and spread out as radii of nearly equal length, each ending in a single flower or dividing again in a similar radiating manner, an umbel is produced, as in fig. 15. From the primary floral axis a the secondary axes come off in a radiating or umbrella-like manner, and end in small umbels b, which are called partial umbels or umbellules. This inflorescence is seen in hemlock and other allied plants, which are hence called umbelliferous. If there are numerous flowers on a flattened, convex or slightly concave receptacle, having either very short pedicels or none, a capitulum (head) is formed, as in dandelion, daisy and other composite plants (fig. 2), also in scabious (fig. 9) and teazel. In the American button-bush the heads are globular, in some species of teazel elliptical, while in scabious and in composite plants, as sunflower, dandelion, thistle, centaury and marigold, they are somewhat hemispherical, with a flattened, slightly hollowed, or convex disk. If the margins of such a receptacle be developed upwards, the centre not developing, a concave receptacle is formed, which may partially or completely enclose a number of flowers that are generally unisexual. This gives rise to the peculiar inflorescence of Dorstenia, or to that of the fig (fig. 6), where the flowers are placed on the inner surface of the hollow receptacle, and are provided with bracteoles. This inflorescence has been called a hypanthodium.
Lastly, we have what are called compound indefinite inflorescences. In these forms the lateral shoots, developed centripetally upon the primary axis, bear numerous bracteoles, from which floral shoots arise which may have a centripetal arrangement similar to that on the mother shoot, or it may be different. Thus we may have a group of racemes, arranged in a racemose manner on a common axis, forming a raceme of racemes or compound raceme, as in Astilbe. In the same way we may have compound umbels, as in hemlock and most Umbelliferae (fig. 15), a compound spike, as in rye-grass, a compound spadix, as in some palms, and a compound capitulum, as in the hen-and-chickens daisy. Again, there may be a raceme of capitula, that is, a group of capitula disposed in a racemose manner, as in Petasites, a raceme of umbels, as in ivy, and so on, all the forms of inflorescence being indefinite in disposition. In Eryngium the shortening of the pedicels changes an umbel into a capitulum.
The simplest form of the definite type of the inflorescence is seen in Anemone nemorosa and in gentianella (Gentiana acaulis), where the axis terminates in a single flower, no other flowers being produced upon the plant. This is a solitary terminal inflorescence. If other flowers were produced, they would arise as lateral shoots from the bracts below the first-formed flower. The general name of cyme is applied to the arrangement of a group of flowers in a definite inflorescence. A cymose inflorescence is an inflorescence where the primary floral axis before terminating in a flower gives off one or more lateral unifloral axes which repeat the process—the development being only limited by the vigour of the plant. The floral axes are thus centrifugally developed. The cyme, according to its development, has been characterized as biparous or uniparous. In fig. 16 the biparous cyme is represented in the flowering branch of Cerastium. Here the primary axis t ends in a flower, which has passed into the state of fruit. At its base two leaves are produced, in each of which arise secondary axes t′ t′, ending in single flowers, and at the base of these axes a pair of opposite leaves is produced, giving rise to tertiary axes t″ t″, ending in single flowers, and so on. The term dichasium has also been applied to this form of cyme.
In the natural order Carophyllaceae (pink family) the dichasial form of inflorescence is very general. In some members of the order, as Dianthus barbatus, D. carthusianorum, &c., in which the peduncles are short, and the flowers closely approximated, with a centrifugal expansion, the inflorescence has the form of a contracted dichasium, and receives the name of fascicle. When the axes become very much shortened, the arrangement is more complicated in appearance, and the nature of the inflorescence can only be recognized by the order of opening of the flowers. In Labiate plants, as the dead-nettle (Lamium), the flowers are produced in the axil of each of the foliage leaves of the plant, and they appear as if arranged in a simple whorl of flowers. But on examination it is found that there is a central flower expanding first, and from its axis two secondary axes spring bearing solitary flowers; the expansion is thus centrifugal. The inflorescence is therefore a contracted dichasium, the flowers being sessile, or nearly so, and the clusters are called verticillasters (fig. 17). Sometimes, especially towards the summit of a dichasium, owing to the exhaustion of the growing power of the plant, only one of the bracts gives origin to a new axis, the other remaining empty; thus the inflorescence becomes unilateral, and further development is arrested. In addition to the dichasial form there are others where more than two lateral axes are produced from the primary floral axis, each of which in turn produces numerous axes. To this form the terms trichasial and polychasial cyme have been applied; but these are now usually designated cymose umbels. They are well seen in some species of Euphorbia. Another term, anthela, has been used to distinguish such forms as occur in several species of Luzula and Juncus, where numerous lateral axes arising from the primary axis grow very strongly and develop in an irregular manner.
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(From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer.) |
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Fig. 16.—Cymose inflorescence (dichasium) of Cerastium collinum; t-t″″, successive axes. (After Duchartre.) |
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Fig. 17.—Flowering stalk of the White Dead-nettle (Lamium album). The bracts are like the ordinary leaves of the plant, and produce clusters of flowers in their axil. The clusters are called verticillasters, and consist of flowers which are produced in a centrifugal manner. |
In the uniparous cyme a number of floral axes are successively developed one from the other, but the axis of each successive generation, instead of producing a pair of bracts, produces only one. The basal portion of the consecutive axes may become much thickened and arranged more or less in a straight line, and thus collectively form an apparent or false axis or sympodium, and the inflorescence thus simulates a raceme. In the true raceme, however, we find only a single axis, producing in succession a series of bracts, from which the floral peduncles arise as lateral shoots, and thus each flower is on the same side of the floral axis as the bract in the axil of which it is developed; but in the uniparous cyme the flower of each of these axes, the basal portions of which unite to form the false axis, is situated on the opposite side of the axis to the bract from which it apparently arises (fig. 18). The bract is not, however, the one from which the axis terminating in the flower arises, but is a bract produced upon it, and gives origin in its axil to a new axis, the basal portion of which, constituting the next part of the false axis, occupies the angle between this bract and its parent axis—the bract from which the axis really does arise being situated lower down upon the same side of the axis with itself. The uniparous cyme presents two forms, the scorpioid or cicinal and the helicoid or bostrychoid.
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Fig. 18.—Helicoid cyme of a species of Alstroemeria. a1, a2, a3, a4, &c., separate axes successively developed in the axils of the corresponding bracts b2, b3, b4, &c., and ending in a flower f2, f3, f4, &c. The whole appears to form a simple raceme of which the axes form the internodes. | ||||||||||||
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Fig. 19.—Scorpioidal or cicinal cyme of Forget-me-not (Myosotis palustris). | ||||||||||||
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Fig. 20.—Diagram of definite floral axes a, b, c, d, e, &c. | ||||||||||||
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Fig. 21.—Flowering stalk of Ragwort (Senecio). The flowers are in heads (capitula), and open from the circumference inwards in an indefinite centripetal manner. The heads of flowers, on the other hand, taken collectively, expand centrifugally—the central one a first. | ||||||||||||
In the scorpioid cyme the flowers are arranged alternately in a double row along one side of the false axis (fig. 19), the bracts when developed forming a second double row on the opposite side; the whole inflorescence usually curves on itself like a scorpion’s tail, hence its name. In fig. 20 is shown a diagrammatic sketch of this arrangement. The false axis, a b c d, is formed by successive generations of unifloral axes, the flowers being arranged along one side alternately and in a double row; had the bracts been developed they would have formed a similar double row on the opposite side of the false axis; the whole inflorescence is represented as curved on itself. The inflorescence in the family Boraginaceae are usually regarded as true scorpioid cymes.
In the helicoid cyme there is also a false axis formed by the basal portion of the separate axes, but the flowers are not placed in a double row, but in a single row, and form a spiral or helix round the false axis. In Alstroemeria, as represented in fig. 18, the axis a1 ends in a flower (cut off in the figure) and bears a leaf. From the axil of this leaf, that is, between it and the primary axis a1 arises a secondary axis a2, ending in a flower f2, and producing a leaf about the middle. From the axil of this leaf a tertiary floral axis a3, ending in a flower f3, takes origin. In this case the axes are not arranged in two rows along one side of the false axis, but are placed at regular intervals, so as to form an elongated spiral round it.
Compound definite inflorescences are by no means common, but in Streptocarpus polyanthus and in several calceolarias we probably have examples. Here there are scorpioid cymes of pairs of flowers, each pair consisting of an older and a younger flower.
Forms of inflorescence occur, in which both the definite and indefinite types are represented—mixed inflorescences. Thus in Composite plants, such as hawk weeds (Hieracia) and ragworts (Senecio, fig. 21), the heads of flowers, taken as a whole, are developed centrifugally, the Mixed inflorescence. terminal head first, while the florets, or small flowers on the receptacle, open centripetally, those at the circumference first. So also in Labiatae, such as dead-nettle (Lamium), the different whorls of inflorescence are developed centripetally, while the florets of the verticillaster are centrifugal. This mixed character presents difficulties in such cases as Labiatae, where the leaves, in place of retaining their ordinary form, become bracts, and thus might lead to the supposition of the whole series of flowers being one inflorescence. In such cases the cymes are described as spiked, racemose, or panicled, according to circumstances. In Saxifraga umbrosa (London-pride) and in the horse-chestnut we meet with a raceme of scorpioid cymes; in sea-pink, a capitulum of contracted scorpioid cymes (often called a glomerulus); in laurustinus, a compound umbel of dichasial cymes; a scorpioid cyme of capitula in Vernonia scorpioides. The so-called catkins of the birch are, in reality, spikes of contracted dichasial cymes. In the bell-flower (Campanula) there is a racemose uniparous cyme. In the privet (Ligustrum vulgare) there are numerous racemes of dichasia arranged in a racemose manner along an axis; the whole inflorescence thus has an appearance not unlike a bunch of grapes, and has been called a thyrsus.
Tabular View of Inflorescences
| A. | Indefinite Centripetal Inflorescence. | |||
| I. | Flowers solitary, axillary. Vinca, Veronica hederifolia. | |||
| II. | Flowers in groups, pedicellate. | |||
| 1. | Elongated form (Raceme), Hyacinth, Laburnum, Currant. | |||
| (Corymb), Ornithogalum. | ||||
| 2. | Contracted or shortened form (Umbel), Cowslip, Astrantia. | |||
| III. | Flowers in groups, sessile. | |||
| 1. | Elongated form (Spike), Plantago. | |||
| (Spikelet), Grasses. | ||||
| (Amentum, Catkin), Willow, Hazel. | ||||
| (Spadix) Arum, some Palms. | ||||
| (Strobilus), Hop. | ||||
| 2. | Contracted or shortened form (Capitulum), Daisy, Dandelion, Scabious. | |||
| IV. | Compound Indefinite Inflorescence. | |||
| a. | Compound Spike, Rye-grass. | |||
| b. | Compound Spadix, Palms. | |||
| c. | Compound Raceme, Astilbe. | |||
| d. | Compound Umbel, Hemlock and most Umbelliferae. | |||
| e. | Raceme of Capitula, Petasites. | |||
| f. | Raceme of Umbels, Ivy. | |||
| B. | Definite Centrifugal Inflorescence. | |||
| I. | Flowers solitary, terminal. Gentianella, Tulip. | |||
| II. | Flowers in Cymes. | |||
| 1. | Uniparous Cyme. | |||
| a. | Helicoid Cyme (axes forming a spiral). | |||
| Elongated form, Alstromeria. | ||||
| Contracted form, Witsenia corymbosa. | ||||
| b. | Scorpioid Cyme (axes unilateral, two rows). | |||
| Elongated form, Forget-me-not, Symphytum, Henbane. | ||||
| Contracted form, Erodium, Alchemilla arvensis. | ||||
| 2. | Biparous Cyme (Dichotomous), including 3-5-chotomous Cymes (Dichasium, Cymose Umbel, Anthela). | |||
| a. | Elongated form, Cerastium, Stellaria. | |||
| b. | Contracted form (Verticillaster), Dead-nettle, Pelargonium. | |||
| 3. | Compound Definite Inflorescence. Streptocarpus polyanthus, many Calceolarias. | |||
| C. | Mixed Inflorescence. | |||
| Raceme of Scorpioid Cymes, Horse-chestnut. | ||||
| Scorpioid Cyme of Capitula, Vernonia scorpioides. | ||||
| Compound Umbel of Dichotomous Cymes, Laurustinus. | ||||
| Capitulum of contracted Scorpioid Cymes (Glomerulus), Sea-pink. | ||||
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Fig. 22.—Flower of Sedum rubens. s, Sepals; p, petals; a, stamens; c, carpels. | |||||||||||||||
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Fig. 23.—Diagram of a completely symmetrical flower, consisting of four whorls, each of five parts, s, Sepals; p, petals; a, stamens; c, carpels. | |||||||||||||||
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Fig. 24.—Monochlamydeous (apetalous) flower of Goosefoot (Chenopodium), consisting of a single perianth (calyx) of five parts, enclosing five stamens, which are opposite the divisions of the perianth, owing to the absence of the petals. | |||||||||||||||
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Fig. 25.—Stamen, consisting of a filament (stalk) f and an anther a, containing the pollen p, which is discharged through slits in the two lobes of the anther. | |||||||||||||||
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Fig. 26.—The pistil of Tobacco (Nicotiana Tabacum), consisting of the ovary o, containing ovules, the style s, and the capitate stigma g. The pistil is placed on the receptacle r, at the extremity of the peduncle. | |||||||||||||||
The flower consists of the floral axis bearing the sporophylls (stamens and carpels), usually with certain protective envelopes. The axis is usually very much contracted, no internodes being developed, and the portion bearing the floral leaves, termed the thalamus or torus, frequently The flower. expands into a conical, flattened or hollowed expansion; at other times, though rarely, the internodes are developed and it is elongated. Upon this torus the parts of the flower are arranged in a crowded manner, usually forming a series of verticils, the parts of which alternate; but they are sometimes arranged spirally especially if the floral axis be elongated. In a typical flower, as in fig. 22, we recognize four distinct whorls of leaves: an outer whorl, the calyx of sepals; within it, another whorl, the parts alternating with those of the outer whorl, the corolla of petals; next a whorl of parts alternating with the parts of the corolla, the androecium of stamens; and in the centre the gynoecium of carpels. Fig. 23 is a diagrammatic representation of the arrangement of the parts of such a flower; it is known as a floral diagram. The flower is supposed to be cut transversely, and the parts of each whorl are distinguished by a different symbol. Of these whorls the two internal, forming the sporophylls, constitute the essential organs of reproduction; the two outer whorls are the protective coverings or floral envelopes. The sepals are generally of a greenish colour; their function is mainly protective, shielding the more delicate internal organs before the flower opens. The petals are usually showy, and normally alternate with the sepals. Sometimes, as usually in monocotyledons, the calyx and corolla are similar; in such cases the term perianth, or perigone, is applied. Thus, in the tulip, crocus, lily, hyacinth, we speak of the parts of the perianth, in place of calyx and corolla, although in these plants there is an outer whorl (calyx), of three parts, and an inner (corolla), of a similar number, alternating with them. When the parts of the calyx are in appearance like petals they are said to be petaloid, as in Liliaceae. In some cases the petals have the appearance of sepals, then they are sepaloid, as in Juncaceae. In plants, as Nymphaea alba, where a spiral arrangement of the floral leaves occurs, it is not easy to say where the calyx ends and the corolla begins, as these two whorls pass insensibly into each other. When both calyx and corolla are present, the plants are dichlamydeous; when one only is present, the flower is termed monochlamydeous or apetalous, having no petals (fig. 24). Sometimes both are absent, when the flower is achlamydeous, or naked, as in willow. The outermost series of the essential organs, collectively termed the androecium, is composed of the microsporophylls known as the staminal leaves or stamens. In their most differentiated form each consists of a stalk, the filament (fig. 25, f), supporting at its summit the anther (a), consisting of the pollen-sacs which contain the powdery pollen (p), the microspores, which is ultimately discharged therefrom. The gynoecium or pistil is the central portion of the flower, terminating the floral axis. It consists of one or more carpels (megasporophylls), either separate (fig. 22, c) or combined (fig. 24). The parts distinguished in the pistil are the ovary (fig. 26, o), which is the lower portion enclosing the ovules destined to become seeds, and the stigma (g), a portion of loose cellular tissue, the receptive surface on which the pollen is deposited, which is either sessile on the apex of the ovary, as in the poppy, or is separated from it by a prolonged portion called the style (s). The androecium and gynoecium are not present in all flowers. When both are present the flower is hermaphrodite; and in descriptive botany such a flower is indicated by the symbol ☿. When only one of those organs is present the flower is unisexual or diclinous, and is either male (staminate), ♁; or female (pistillate), ♀. A flower then normally consists of the four series of leaves—calyx, corolla, androecium and gynoecium—and when these are all present the flower is complete. These are usually densely crowded upon the thalamus, but in some instances, after apical growth has ceased in the axis, an elongation of portions of the receptacle by intercalary growth occurs, by which changes in the position of the parts may be brought about. Thus in Lychnis an elongation of the axis betwixt the calyx and the corolla takes place, and in this way they are separated by an interval. Again, in the passion-flower (Passiflora) the stamens are separated from the corolla by an elongated portion of the axis, which has consequently been termed the androphore, and in Passiflora also, fraxinella (fig. 27), Capparidaceae, and some other plants, the ovary is raised upon a distinct stalk termed the gynophore; it is thus separated from the stamens, and is said to be stipitate. Usually the successive whorls of the flower, disposed from below upwards or from without inwards upon the floral axis, are of the same number of parts, or are a multiple of the same number of parts, those of one whorl alternating with those of the whorls next it.
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Fig. 27.—Calyx and pistil of Fraxinella (Dictamnus Fraxinella). The pistil consists of several carpels, which are elevated on a stalk or gynophore prolonged from the receptacle. |
In the more primitive types of flowers the torus is more or less convex, and the series of organs follow in regular succession, culminating in the carpels, in the formation of which the growth of the axis is closed (fig. 28). This arrangement is known as hypogynous, the other series (calyx, corolla and stamens) being beneath (hypo-) the gynoecium. In other cases, the apex of the growing point ceases to develop, and the parts below form a cup around it, from the rim of which the outer members of the flower are developed around (peri-) the carpels, which are formed from the apex of the growing-point at the bottom of the cup. This arrangement is known as perigynous (fig. 29). In many cases this is carried farther and a cavity is formed which is roofed over by the carpels, so that the outer members of the flower spring from the edge of the receptacle which is immediately above the ovary (epigynous), hence the term epigyny (fig. 30).
When a flower consists of parts arranged in whorls it is said to be cyclic, and if all the whorls have an equal number of parts and are alternate it is eucyclic (figs. 22, 23). In contrast to the cyclic flowers are those, as in Magnoliaceae, Symmetry of the flower. where the parts are in spirals (acyclic). Flowers which are cyclic at one portion and spiral at another, as in many Ranunculaceae, are termed hemicyclic. In spiral flowers the distinction into series is by no means easy, and usually there is a gradual passage from sepaloid through petaloid to staminal parts, as in the water-lily family, Nymphaeaceae (figs. 31, 32), although in some plants there is no such distinction, the parts being all petaloid, as in Trollius. Normally, the parts of successive whorls alternate; but in some cases we find the parts of one whorl opposite or superposed to those of the next whorl. In some cases, as in the vine-family Ampelidaceae, this seems to be the ordinary mode of development, but the superposition of the stamens on the sepals in many plants, as in the pink family, Caryophyllaceae, is due to the suppression or abortion of the whorl of petals, and this idea is borne out by the development, in some plants of the order, of the suppressed whorl. As a rule, whenever we find the parts of one whorl superposed on those of another we may suspect some abnormality.
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| Fig. 28. | Fig. 29. | Fig. 30. |
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Figs. 28, 29 and 30.—Diagrams illustrating hypogyny, perigyny and epigyny of the flower. a, Stamens; c, carpels; p, petals; s, sepals. | ||
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| Fig. 32.Fig. 31. |
| From Strasburger’s Text-Book of Botany, by permission of Macmillan & Co., Ltd. |
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Figs. 31 and 32.—White Water Lily. Fig. 31, flower; fig. 32, successive stages, a-f, in the transition from petals to stamens. (After Wossidlo.) |
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| Fig. 33. | Fig. 34. |
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| Fig. 35. | Fig. 36. |
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| Fig. 37. | Fig. 38. |
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Fig. 33.—Diagrammatic section of a symmetrical pentamerous flower of Stone-crop (Sedum), consisting of five sepals (s), five petals (p) alternating with the sepals, ten stamens (a) in two rows, and five carpels (c) containing ovules. The dark lines (d) on the outside of the carpels are glands. Fig. 34.—Diagram of the flower of Flax (Linum), consisting of five sepals (s), five petals (p), five stamens (a), and five carpels (c), each of which is partially divided into two. The dots represent a whorl of stamens which has disappeared. It is pentamerous, complete, symmetrical and regular. Fig. 35.—Diagram of the flower of Heath (Erica), a regular tetramerous flower. Fig. 36.—Diagram of the trimerous symmetrical flower of Iris. Fig. 37.—Diagram of the symmetrical trimerous flower of Fritillary (Fritillaria). Fig. 38.—Diagram of the flower of Saxifrage (Saxifraga tridactylites). The calyx and corolla consist of five parts, the stamens are ten in two rows, while the pistil has only two parts developed. | |
A flower is said to be symmetrical when each of its whorls consists of an equal number of parts, or when the parts of any one whorl are multiples of that preceding it. Thus, a symmetrical flower may have five sepals, five petals, five stamens and five carpels, or the number of any of these parts may be ten, twenty or some multiple of five. Fig. 23 is a diagram of a symmetrical flower, with five parts in each whorl, alternating with each other. Fig. 33 is a diagram of a symmetrical flower of stone-crop, with five sepals, five alternating petals, ten stamens and five carpels. Here the number of parts in the staminal whorl is double that in the others, and in such a case the additional five parts form a second row alternating with the others. In the staminal whorl especially it is common to find additional rows. Fig. 34 shows a symmetrical flower, with five parts in the three outer rows, and ten divisions in the inner. In this case it is the gynoecium which has an additional number of parts. Fig. 35 shows a flower of heath, with four divisions of the calyx and corolla, eight stamens in two rows, and four divisions of the pistil. In fig. 36 there are three parts in each whorl; and in fig. 37 there are three divisions of the calyx, corolla and pistil, and six stamens in two rows. In all these cases the flower is symmetrical. In Monocotyledons it is usual for the staminal whorl to be double, it rarely having more than two rows, whilst amongst dicotyledons there are often very numerous rows of stamens. The floral envelopes are rarely multiplied. Flowers in which the number of parts in each whorl is the same, are isomerous (of equal number); when the number in some of the whorls is different, the flower is anisomerous (of unequal number). The pistillate whorl is very liable to changes. It frequently happens that when it is fully formed, the number of its parts is not in conformity with that of the other whorls. In such circumstances, however, a flower has been called symmetrical, provided the parts of the other whorls are normal,—the permanent state of the pistil not being taken into account in determining symmetry. Thus fig. 38 shows a pentamerous symmetrical flower, with dimerous pistil. Symmetry, then, in botanical language, has reference to a certain definite numerical relation of parts. A flower in which the parts are arranged in twos is called dimerous; when the parts of the whorls are three, four or five, the flower is trimerous, tetramerous or pentamerous, respectively. The symmetry which is most commonly met with is trimerous and pentamerous—the former occurring generally among monocotyledons, the latter among dicotyledons. Dimerous and tetramerous symmetry occur also among dicotyledons.
The various parts of the flower have a certain definite relation to the axis. Thus, in axillary tetramerous flowers (fig. 35), one sepal is next the axis, and is called superior or posterior; another is next the bract, and is inferior or anterior, and the other two are lateral; and certain terms are used to indicate that position. A plane passing through the anterior and posterior sepal and through the floral axis is termed the median plane of the flower; a plane cutting it at right angles, and passing through the lateral sepals, is the lateral plane; whilst the planes which bisect the angles formed by the lateral and median planes are the diagonal planes, and in these flowers the petals which alternate with the sepals are cut by the diagonal planes.
In a pentamerous flower one sepal may be superior, as in the calyx of Rosaceae and Labiatae; or it may be inferior, as in the calyx of Leguminosae (fig. 39)—the reverse, by the law of alternation, being the case with the petals. Thus, in the blossom of the pea (figs. 39, 40), the odd petal (vexillum) st is superior, while the odd sepal is inferior. In the order Scrophulariaceae one of the two carpels is posterior and the other anterior, whilst in Convolvulaceae the carpels are arranged laterally. Sometimes the twisting of a part makes a change in the position of other parts, as in Orchids, where the twisting of the ovary changes the position of the labellum.
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Fig. 39.—Diagram of flower of Sweet-pea (Lathyrus), showing five sepals (s), two superior, one inferior, and two lateral; five petals (p), one superior, two inferior, and two lateral; ten stamens in two rows (a); and one carpel (c). |
Fig. 40.—Flower of Pea (Pisum sativum), showing a papilionaceous corolla, with one petal superior (st) called the standard (vexillum), two inferior (car) called the keel (carina), and two lateral (a) called wings (alae). The calyx is marked c. |
When the different members of each whorl are like in size and shape, the flower is said to be regular; while differences in the size and shape of the parts of a whorl make the flower irregular, as in the papilionaceous flower, represented in fig. 39. When a flower can be divided by a single plane into two exactly similar parts; then it is said to be zygomorphic. Such flowers as Papilionaceae, Labiatae, are examples. In contrast with this are polysymmetrical or actinomorphic flowers, which have a radial symmetry and can be divided by several planes into several exactly similar portions; such are all regular, symmetrical flowers. When the parts of any whorl are not equal to or some multiple of the others, then the flower is asymmetrical. This want of symmetry may be brought about in various ways. Alteration in the symmetrical arrangement as well as in the completeness and regularity of flowers has been traced to suppression or the non-development of parts, degeneration or imperfect formation, cohesion or union of parts of the same whorl, adhesion or union of the parts of different whorls, multiplication of parts, and deduplication (sometimes called chorisis) or splitting of parts.
By suppression or non-appearance of a part at the place where it ought to appear if the structure was normal, the symmetry or completeness of the flower is disturbed. This suppression when confined to the parts of certain verticils makes the flower asymmetrical. Thus, in many Caryophyllaceae, as Polycarpon and Holosteum, while the calyx and corolla are pentamerous, there are only three or four stamens and three carpels; in Impatiens Noli-me-tangere the calyx is composed of three parts, while the other verticils have five; in labiate flowers there are five parts of the calyx and corolla, and only four stamens; and in Tropaeolum pentaphyllum there are five sepals, two petals, eight stamens and three carpels. In all these cases the want of symmetry is traced to the suppression of certain parts. In the last-mentioned plant the normal number is five, hence it is said that there are three petals suppressed, as shown by the position of the two remaining ones; there are two rows of stamens, in each of which one is wanting; and there are two carpels suppressed. In many instances the parts which are afterwards suppressed can be seen in the early stages of growth, and occasionally some vestiges of them remain in the fully developed flower. By the suppression of the verticil of the stamens, or of the carpels, flowers become unisexual or diclinous, and by the suppression of one or both of the floral envelopes, monochlamydeous and achlamydeous flowers are produced. The suppression of parts of the flower may be carried so far that at last a flower consists of only one part of one whorl. In the Euphorbiaceae we have an excellent example of the gradual suppression of parts, where from an apetalous, trimerous, staminal flower we pass to one where one of the stamens is suppressed, and then to forms where two of them are wanting. We next have flowers in which the calyx is suppressed, and its place occupied by one, two or three bracts (so that the flower is, properly speaking, achlamydeous), and only one or two stamens are produced. And finally, we find flowers consisting of a single stamen with a bract. There is thus traced a degradation, as it is called, from a flower with three stamens and three divisions of the calyx, to one with a single bract and a single stamen.
Degeneration, or the transformation of parts, often gives rise either to an apparent want of symmetry or to irregularity in form. In unisexual flowers it is not uncommon to find vestiges of the undeveloped stamens in the form of filiform bodies or scales. In double flowers transformations of the stamens and pistils take place, so that they appear as petals. In Canna, what are called petals are in reality metamorphosed stamens. In the capitula of Compositae we sometimes find the florets converted into green leaves. The limb of the calyx may appear as a rim, as in some Umbelliferae; or as pappus, in Compositae and Valeriana. In Scrophularia the fifth stamen appears as a scale-like body; in other Scrophulariaceae, as in Pentstemon, it assumes the form of a filament, with hairs at its apex in place of an anther.
Cohesion, or the union of parts of the same whorl, and adhesion, or the growing together of parts of different whorls, are causes of change both as regards form and symmetry. Thus in Cucurbita the stamens are originally five in number, but subsequently some cohere, so that three stamens only are seen in the mature flower. Adhesion is well seen in the gynostemium of orchids, where the stamens and stigmas adhere. In Capparidaceae the calyx and petals occupy their usual position, but the axis is prolonged in the form of a gynophore, to which the stamens are united.
Multiplication, or an increase of the number of parts, gives rise to changes. We have already alluded to the interposition of new members in a whorl. This takes place chiefly in the staminal whorl, but usually the additional parts produced form a symmetrical whorl with the others. In some instances, however, this is not the case. Thus in the horse-chestnut there is an interposition of two stamens, and thus seven stamens are formed in the flower, which is asymmetrical.
Parts of the flower are often increased by a process of deduplication, or chorisis, i.e. the splitting of a part so that two or more parts are formed out of what was originally one. Thus in Cruciferous plants the staminal whorl consists of four long stamens and two short ones (tetradynamous). The symmetry in the flower is evidently dimerous, and the abnormality in the androecium, where the four long stamens are opposite the posterior sepals, takes place by a splitting, at a very early stage of development, of a single outgrowth into two. Many cases of what was considered chorisis are in reality due to the development of stipules from the staminal leaf. Thus in Dicentra and Corydalis there are six stamens in two bundles; the central one of each bundle alone is perfect, the lateral ones have each only half an anther, and are really stipules formed from the staminal leaf. Branching of stamens also produces apparent want of symmetry; thus, in the so-called polyadelphous stamens of Hypericaceae there are really only five stamens which give off numerous branches, but the basal portion remaining short, the branches have the appearance of separate stamens, and the flower thus seems asymmetrical.
Cultivation has a great effect in causing changes in the various parts of plants. Many alterations in form, size, number and adhesion of parts are due to the art of the horticulturist. The changes in the colour and forms of flowers thus produced are endless. In the dahlia the florets are rendered quilled, and are made to assume many glowing colours. In pelargonium the flowers have been rendered larger and more showy; and such is also the case with the Ranunculus, the auricula and the carnation. Some flowers, with spurred petals in their usual state, as columbine, are changed so that the spurs disappear; and others, as Linaria, in which one petal only is usually spurred, are altered so as to have all the petals spurred, and to present what are called pelorian varieties.
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| Fig. 41. | Fig. 42. | Fig. 43. |
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Fig. 41.—Tetramerous monochlamydeous male flower of the Nettle (Urtica). | ||
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Fig. 42.—Diagram to illustrate valvular or valvate aestivation, in which the parts are placed in a circle, without overlapping or folding. | ||
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Fig. 43.—Diagram to illustrate induplicative or induplicate aestivation, in which the parts of the verticil are slightly turned inwards at the edges. | ||
As a convenient method of expressing the arrangement of the parts of the flower, floral formulae have been devised. Several modes of expression are employed. The following is a very simple mode which has been proposed:—The several whorls are represented by the letters S (sepals), P (petals), St (stamens), C (carpels), and a figure marked after each indicates the number of parts in that whorl. Thus the formula S5P5St5C5 means that the flower is perfect, and has pentamerous symmetry, the whorls being isomerous. Such a flower as that of Sedum (fig. 33) would be represented by the formula S5P5St5+5C5, where St5+5 indicates that the staminal whorl consists of two rows of five parts each. A flower such as the male flower of the nettle (fig. 41) would be expressed S4P0St4C0. When no other mark is appended the whorls are supposed to be alternate; but if it is desired to mark the position of the whorls special symbols are employed. Thus, to express the superposition of one whorl upon another, a line is drawn between them, e.g. the symbol S5P5 | St5C5 is the formula of the flower of Primulaceae.
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| Fig. 44. | Fig. 45. | Fig. 46. |
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Fig. 44.—Diagram to illustrate reduplicative or reduplicate aestivation, in which the parts of the whorl are slightly turned outwards at the edges. | ||
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Fig. 45.—Diagram to illustrate contorted or twisted aestivation, in which the parts of the whorl are overlapped by each other in turn, and are twisted on their axis. | ||
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Fig. 46.—Diagram to illustrate the quincuncial aestivation, in which the parts of the flower are arranged in a spiral cycle, so that 1 and 2 are wholly external, 4 and 5 are internal, and 3 is partly external and partly overlapped by 1. | ||
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| Fig. 47. | Fig. 48. |
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Fig. 47.—Diagram to illustrate imbricated aestivation, in which the parts are arranged in a spiral cycle, following the order indicated by the figures 1, 2, 3, 4, 5. | |
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Fig. 48.—Diagram of a papilionaceous flower, showing vexillary aestivation. 1 and 2, The alae or wings. 3, A part of the carina or keel. 4, The vexillum or standard, which, in place of being internal, as marked by the dotted line, becomes external. 5, The remaining part of the keel. The order of the cycle is indicated by the figures. | |
The manner in which the parts are arranged in the flower-bud with respect to each other before opening is the aestivation or praefloration. The latter terms are applied to the flower-bud in the same way as vernation is to the leaf-bud, and distinctive names have been given to the different arrangements exhibited, both by the leaves individually and in their relations to each other. As regards each leaf of the flower, it is either spread out, as the sepals in the bud of the lime-tree, or folded upon itself (conduplicate), as in the petals of some species of Lysimachia, or slightly folded inwards or outwards at the edges, as in the calyx of some species of clematis and of some herbaceous plants, or rolled up at the edges (involute or revolute), or folded transversely, becoming crumpled or corrugated, as in the poppy. When the parts of a whorl are placed in an exact circle, and are applied to each other by their edges only, without overlapping or being folded, thus resembling the valves of a seed-vessel, the aestivation is valvate (fig. 42). The edges of each of the parts may be turned either inwards or outwards; in the former case the aestivation is induplicate (fig. 43), in the latter case reduplicate (fig. 44). When the parts of a single whorl are placed in a circle, each of them exhibiting a torsion of its axis, so that by one of its sides it overlaps its neighbour, whilst its side is overlapped in like manner by that standing next to it, the aestivation is twisted or contorted (fig. 45). This arrangement is characteristic of the flower-buds of Malvaceae and Apocynaceae, and it is also seen in Convolvulaceae and Caryophyllaceae. When the flower expands, the traces of twisting often disappear, but sometimes, as in Apocynaceae, they remain. Those forms of aestivation are such as occur in cyclic flowers, and they are included under circular aestivation. But in spiral flowers we have a different arrangement; thus the leaves of the calyx of Camellia japonica cover each other partially like tiles on a house. This aestivation is imbricate. At other times, as in the petals of Camellia, the parts envelop each other completely, so as to become convolute. This is also seen in a transverse section of the calyx of Magnolia grandiflora, where each of the three leaves embraces that within it. When the parts of a whorl are five, as occurs in many dicotyledons, and the imbrication is such that there are two parts external, two internal, and a fifth which partially covers one of the internal parts by its margin, and is in its turn partially covered by one of the external parts, the aestivation is quincuncial (fig. 46). This quincunx is common in the corolla of Rosaceae. In fig. 47 a section is given of the bud of Antirrhinum majus, showing the imbricate spiral arrangement. In this case it will be seen that the part marked 5 has, by a slight change in position, become overlapped by 1. This variety of imbricate aestivation has been termed cochlear. In flowers such as those of the pea (fig. 40), one of the parts, the vexillum, is often large and folded over the others, giving rise to vexillary aestivation (fig. 48), or the carina may perform a similar office, and then the aestivation is carinal, as in the Judas-tree (Cercis Siliquastrum). The parts of the several verticils often differ in their mode of aestivation. Thus, in Malvaceae the corolla is contorted and the calyx valvate, or reduplicate; in St John’s-wort the calyx is imbricate, and the corolla contorted. In Convolvulaceae, while the corolla is twisted, and has its parts arranged in a circle, the calyx is imbricate, and exhibits a spiral arrangement. In Guazuma the calyx is valvate, and the corolla induplicate. The circular aestivation is generally associated with a regular calyx and corolla, while the spiral aestivations are connected with irregular as well as with regular forms.
The sepals are sometimes free or separate from each other, at other times they are united to a greater or less extent; in the former case, the calyx is polysepalous, in the latter gamosepalous or monosepalous. The divisions of the Calyx. calyx present usually the characters of leaves, and in some cases of monstrosity they are converted into leaf-like organs, as not infrequently happens in primulas. They are usually entire, but occasionally they are cut in various ways, as in the rose; they are rarely stalked. Sepals are generally of a more or less oval, elliptical or oblong form, with their apices either blunt or acute. In their direction they are erect or reflexed (with their apices downwards), spreading outwards (divergent or patulous), or arched inwards (connivent). They are usually of a greenish colour (herbaceous); but sometimes they are coloured or petaloid, as in the fuchsia, tropāeolum, globe-flower and pomegranate. Whatever be its colour, the external envelope of the flower is considered as the calyx. The vascular bundles sometimes form a prominent rib, which indicates the middle of the sepal; at other times they form several ribs. The venation is useful as pointing out the number of leaves which constitute a gamosepalous calyx. In a polysepalous calyx the number of the parts is indicated by Greek numerals prefixed; thus, a calyx which has three sepals is trisepalous; one with five sepals is pentasepalous. The sepals occasionally are of different forms and sizes. In Aconite one of them is shaped like a helmet (galeate). In a gamosepalous calyx the sepals are united in various ways, sometimes very slightly, and their number is marked by the divisions at the apex. These divisions either are simple projections in the form of acute or obtuse teeth (fig. 49); or they extend down the calyx as fissures about half-way, the calyx being trifid (three-cleft), quinquefid (five-cleft), &c., according to their number; or they reach to near the base in the form of partitions, the calyx being tripartite, quadripartite, quinquepartite, &c. The union of the parts may be complete, and the calyx may be quite entire or truncate, as in some Correas, the venation being the chief indication of the different parts. The cohesion is sometimes irregular, some parts uniting to a greater extent than others; thus a two-lipped or labiate calyx is formed. The upper lip is often composed of three parts, which are thus posterior or next the axis, while the lower has two, which are anterior. The part formed by the union of the sepals is called the tube of the calyx; the portion where the sepals are free is the limb.
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| Fig. 49. | Fig. 50. | |||||||
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| Fig. 51. | Fig. 52. | Fig. 53. | ||||||
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Fig. 49.—Gamosepalous five-toothed calyx of Campion (Lychnis). | ||||||||
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Fig. 50.—Obsolete calyx (c) of Madder (Rubia) adherent to the pistil, in the form of a rim. | ||||||||
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Fig. 51.—Feathery pappus attached to the fruit of Groundsel (Senecio vulgaris). | ||||||||
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Fig. 52.—Caducous calyx (c) of Poppy. There are two sepals which fall off before the petals expand. | ||||||||
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Fig. 53.—Fruit of Physalis Alkekengi, consisting of the persistent calyx (s), surrounding the berry (fr), derived from the ovary. (After Duchartre.) | ||||||||
Occasionally, certain parts of the sepals undergo marked enlargement. In the violet the calycine segments are prolonged downwards beyond their insertions, and in the Indian cress (Tropaeolum) this prolongation is in the form of a spur (calcar), formed by three sepals; in Delphinium it is formed by one. In Pelargonium the spur from one of the sepals is adherent to the flower-stalk. In Potentilla and allied genera an epicalyx is formed by the development of stipules from the sepals, which form an apparent outer calyx, the parts of which alternate with the true sepals. In Malvaceae an epicalyx is formed by the bracteoles. Degenerations take place in the calyx, so that it becomes dry, scaly and glumaceous (like the glumes of grasses), as in the rushes (Juncaceae); hairy, as in Compositae; or a mere rim, as in some Umbelliferae and Acanthaceae, and in Madder (Rubia tinctorum, fig. 50), when it is called obsolete or marginate. In Compositae, Dipsacaceae and Valerianaceae the calyx is attached to the pistil, and its limb is developed in the form of hairs called pappus (fig. 51). This pappus is either simple (pilose) or feathery (plumose). In Valeriana the superior calyx is at first an obsolete rim, but as the fruit ripens it is shown to consist of hairs rolled inwards, which expand so as to waft the fruit. The calyx sometimes falls off before the flower expands, as in poppies, and is caducous (fig. 52); or along with the corolla, as in Ranunculus, and is deciduous; or it remains after flowering (persistent) as in Labiatae, Scrophulariaceae, and Boraginaceae; or its base only is persistent, as in Datura Stramonium. In Eschscholtzia and Eucalyptus the sepals remain united at the upper part, and become disarticulated at the base or middle, so as to come off in the form of a lid or funnel. Such a calyx is operculate or calyptrate. The existence or non-existence of an articulation determines the deciduous or persistent nature of the calyx.
The receptacle bearing the calyx is sometimes united to the pistil, and enlarges so as to form a part of the fruit, as in the apple, pear, &c. In these fruits the withered calyx is seen at the apex. Sometimes a persistent calyx increases much after flowering, and encloses the fruit without being incorporated with it, becoming accrescent, as in various species of Physalis (fig. 53); at other times it remains in a withered or marcescent form, as in Erica; sometimes it becomes inflated or vesicular, as in sea campion (Silene maritima).
The corolla is the more or less coloured attractive inner floral envelope; generally the most conspicuous whorl. It is present in the greater number of Dicotyledons. Petals differ more from ordinary leaves than sepals do, and are Corolla. much more nearly allied to the staminal whorl. In some cases, however, they are transformed into leaves, like the calyx, and occasionally leaf-buds are developed in their axil They are seldom green, although occasionally that colour is met with, as in some species of Cobaea, Hoya viridiflora, Gonolobus viridiflorus and Pentatropis spiralis. As a rule they are highly coloured, the colouring matter being contained in the cell-sap, as in blue or red flowers, or in plastids (chromoplasts), as generally in yellow flowers, or in both forms, as in many orange-coloured or reddish flowers. The attractiveness of the petal is often due wholly or in part to surface markings; thus the cuticle of the petal of a pelargonium, when viewed with a ½ or ¼-in. object-glass, shows beautiful hexagons, the boundaries of which are ornamented with several inflected loops in the sides of the cells.
Petals are generally glabrous or smooth; but, in some instances, hairs are produced on their surface. Petaline hairs, though sparse and scattered, present occasionally the same arrangement as those which occur on the leaves; thus, in Bombaceae they are stellate. Coloured hairs are seen on the petals of Menyanthes, and on the segments of the perianth of Iris. They serve various purposes in the economy of the flower, often closing the way to the honey-secreting part of the flower to small insects, whose visits would be useless for purposes of pollination. Although petals are usually very thin and delicate in their texture, they occasionally become thick and fleshy, as in Stapelia and Rafflesia; or dry, as in heaths; or hard and stiff, as in Xylopia. A petal often consists of two portions—the lower narrow, resembling the petiole of a leaf, and called the unguis or claw; the upper broader, like the blade of a leaf, and called the lamina or limb. These parts are seen in the petals of the wallflower (fig. 54). The claw is often wanting, as in the crowfoot (fig. 55) and the poppy, and the petals are then sessile. According to the development of veins and the growth of cellular tissue, petals present varieties similar to those of leaves. Thus the margin is either entire or divided into lobes or teeth. These teeth sometimes form a regular fringe round the margin, and the petal becomes fimbriated, as in the pink; or laciniated, as in Lychnis Flos-cuculi; or crested, as in Polygala. Sometimes the petal becomes pinnatifid, as in Schizopetalum. The median vein is occasionally prolonged beyond the summit of the petals in the form of a long process, as in Strophanthus hispidus, where it extends for 7 in.; or the prolonged extremity is folded downwards or inflexed, as in Umbelliferae, so that the apex approaches the base. The limb of the petal may be flat or concave, or hollowed like a boat. In Hellebore the petals become folded in a tubular form, resembling a horn (fig. 56); in aconite (fig. 58) some of the petals resemble a hollow-curved horn, supported on a grooved stalk; while in columbine, violet (fig. 57), snapdragon and Centranthus, one or all of them are prolonged in the form of a spur, and are calcarate. In Valeriana, Antirrhinum and Corydalis, the spur is very short, and the corolla or petal is said to be gibbous, or saccate, at the base. These spurs, tubes and sacs serve as receptacles for the secretion or containing of nectar.
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| Fig. 54. | Fig. 55. | |||
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| Fig. 56. | Fig. 57. | Fig. 58. | ||
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Fig. 54.—Unguiculate or clawed petal of Wallflower (Cheiranthus Cheiri). c, The claw or unguis; l, the blade or lamina. | ||||
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Fig. 55.—Petal of Crowfoot (Ranunculus), without a claw, and thus resembling a sessile leaf. At the base of the petal a nectariferous scale is seen. | ||||
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Fig. 56.—Tubular petal of Hellebore (Helleborus). | ||||
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Fig. 57.—Pansy (Viola tricolor). Longitudinal section of flower; v, bracteole on the peduncle; l, sepals; ls, appendage of sepal; c, petals; cs, spur of the lower petals; fs, glandular appendage of the lower stamens; a, anthers. (After Sachs.) | ||||
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(From Vines’ Students’ Text-Book of Botany, by permission of Swan Sonnenschein & Co.) | ||||
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Fig. 58.—Part of the flower of Aconite (Aconitum Napellus), showing two irregular horn-like petals (p) supported on grooved stalks (o). These serve as nectaries, s, the whorl of stamens inserted on the thalamus and surrounding the pistil. | ||||
A corolla is dipetalous, tripetalous, tetrapetalous or pentapetalous according as it has two, three, four or five separate petals. The general name of polypetalous is given to corollas having separate petals, while monopetalous, gamopetalous or sympetalous is applied to those in which the petals are united. This union generally takes place at the base, and extends more or less towards the apex; in Phyteuma the petals are united at their apices also. In some polypetalous corollas, as that of the vine, the petals are separate at the base and adhere by the apices. When the petals are equal as regards their development and size, the corolla is regular; when unequal, it is irregular. When a corolla is gamopetalous it usually happens that the lower portion forms a tube, while the upper parts are either free or partially united, so as to form a common limb, the point of union of the two portions being the throat, which often exhibits a distinct constriction or dilatation. The number of parts forming such a corolla can be determined by the divisions, whether existing as teeth, crenations, fissures or partitions, or if, as rarely happens, the corolla is entire, by the venation. The union may be equal among the parts, or some may unite more than others.
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Fig. 59.—Rosaceous corolla (c) of the Strawberry (Fragaria vesca), composed of five petals without claws. |
Amongst regular polypetalous corollas may be noticed the rosaceous corolla (fig. 59), in which there are five spreading petals, having no claws, and arranged as in the rose, strawberry and Potentilla; the caryophyllaceous corolla, in which there are five petals with long, narrow, tapering claws, as in many of the pink tribe; the cruciform, having four petals, often unguiculate, placed opposite in the form of a cross, as seen in wallflower, and in other plants called cruciferous. Of irregular polypetalous corollas the most marked is the papilionaceous (fig. 40), in which there are five petals:—one superior (posterior), st, placed next to the axis, usually larger than the rest, called the vexillum or standard; two lateral, a, the alae or wings; two inferior (anterior), partially or completely covered by the alae, and often united slightly by their lower margins, so as to form a single keel-like piece, car, called carina, or keel, which embraces the essential organs. This form of corolla is characteristic of British leguminous plants.
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From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. |
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Fig. 60.—Flower of Campanula medium; |
Regular gamopetalous corollas are sometimes campanulate or bell-shaped, as in (Campanula) (fig. 60); infundibuliform or funnel-shaped, when the tube is like an inverted cone, and the limb becomes more expanded at the apex, as in tobacco; hypocrateriform or salver-shaped, when there is a straight tube surmounted by a flat spreading limb, as in primula (fig. 61); tubular, having a long cylindrical tube, appearing continuous with the limb, as in Spigelia and comfrey; rotate or wheel-shaped, when the tube is very short, and the limb flat and spreading, as in forget-me-not, Myosotis (when the divisions of the rotate corolla are very acute, as in Galium, it is sometimes called stellate or star-like); urceolate or urn-shaped, when there is scarcely any limb, and the tube is narrow at both ends, and expanded in the middle, as in bell-heath (Erica cinerea). Some of these forms may become irregular in consequence of certain parts being more developed than others. Thus, in Veronica, the rotate corolla has one division much smaller than the rest, and in foxglove (Digitalis) there is a slightly irregular companulate corolla. Of irregular gamopetalous corollas there may be mentioned the labiate or lipped (fig. 62), having two divisions of the limb in the form of lips (the upper one, u, composed usually of two united petals, and the lower, l, of three), separated by a gap. In such cases the tube varies in length, and the parts in their union follow the reverse order of what occurs in the calyx, where two sepals are united in the lower lip and three in the upper. When the upper lip of a labiate corolla is much arched, and the lips separated by a distinct gap, it is called ringent (fig. 62). The labiate corolla characterizes the natural order Labiatae. When the lower lip is pressed against the upper, so as to leave only a chink between them, the corolla is said to be personate, as in snapdragon, and some other Scrophulariaceae. In some corollas the two lips become hollowed out in a remarkable manner, as in calceolaria, assuming a slipper-like appearance, similar to what occurs in the labellum of some orchids, as Cypripedium. When a tubular corolla is split in such a way as to form a strap-like process on one side with several tooth-like projections at its apex, it becomes ligulate or strap-shaped (fig. 63). This corolla occurs in many composite plants, as in the florets of dandelion, daisy and chicory. The number of divisions at the apex indicates the number of united petals, some of which, however, may be abortive. Occasionally some of the petals become more united than others, and then the corolla assumes a bilabiate or two-lipped form, as seen in the division of Compositae called Labiatiflorae.
Petals are sometimes suppressed, and sometimes the whole corolla is absent. In Amorpha and Afzelia the corolla is reduced to a single petal, and in some other Leguminous plants it is entirely wanting. In the natural order Ranunculaceae, some genera, such as Ranunculus, globe-flower and paeony, have both calyx and corolla, while others, such as clematis, anemone and Caltha, have only a coloured calyx. Flowers become double by the multiplication of the parts of the corolline whorl; this arises in general from a metamorphosis of the stamens.
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| Fig. 61. | Fig. 62. | Fig. 63. |
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Fig. 61.—Flower of cowslip (Primula veris) cut vertically. s, Sepals joined to form a gamosepalous calyx; c, corolla consisting of tube and spreading limb; a, stamens springing from the mouth of the tube; p, pistil. | ||
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Fig. 62.—Irregular gamopetalous labiate corolla of the Dead-nettle (Lamium album). The upper lip u is composed of two petals united, the lower lip (l) of three. Between the two lips there is a gap. The throat is the part where the tube and the labiate limb join. From the arching of the upper lip this corolla is called ringent. | ||
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Fig. 63.—Irregular gamopetalous ligulate flower of Ragwort (Senecio). It is a tubular floret, split down on one side, with the united petals forming a straplike projection. The lines on the flat portion indicate the divisions of the five petals. From the tubular portion below, the bifid style projects slightly. | ||
Certain structures occur on the petals of some flowers, which received in former days the name of nectaries. The term nectary was very vaguely applied by Linnaeus to any part of the flower which presented an unusual aspect, as the crown (corona) of narcissus, the fringes of the Passion-flower, &c. If the name is retained it ought properly to include only those parts which secrete a honey-like substance, as the glandular depression at the base of the perianth of the fritillary, or on the petal of Ranunculus (fig. 55), or on the stamens of Rutaceae. The honey secreted by flowers attracts insects, which, by conveying the pollen to the stigma, effect fertilization. The horn-like nectaries under the galeate sepal of aconite (fig. 58) are modified petals, so also are the tubular nectaries of hellebore (fig. 56). Other modifications of some part of the flower, especially of the corolla and stamens, are produced either by degeneration or outgrowth, or by chorisis, or deduplication. Of this nature are the scales on the petals in Lychnis, Silene and Cynoglossum, which are formed in the same way as the ligules of grasses. In other cases, as in Samolus, the scales are alternate with the petals, and may represent altered stamens. In Narcissus the appendages are united to form a crown, consisting of a membrane similar to that which unites the stamens in Pancratium. It is sometimes difficult to say whether these structures are to be referred to the corolline or to the staminal row.
Petals are attached to the axis usually by a narrow base. When this attachment takes place by an articulation, the petals fall off either immediately after expansion (caducous) or after fertilization (deciduous). A corolla which is continuous with the axis and not articulated to it, as in campanula and heaths, may be persistent, and remain in a withered or marcescent state while the fruit is ripening. A gamopetalous corolla falls off in one piece; but sometimes the base of the corolla remains persistent, as in Rhinanthus and Orobanche.
The stamens and the pistil are sometimes spoken of as the essential organs of the flower, as the presence of both is required in order that perfect seed may be produced. As with few exceptions the stamen represents a leaf which has been specially developed to bear the pollen or microspores, it is spoken of in comparative morphology as a microsporophyll; similarly the carpels which make up the pistil are the megasporophylls (see Angiosperms). Hermaphrodite or bisexual flowers are those in which both these organs are found; unisexual or diclinous are those in which only one of these organs appears,—those bearing stamens only, being staminiferous or “male”; those having the pistil only, pistilliferous or “female.” But even in plants with hermaphrodite flowers self-fertilization is often provided against by the structure of the parts or by the period of ripening of the organs. For instance, in Primula and Linum some flowers have long stamens and a pistil with a short style, the others having short stamens and a pistil with a long style. The former occur in the so-called thrum-eyed primroses (fig. 61), the latter in the “pin-eyed.” Such plants are called dimorphic. Other plants are trimorphic, as species of Lythrum, and proper fertilization is only effected by combination of parts of equal length. In some plants the stamens are perfected before the pistil; these are called proterandrous, as in Ranunculus repens, Silene maritima, Zea Mays. In other plants, but more rarely, the pistil is perfected before the stamens, as in Potentilla argentea, Plantago major, Coix Lachryma, and they are termed proterogynous. Plants in which proterandry or proterogyny occurs are called dichogamous. When in the same plant there are unisexual flowers, both male and female, the plant is said to be monoecious, as in the hazel and castor-oil plant. When the male and female flowers of a species are found on separate plants, the term dioecious is applied, as in Mercurialis and hemp; and when a species has male, female and hermaphrodite flowers on the same or different plants, as in Parietaria, it is polygamous.
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From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. |
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Fig. 64.—Flower of Paeonia peregrina, in longitudinal section. k, Sepal; c, petal; a, stamens; g, pistil. (½ nat. size.) |
The stamens arise from the thalamus or torus within the petals, with which they generally alternate, forming one or more whorls, which collectively constitute the androecium. Their normal position is below the pistil, and when Stamens. they are so placed (fig. 64, a) upon the thalamus they are hypogynous. Sometimes they become adherent to the petals, or are epipetalous, and the insertion of both is looked upon as similar, so that they are still hypogynous, provided they are independent of the calyx and the pistil. In other cases they are perigynous or epigynous (fig. 65). Numerous intermediate forms occur, especially amongst Saxifragaceae, where the parts are half superior or half inferior. Where the stamens become adherent to the pistil so as to form a column, the flowers are said to be gynandrous, as in Aristolochia (fig. 66). These arrangements of parts are of great importance in classification. The stamens vary in number from one to many hundreds. In acyclic flowers there is often a gradual transition from petals to stamens, as in the white water-lily (fig. 31). When flowers become double by cultivation, the stamens are converted into petals, as in the paeony, camellia, rose, &c. When there is only one whorl the stamens are usually equal in number to the sepals or petals, and are arranged opposite to the former, and alternate with the latter. The flower is then isostemonous. When the stamens are not equal in number to the sepals or petals, the flower is anisostemonous. When there is more than one whorl of stamens, then the parts of each successive whorl alternate with those of the whorl preceding it. The staminal row is more liable to multiplication of parts than the outer whorls. A flower with a single row of stamens is haplostemonous. If the stamens are double the sepals or petals as regards number, the flower is diplostemonous; if more than double, polystemonous. The additional rows of stamens may be developed in the usual centripetal (acropetal) order, as in Rhamnaceae; or they may be interposed between the pre-existing ones or be placed outside them, i.e. develop centrifugally (basipetally), as in geranium and oxalis, when the flower is said to be obdiplostemonous. When the stamens are fewer than twenty they are said to be definite; when above twenty they are indefinite, and are represented by the symbol ∞. The number of stamens is indicated by the Greek numerals prefixed to the term androus; thus a flower with one stamen is monandrous, with two, three, four, five, six or many stamens, di-, tri-, tetr-, pent-, hex- or polyandrous, respectively.
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From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. |
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Fig. 66.—Flowers of Aristolochia Clematitis cut through longitudinally. I. Young flower in which the stigma (N) is receptive and the stamens (S) have not yet opened; II. Older flower with the stamens (S) opened, the stigma withered, and the hairs on the corolla dried up. |
 Fig. 65.—Flower of Aralia in vertical section. c, Calyx; p, petal; e, stamen; s, stigmas. The calyx, petals and stamens spring from above the ovary (o) in which two chambers are shown each with a pendulous ovule; d, disc between the stamens and stigmas. |
The function of the stamen is the development and distribution of the pollen. The stamen usually consists of two parts, a contracted portion, often thread-like, termed the filament (fig. 25 f), and a broader portion, usually of two lobes, termed the anther (a), containing the powdery pollen (p), and supported upon the end of the filament. That portion of the filament in contact with the anther-lobes is termed the connective. If the anther is absent the stamen is abortive, and cannot perform its functions. The anther is developed before the filament, and when the latter is not produced, the anther is sessile, as in the mistletoe.
The filament is usually, as its name imports, filiform or thread-like, and cylindrical, or slightly tapering towards its summit. It is often, however, thickened, compressed and flattened in various ways, becoming petaloid in Canna, Marania, water-lily (fig. 32); subulate or slightly broadened at the base and drawn out into a point like an awl, as in Butomus umbellatus; or clavate, that is, narrow below and broad above, as in Thalictrum. In some instances, as in Tamarix gallica, Peganum Harmala, and Campanula, the base of the filament is much dilated, and ends suddenly in a narrow thread-like portion. In these cases the base may give off lateral stipulary processes, as in Allium and Alyssum calycinum. The filament varies much in length and in firmness. The length sometimes bears a relation to that of the pistil, and to the position of the flower, whether erect or drooping. The filament is usually of sufficient solidity to support the anther in an erect position; but sometimes, as in grasses, and other wind-pollinated flowers, it is very delicate and hair-like, so that the anther is pendulous (fig. 105). The filament is generally continuous from one end to the other, but in some cases it is bent or jointed, becoming geniculate; at other times, as in the pellitory, it is spiral. It is colourless, or of different colours. Thus in fuchsia and Poinciana, it is red; in Adamia and Tradescantia virginica, blue; in Oenothera and Ranunculus acris, yellow.
 Fig. 67.—Spikelet of Reed (Phragmites communis) opened out. a, b, Barren glumes; c, fertile glumes, each enclosing one flower with its pale, d; the zigzag axis (rhachilla) bears long silky hairs. |
Hairs, scales, teeth or processes of different kinds are sometimes times developed on the filament. In spiderwort (Tradescantia virginica) the hairs are beautifully coloured, moniliform or necklace-like, and afford good objects for studying rotation of the protoplasm. Filaments are usually articulated to the thalamus or torus, and the stamens fall off after fertilization; but in Campanula and some other plants they are continuous with the torus, and the stamens remain persistent, although in a withered state. Changes are produced in the whorl of stamens by cohesion of the filaments to a greater or less extent, while the anthers remain free; thus, all the filaments of the androecium may unite, forming a tube round the pistil, or a central bundle when the pistil is abortive, the stamens becoming monadelphous, as occurs in plants of the Mallow tribe; or they may be arranged in two bundles, the stamens being diadelphous, as in Polygala, Fumaria and Pea; in this case the bundles may be equal or unequal. It frequently happens, especially in Papilionaceous flowers, that out of ten stamens nine are united by their filaments, while one (the posterior one) is free (fig. 68). When there are three or more bundles the stamens are triadelphous, as in Hypericum aegyptiacum, or polyadelphous, as in Ricinus communis (castor-oil). In some cases, as in papilionaceous flowers, the stamens cohere, having been originally separate, but in most cases each bundle is produced by the branching of a single stamen. When there are three stamens in a bundle we may conceive the lateral ones as of a stipulary nature. In Lauraceae there are perfect stamens, each having at the base of the filament two abortive stamens or staminodes, which may be analogous to stipules. Filaments sometimes are adherent to the pistil, forming a column (gynostemium), as in Stylidium, Asclepiadaceae, Rafflesia, and Aristolochiaceae (fig. 66); the flowers are then termed gynandrous.
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| Fig. 69. | Fig. 70. |
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Fig. 68.—Stamens and pistil of Sweet Pea (Lathyrus). The stamens are diadelphous, nine of them being united by their filaments (f), while one of them (e) is free; st, stigma; c, calyx. | |
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Fig. 69.—Portion of wall of anther of Wallflower (Cheiranthus). ce, Exothecium; cf, endothecium; highly magnified. | |
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Fig. 70.—Quadrilocular or tetrathecal anther of the flowering Rush (Butomus umbellatus). The anther entire (a) with its filament; section of anther (b) showing the four loculi. | |
The anther consists of lobes containing the minute powdery pollen grains, which, when mature, are discharged by a fissure or opening of some sort. There is a double covering of the anther—the outer, or exothecium, resembles the The anther. epidermis, and often presents stomata and projections of different kinds (fig. 69); the inner, or endothecium, is formed by a layer or layers of cellular tissue (fig. 69, cf), the cells of which have a spiral, annular, or reticulated thickening of the wall. The endothecium varies in thickness, generally becoming thinner towards the part where the anther opens, and there disappears entirely. The walls of the cells are frequently absorbed, so that when the anther attains maturity the fibres are alone left, and these by their elasticity assist in discharging the pollen. The anther is developed before the filament, and is always sessile in the first instance, and sometimes continues so. It appears at first as a simple cellular papilla of meristem, upon which an indication of two lobes soon appears. Upon these projections the rudiments of the pollen-sacs are then seen, usually four in number, two on each lobe. In each a differentiation takes place in the layers beneath the epidermis, by which an outer layer of small-celled tissue surrounds an inner portion of large cells. Those central cells are the mother-cells of the pollen, whilst the small-celled layer of tissue external to them becomes the endothecium, the exothecium being formed from the epidermal layer.
In the young state there are usually four pollen-sacs, two for each anther-lobe, and when these remain permanently complete it is a quadrilocular or tetrathecal anther (fig. 70). Sometimes, however, only two cavities remain in the anther, by union of the sacs in each lobe, in which case the anther is said to be bilocular or dithecal. Sometimes the anther has a single cavity, and becomes unilocular, or monothecal, or dimidiate, either by the disappearance of the partition between the two lobes, or by the abortion of one of its lobes, as in Styphelia laeta and Althaea officinalis (hollyhock). Occasionally there are numerous cavities in the anther, as in Viscum and Rafflesia. The form of the anther-lobes varies. They are generally of a more or less oval or elliptical form, or they may be globular, as in Mercurialis annua; at other times linear or clavate: curved, flexuose, or sinuose, as in bryony and gourd. According to the amount of union of the lobes and the unequal development of different parts of their surface an infinite variety of forms is produced. That part of the anther to which the filament is attached is the back, the opposite being the face. The division between the lobes is marked on the face of the anther by a groove or furrow, and there is usually on the face a suture, indicating the line of dehiscence. The suture is often towards one side in consequence of the valves being unequal. The stamens may cohere by their anthers, and become syngenesious, as in composite flowers, and in lobelia, jasione, &c.
The anther-lobes are united to the connective, which is either continuous with the filament or articulated with it. When the filament is continuous with the connective, and is prolonged so that the anther-lobes appear to be united The connective. to it throughout their whole length, and lie in apposition to it and on both sides of it, the anther is said to be adnate or adherent; when the filament ends at the base of the anther, then the latter is innate or erect. In these cases the anther is to a greater or less degree fixed. When, however, the attachment is very narrow, and an articulation exists, the anthers are movable (versatile) and are easily turned by the wind, as in Tritonia, grasses (fig. 105), &c., where the filament is attached only to the middle of the connective. The connective may unite the anther-lobes completely or only partially. It is sometimes very short and is reduced to a mere point, so that the lobes are separate or free. At other times it is prolonged upwards beyond the lobes, assuming various forms, as in Acalypha and oleander; or it is extended backwards and downwards, as in violet (fig. 71), forming a nectar-secreting spur. In Salvia officinalis the connective is attached to the filament in a horizontal manner, so as to separate the two anther-lobes (fig. 72), one only of which contains pollen, the other being imperfectly developed and sterile. The connective is joined to the filament by a movable joint forming a lever which plays an important part in the pollination-mechanism. In Stachys the connective is expanded laterally, so as to unite the bases of the anther-lobes and bring them into a horizontal line.
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| Fig. 71. | Fig. 73. | Fig. 74. | ||||
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Fig. 71.—Two stamens of Pansy (Viola tricolor), with their two anther-lobes and the connectives (p) extending beyond them. One of the stamens has been deprived of its spur, the other shows its spur c. | ||||||
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Fig. 72.—Anther of Salvia officinalis. lf, fertile lobe full of pollen; ls, barren lobe without pollen; e, connective; f, filament. | ||||||
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Fig. 73.—Stamen of Lady’s Mantle (Alchemilla), with the anther opening transversely. | ||||||
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Fig. 74.—Stamen of a species of Nightshade (Solanum), showing the divergence of the anther-lobes at the base, and the dehiscence by pores at the apex. | ||||||
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Fig. 75.—The stamen of the Barberry (Berberis vulgaris), showing one of the valves of the anther (v) curved upwards, bearing the pollen on its inner surface. | ||||||
The opening or dehiscence of the anthers to discharge their contents takes place either by clefts, by valves, or by pores. When the anther-lobes are erect, the cleft is lengthwise along the line of the suture—longitudinal dehiscence (fig. 25). At other Antherdehiscence. times the slit is horizontal, from the connective to the side, as in Alchemilla arvensis (fig. 73) and in Lemna; the dehiscence is then transverse. When the anther-lobes are rendered horizontal by the enlargement of the connective, then what is really longitudinal dehiscence may appear to be transverse. The cleft does not always proceed the whole length of the anther-lobe at once, but often for a time it extends only partially. In other instances the opening is confined to the base or apex, each loculament opening by a single pore, as in Pyrola, Tetratheca juncea, Rhododendron, Vaccinium and Solanum (fig. 74), where there are two, and Poranthera, where there are four; whilst in the mistletoe the anther has numerous pores for the discharge of the pollen. Another mode of dehiscence is the valvular, as in the barberry (fig. 75), where each lobe opens by a valve on the outer side of the suture, separately rolling up from base to apex; in some of the laurel tribe there are two such valves for each lobe, or four in all. In some Guttiferae, as Hebradendron cambogioides (the Ceylon gamboge plant), the anther opens by a lid separating from the apex (circumscissile dehiscence).
The anthers dehisce at different periods during the process of flowering; sometimes in the bud, but more commonly when the pistil is fully developed and the flower is expanded. They either dehisce simultaneously or in succession. In the latter case individual stamens may move in succession towards the pistil and discharge their contents, as in Parnassia palustris, or the outer or the inner stamens may first dehisce, following thus a centripetal or centrifugal order. These variations are intimately connected with the arrangements for transference of pollen. The anthers are called introrse when they dehisce by the surface next to the centre of the flower; they are extrorse when they dehisce by the outer surface; when they dehisce by the sides, as in Iris and some grasses, they are laterally dehiscent. Sometimes, from their versatile nature, anthers originally introrse become extrorse, as in the Passion-flower and Oxalis.
The usual colour of anthers is yellow, but they present a great variety in this respect. They are red in the peach, dark purple in the poppy and tulip, orange in Eschscholtzia, &c. The colour and appearance of the anthers often change after they have discharged their functions.
Stamens occasionally become sterile by the degeneration or non-development of the anthers, when they are known as staminodia, or rudimentary stamens. In Scrophularia the fifth stamen appears in the form of a scale; and in many Pentstemons it is reduced to a filament with hairs or a shrivelled membrane at the apex. In other cases, as in double flowers, the stamens are converted into petals; this is also probably the case with such plants as Mesembryanthemum, where there is a multiplication of petals in several rows. Sometimes, as in Canna, one of the anther-lobes becomes abortive, and a petaloid appendage is produced. Stamens vary in length as regards the corolla. Some are enclosed within the tube of the flower, as in Cinchona (included); others are exserted, or extend beyond the flower, as in Littorella or Plantago. Sometimes the stamens in the early state of the flower project beyond the petals, and in the progress of growth become included, as in Geranium striatum. Stamens also vary in their relative lengths. When there is more than one row or whorl in a flower, those on the outside are sometimes longest, as in many Rosaceae; at other times those in the interior are longest, as in Luhea. When the stamens are in two rows, those opposite the petals are usually shorter than those which alternate with the petals. It sometimes happens that a single stamen is longer than all the rest. A definite relation, as regards number, sometimes exists between the long and the short stamens. Thus, in some flowers the stamens are didynamous, having only four out of five stamens developed, and the two corresponding to the upper part of the flower longer than the two lateral ones. This occurs in Labiatae and Scrophulariaceae (fig. 76). Again, in other cases there are six stamens, whereof four long ones are arranged in pairs opposite to each other, and alternate with two isolated short ones (fig. 77), giving rise to tetradynamous flowers, as in Cruciferae. Stamens, as regards their direction, may be erect, turned inwards, outwards, or to one side. In the last-mentioned case they are called declinate, as in amaryllis, horse-chestnut and fraxinella.
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Fig. 76.—Corolla of foxglove (Digitalis purpurea), cut in order to show the didynamous stamens (two long and two short) which are attached to it. |
From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. | |
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Fig. 77.—Tetradynamous stamens (four long and two short) of wallflower (Cheiranthus Cheiri). |
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| Fig. 78. | Fig. 79. | Fig. 80. |
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Fig. 78.—Pollinia, or pollen-masses, with their retinacula (g) or viscid matter attaching them at the base. The pollen masses (p) are supported on stalks or caudicles (c). These masses are easily detached by the agency of insects. Much enlarged. | ||
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Fig. 79.—Pistil of Asclepias (a) with pollen-masses (p) adhering to the stigma (s). b, pollen-masses, removed from the stigma, united by a gland-like body. Enlarged. | ||
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Fig. 80.—Stamen of Asclepias, showing filament f, anther a, and appendages p. Enlarged. | ||
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Fig. 81.—Pollen of Hollyhock (Althaea rosea), highly magnified. |
The pollen-grains or microspores contained in the anther consist of small cells, which are developed in the large thick-walled mother-cells formed in the interior of the pollen-sacs (microsporangia) of the young anther. These mother-cells are either separated from one another and float in the granular fluid which fills up the cavity of the pollen-sac, or are not so isolated. A division takes place, by which four cells are formed in each, the exact mode of division differing in dicotyledons and monocotyledons. These cells are the pollen-grains. They increase in size and acquire a cell-wall, which becomes differentiated into an outer cuticular layer, or extine, and an inner layer, or intine. Then the walls of the mother-cells are absorbed, and the pollen-grains float freely in the fluid of the pollen-sacs, which gradually disappears, and the mature grains form a powdery mass within the anther. They then either remain united in fours, or multiples of four, as in some acacias, Periploca graeca and Inga anomala, or separate into individual grains, which by degrees become mature pollen. Occasionally the membrane of the mother-cell is not completely absorbed, and traces of it are detected in a viscid matter surrounding the pollen-grains, as in Onagraceae. In orchidaceous plants the pollen-grains are united into masses, or pollinia (fig. 78), by means of viscid matter. In orchids each of the pollen-masses has a prolongation or stalk (caudicle) which adheres to a prolongation at the base of the anther (rostellum) by means of a viscid gland (retinaculum) which is either naked or covered. The term clinandrium is sometimes applied to the part of the column in orchids where the stamens are situated. In some orchids, as Cypripedium, the pollen has its ordinary character of separate grains. The number of pollinia varies; thus, in Orchis there are usually two, in Cattleya four, and in Laelia eight. The two pollinia in Orchis Morio contain each about 200 secondary smaller masses. These small masses, when bruised, divide into grains which are united in fours. In Asclepiadaceae the pollinia are usually united in pairs (fig. 79), belonging to two contiguous anther-lobes—each pollen-mass having a caudicular appendage, ending in a common gland, by means of which they are attached to a process of the stigma. The pollinia are also provided with an appendicular staminal covering (fig. 80). The extine is a firm membrane, which defines the figure of the pollen-grain, and gives colour to it. It is either smooth, or covered with numerous projections (fig. 81), granules, points or crested reticulations. The colour is generally yellow, and the surface is often covered with a viscid or oily matter. The intine is uniform in different kinds of pollen, thin and transparent, and possesses great power of extension. In some aquatics, as Zostera, Zannichellia, Naias, &c., only one covering exists.
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From Vines’ Students’ Text-Book of Botany, by permission of Swan Sonnenschein & Co. |
Fig. 83.—Male flower of Pellitory (Parietaria officinalis), having four stamens with in-curved elastic filaments, and an abortive pistil in the centre. When the perianth (p) expands, the filaments are thrown out with force as at a, so as to scatter the pollen. | |
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Fig. 82.—Germinating pollen-grain of Epilobium (highly mag.) bearing a pollen-tube s; e, exine; i, intine; abc, the three spots where the exine is thicker in anticipation of the formation of the pollen-tube developed in this case at a. |
Pollen-grains vary from 1300 to 1700 of an inch or less in diameter. Their forms are various. The most common form of grain is ellipsoidal, more or less narrow at the extremities, which are called its poles, in contradistinction to a line equidistant from the extremities, which is its equator. Pollen-grains are also spherical; cylindrical and curved, as in Tradescantia virginica; polyhedral in Dipsacaceae and Compositae; nearly triangular in section in Proteaceae and Onagraceae (fig. 82). The surface of the pollen-grain is either uniform and homogeneous, or it is marked by folds formed by thinnings of the membrane. There are also rounded portions of the membrane or pores visible in the pollen-grain; these vary in number from one to fifty, and through one or more of them the pollen-tube is extended in germination of the spore. In Monocotyledons, as in grasses, there is often only one, while in Dicotyledons they number from three upwards; when numerous, the pores are either scattered irregularly, or in a regular order, frequently forming a circle round the equatorial surface. Sometimes at the place where they exist, the outer membrane, in place of being thin and transparent, is separated in the form of a lid, thus becoming operculate, as in the passion-flower and gourd. Within the pollen-grain is the granular protoplasm with some oily particles, and occasionally starch. Before leaving the pollen-sac a division takes place in the pollen-grain into a vegetative cell or cells, from which the tube is developed, and a generative cell, which ultimately divides to form the male cells (see Angiosperms and Gymnosperms).
When the pollen-grains are ripe, the anther dehisces and the pollen is shed. In order that fertilization may be effected the pollen must be conveyed to the stigma of the pistil. This process, termed pollination (see Pollination), Pollination. is promoted in various ways,—the whole form and structure of the flower having relation to the process. In some plants, as Kalmia and Pellitory (fig. 83), the mere elasticity of the filaments is sufficient to effect this; in other plants pollination is effected by the wind, as in most of our forest trees, grasses, &c., and in such cases enormous quantities of pollen are produced. These plants are anemophilous. But the common agents for pollination are insects. To allure and attract them to visit the flower the odoriferous secretions and gay colours are developed, and the position and complicated structure of the parts of the flower are adapted to the perfect performance of the process. It is comparatively rare in hermaphrodite flowers for self-fertilization to occur, and the various forms of dichogamy, dimorphism and trimorphism are fitted to prevent this.
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Fig. 84.—Flower of Tree Paeony (Paeonia Moutan), deprived of its corolla, and showing the disk in the form of a fleshy expansion (d) covering the ovary. |
Under the term disk is included every structure intervening between the stamens and the pistil. It was to such structures that the name of nectary was applied by old authors. It presents great varieties of form, such as a ring, scales, Disk. glands, hairs, petaloid appendages, &c., and in the progress of growth it often contains saccharine matter, thus becoming truly nectariferous. The disk is frequently formed by degeneration or transformation of the staminal row. It may consist of processes rising from the torus, alternating with the stamens, and thus representing an abortive whorl; or its parts may be opposite to the stamens. In some flowers, as Jatropha Curcas, in which the stamens are not developed, their place is occupied by glandular bodies forming the disk. In Gesneraceae and Cruciferae the disk consists of tooth-like scales at the base of the stamens. The parts composing the disk sometimes unite and form a glandular ring, as in the orange; or they form a dark-red lamina covering the pistil, as in Paeonia Moutan (fig. 84); or a waxy lining of the hollow receptacle, as in the rose; or a swelling at the top of the ovary, as in Umbelliferae, in which the disk is said to be epigynous. The enlarged torus covering the ovary in Nymphaea (Castalia) and Nelumbium may be regarded as a form of disk.
The pistil or gynoecium occupies the centre or apex of the flower, and is surrounded by the stamens and floral envelopes when these are present. It constitutes the innermost whorl, which after flowering is changed into the fruit The pistil. and contains the seeds. It consists essentially of two parts, a basal portion forming a chamber, the ovary, containing the ovules attached to a part called the placenta, and an upper receptive portion, the stigma, which is either seated on the ovary (sessile), as in the tulip and poppy, or is elevated on a stalk called the style, interposed between the ovary and stigma. The pistil consists of one or more modified leaves, the carpels (or megasporophylls). When a pistil consists of a single carpel it is simple or monocarpellary (fig. 85). When it is composed of several carpels, more or less united, it is compound or polycarpellary (fig. 86). In the first-mentioned case the terms carpel and pistil are synonymous. Each carpel has its own ovary, style (when present), and stigma, and may be regarded as formed by a folded leaf, the upper surface of which is turned inwards towards the axis, and the lower outwards, while from its margins are developed one or more ovules. This comparison is borne out by an examination of the flower of the double-flowering cherry. In it no fruit is produced, and the pistil consists merely of sessile leaves, the limb of each being green and folded, with a narrow prolongation upwards, as if from the midrib, and ending in a thickened portion. In Cycas the carpels are ordinary leaves, with ovules upon their margin.
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| Fig. 85. | Fig. 87. | ||||
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| Fig. 86. | Fig. 89. | Fig. 88. | Fig. 90. | ||
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Fig. 85.—Pistil of Broom (Cytisus) consisting of ovary o, style s, and stigma t. It is formed by a single carpel. | |||||
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Fig. 86.—Vertical section of the flower of Black Hellebore (Helleborus niger). The pistil is apocarpous, consisting of several distinct carpels, each with ovary, style and stigma. The stamens are indefinite, and are inserted below the pistil (hypogynous). | |||||
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Fig. 87.—Fruit of the Strawberry (Fragaria vesca), consisting of an enlarged succulent receptacle, bearing on its surface the small dry seed-like fruits (achenes). | |||||
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Fig. 88.—Fruit of Rosa alba, consisting of the fleshy hollowed axis s′, the persistent sepals s, and the carpels fr. The stamens (c) have withered. (After Duchartre.) | |||||
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Fig. 89.—Pistil of Ranunculus. x, Receptacle with the points of insertion of the stamens a, most of which have been removed. | |||||
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Fig. 90.—Syncarpous Pistil of Flax (Linum), consisting of five carpels, united by their ovaries, while their styles and stigmas are separate. | |||||
A pistil is usually formed by more than one carpel. The carpels may be arranged either at the same or nearly the same height in a verticil, or at different heights in a spiral cycle. When they remain separate and distinct, thus showing at once the composition of the pistil, as in Caltha, Ranunculus, hellebore (fig. 86), and Spiraea, the term apocarpous is applied. Thus, in Sedum (fig. 22) the pistil consists of five verticillate carpels o, alternating with the stamens e. In magnolia and Ranunculus (fig. 89) the separate carpels are numerous and are arranged in a spiral cycle upon an elongated axis or receptacle. In the raspberry the carpels are on a conical receptacle; in the strawberry, on a swollen succulent one (fig. 87); and in the rose (fig. 88), on a hollow one. When the carpels are united, as in the pear, arbutus and chickweed, the pistil becomes syncarpous. The number of carpels in a pistil is indicated by the Greek numeral. A flower with a simple pistil is monogynous; with two carpels, digynous; with three carpels, trigynous, &c.
The union in a syncarpous pistil is not always complete; it may take place by the ovaries alone, while the styles and stigmas remain free (fig. 90), and in this case, when the ovaries form apparently a single body, the organ receives the name of compound ovary; or the union may take place by the ovaries and styles while the stigmas are disunited; or by the stigmas and the summit of the style only. Various intermediate states exist, such as partial union of the ovaries, as in the rue, where they coalesce at their base; and partial union of the styles, as in Malvaceae. The union is usually most complete at the base; but in Labiatae the styles are united throughout their length, and in Apocynaceae and Asclepiadaceae the stigmas only. When the union is incomplete, the number of the parts of a compound pistil may be determined by the number of styles and stigmas; when complete, the external venation, the grooves on the surface, and the internal divisions of the ovary indicate the number.
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| Fig. 92. | Fig. 93. | Fig. 94. |
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Fig. 92.—Trilocular ovary of the Lily (Lilium), cut transversely. s, Septum; o, ovules, which form a double row in the inner angle of each chamber. Enlarged. | ||
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Fig. 93.—Diagrammatic section of a quinquelocular ovary, composed of five carpels, the edges of which are folded inwards, and meet in the centre forming the septa, s. The ovules (o) are attached to a central placenta, formed by the union of the five ventral sutures. Dorsal suture, l. | ||
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Fig. 94.—Diagrammatic section of a five-carpellary ovary, in which the edges of the carpels, bearing the placentas and ovules o, are not folded inwards. The placentas are parietal, and the ovules appear sessile on the walls of the ovary. The ovary is unilocular. | ||
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| Fig. 97. | Fig. 98. | ||||||
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Fig. 95.—Diagrammatic section of a five-carpellary ovary, in which the septa (s) proceed inwards for a certain length, bearing the placentas and ovules (o). In this case the ovary is unilocular, and the placentas are parietal. Dorsal suture, l. | |||||||
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Fig. 96.—Pistil of Pansy (Viola tricolor), enlarged. 1, Vertical; 2, horizontal section; c, calyx; d, wall of ovary; o, ovules; p, placenta; s, stigma. | |||||||
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Fig. 97.—Transverse section of the fruit of the Melon (Cucumis Melo), showing the placentas with the seeds attached to them. The three carpels forming the pepo are separated by partitions. From the centre, processes go to circumference, ending in curved placentas bearing the ovules. | |||||||
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Fig. 98.—Diagrammatic section of a compound unilocular ovary, in which there are no indications of partitions. The ovules (o) are attached to a free central placenta, which has no connexion with the walls of the ovary. | |||||||
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Fig. 91.—Pistil of Pea after fertilization of the ovules, developing to form the fruit. f, Funicle or stalk of ovule (ov); pl, placenta; s, withered style and stigma; c, persistent calyx. | |
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| Fig. 99. | Fig. 100. |
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Fig. 99.—Pistil of Cerastium hirsutum cut vertically. o, Ovary; p, free central placenta; g, ovules; s, styles. | |
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Fig. 100.—The same cut horizontally, and the halves separated so as to show the interior of the cavity of the ovary o, with the free central placenta p, covered with ovules g. | |
The ovules are attached to the placenta, which consists of a mass of cellular tissue, through which the nourishing vessels pass to the ovule. The placenta is usually formed on the edges of the carpellary leaf (fig. 91)—marginal. The placenta. In many cases, however, the placentas are formations from the axis (axile), and are not connected with the carpellary leaves. In marginal placentation the part of the carpel bearing the placenta is the inner or ventral suture, corresponding to the margin of the folded carpellary leaf, while the outer or dorsal suture corresponds to the midrib of the carpellary leaf. As the placenta is formed on each margin of the carpel it is essentially double. This is seen in cases where the margins of the carpel do not unite, but remain separate, and consequently two placentas are formed in place of one. When the pistil is formed by one carpel the inner margins unite and form usually a common marginal placenta, which may extend along the whole margin of the ovary as far as the base of the style (fig. 91), or may be confined to the base or apex only. When the pistil consists of several separate carpels, or is apocarpous, there are generally separate placentas at each of their margins. In a syncarpous pistil, on the other hand, the carpels are so united that the edges of each of the contiguous ones, by their union, form a septum or dissepiment, and the number of these septa consequently indicates the number of carpels in the compound pistil (fig. 92). When the dissepiments extend to the centre or axis, the ovary is divided into cavities or cells, and it may be bilocular, triloculur (fig. 92), quadrilocular, quinquelocular, or multilocular, according as it is formed by two, three, four, five or many carpels, each carpel corresponding to a single cell. In these cases the marginal placentas meet in the axis, and unite so as to form a single central one (figs. 92, 93), and the ovules appear in the central angle of the loculi. When the carpels in a syncarpous pistil do not fold inwards so that the placentas appear as projections on the walls of the ovary, then the ovary is unilocular (fig. 95) and the placentas are parietal, as in Viola (fig. 96). In these instances the placentas may be formed at the margin of the united contiguous leaves, so as to appear single, or the margins may not be united, each developing a placenta. Frequently the margins of the carpels, which fold in to the centre, split there into two lamellae, each of which is curved outwards and projects into the loculament, dilating at the end into a placenta. This is well seen in Cucurbitaceae (fig. 97), Pyrola, &c. The carpellary leaves may fold inwards very slightly, or they may be applied in a valvate manner, merely touching at their margins, the placentas then being parietal (fig. 94), and appearing as lines or thickenings along the walls. Cases occur, however, in which the placentas are not connected with the walls of the ovary, and form what is called a free central placenta (fig. 98). This is seen in many of the Caryophyllaceae and Primulaceae (figs. 99, 100). In Caryophyllaceae, however, while the placenta is free in the centre, there are often traces found at the base of the ovary of the remains of septa, as if rupture had taken place, and, in rare instances, ovules are found on the margins of the carpels. But in Primulaceae no vestiges of septa or marginal ovules can be perceived at any period of growth; the placenta is always free, and rises in the centre of the ovary. Free central placentation, therefore, has been accounted for in two ways: either by supposing that the placentas in the early state were formed on the margins of carpellary leaves, and that in the progress of development these leaves separated from them, leaving the placentas and ovules free in the centre; or by supposing that the placentas are not marginal but axile formations, produced by an elongation of the axis, and the carpels verticillate leaves, united together around the axis. The first of these views applies to Caryophyllaceae, the second to Primulaceae.
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| Fig. 101. | Fig. 102. | |
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| Fig. 104. | Fig. 103. | |
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Fig. 101.—Carpel of Lady’s-mantle (Alchemilla) with lateral style s; o, ovary, st, stigma. Enlarged. | ||
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Fig. 102.—Pistil of Primrose (Primula) composed of five carpels which are completely united; o, ovary; s, style; st, stigma. Enlarged. | ||
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Fig. 103.—Gynoecium of the Flower-de-Luce (Iris), consisting of an inferior ovary (o) and a style which divides into three petaloid segments (s), each bearing a stigma (st). | ||
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Fig. 104.—Capsule of Poppy, opening by pores (p), under the radiating peltate stigma (s). | ||
Occasionally, divisions take place in ovaries which are not formed by the edges of contiguous carpels. These are called spurious dissepiments. They are often horizontal, as in Cathartocarpus Fistula, where they consist of transverse cellular prolongations from the walls of the ovary, only developed after fertilization, and therefore more properly noticed under fruit. At other times they are vertical, as in Datura, where the ovary, in place of being two-celled, becomes four-celled; in Cruciferae, where the prolongation of the placentas forms a vertical partition; in Astragalus and Thespesia, where the dorsal suture is folded inwards; and in Oxytropis, where the ventral suture is folded inwards.
The ovary is usually of a more or less spherical or curved form, sometimes smooth and uniform on its surface, at other times hairy and grooved. The grooves usually indicate the divisions between the carpels and correspond to the dissepiments. The dorsal suture may be marked by a slight projection or by a superficial groove. When the ovary is situated on the centre of the receptacle, free from the other whorls, so that its base is above the insertion of the stamens, it is termed superior, as in Lychnis, Primula (fig. 61) and Peony (fig. 64) (see also fig. 28). When the margin of the receptacle is prolonged upwards, carrying with it the floral envelopes and staminal leaves, the basal portion of the ovary being formed by the receptacle, and the carpellary leaves alone closing in the apex, the ovary is inferior, as in pomegranate, aralia (fig. 65), gooseberry and fuchsia (see fig. 30). In some plants, as many Saxifragaceae, there are intermediate forms, in which the term half-inferior is applied to the ovary, whilst the floral whorls are half-superior.
The style proceeds from the summit of the carpel (fig. 102), and is traversed by a narrow canal, in which there are some loose projecting cells, a continuation of the placenta, constituting what is called conducting tissue, which ends in the stigma. This is particularly abundant when The style. the pistil is ready for fertilization. In some cases, owing to more rapid growth of the dorsal side of the ovary, the style becomes lateral (fig. 101); this may so increase that the style appears to arise from near the base, as in the strawberry, or from the base, as in Chrysobalanus Icaco, when it is called basilar. In all these cases the style still indicates the organic apex of the ovary, although it may not be the apparent apex. When in a compound pistil the style of each carpel is thus displaced, it appears as if the ovary were depressed in the centre, and the style rising from the depression in the midst of the carpels seems to come from the torus. Such a style is gynobasic, and is well seen in Boraginaceae. The form of the style is usually cylindrical, more or less filiform and simple; sometimes it is grooved on one side, at other times it is flat, thick, angular, compressed and even petaloid, as in Iris (fig. 103) and Canna. In Goodeniaceae it ends in a cuplike expansion, enclosing the stigma. It sometimes bears hairs, which aid in the application of the pollen to the stigma, and are called collecting hairs, as in Campanula, and also in Aster and other Compositae. These hairs, during the upward growth of the style, come into contact with the already ripened pollen, and carry it up along with them, ready to be applied by insects to the mature stigma of other flowers. In Vicia and Lobelia the hairs frequently form a tuft below the stigma. The styles of a syncarpous pistil are either separate or united; when separate, they alternate with the septa; when united completely, the style is said to be simple (fig. 102). The style of a single carpel, or of each carpel of a compound pistil, may also be divided. Each division of the tricarpellary ovary of Jatropha Curcas has a bifurcate or forked style, and the ovary of Emblica officinalis has three styles, each of which is twice forked. The length of the style is determined by the relation which should subsist between the position of the stigma and that of the anthers, so as to allow the proper application of the pollen. The style is deciduous or persists after fertilization.
The stigma is the termination of the conducting tissue of the style, and is usually in direct communication with the placenta. It consists of loose cellular tissue, and secretes a viscid matter which detains the pollen, and causes it to The stigma. germinate. This secreting portion is, strictly speaking, the true stigma, but the name is generally applied to all the divisions of the style on which the stigmatic apparatus is situated. The stigma alternates with the dissepiments of a syncarpous pistil, or, in other words, corresponds with the back of the loculaments; but in some cases it would appear that half the stigma of one carpel unites with half that of the contiguous carpel, and thus the stigma is opposite the dissepiments, that is, alternates with the loculaments, as in the poppy.
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Fig. 105.—Flower of a grass with glumes removed, showing three stamens and two feathery styles. p, Pale; l, lodicules. Enlarged. |
The divisions of the stigma mark the number of carpels which compose the pistil. Thus in Campanula a five-cleft stigma indicates five carpels; in Bignoniaceae, Scrophulariaceae and Acanthaceae, the two-lobed or bilamellar stigma indicates a bilocular ovary. Sometimes, however, as in Gramineae, the stigma of a single carpel divides. Its position may be terminal or lateral. In Iris it is situated on a cleft on the back of the petaloid divisions of the style (fig. 103). Some stigmas, as those of Mimulus, present sensitive flattened laminae, which close when touched. The stigma presents various forms. It may be globular, as in Mirabilis Jalapa; orbicular, as in Arbutus Andrachne; umbrella-like, as in Sarracenia, where, however, the proper stigmatic surface is beneath the angles of the large expansion of the apex of the style; ovoid, as in fuchsia; hemispherical; polyhedral; radiating, as in the poppy (fig. 104), where the true stigmatic rays are attached to a sort of peltate or shield-like body, which may represent depressed or flattened styles; cucullate, i.e. covered by a hood, in calabar bean. The lobes of a stigma are flat and pointed as in Mimulus and Bignonia, fleshy and blunt, smooth or granular, or they are feathery, as in many grasses (fig. 105) and other wind-pollinated flowers. In Orchidaceae the stigma is situated on the anterior surface of the column beneath the anther. In Asclepiadaceae the stigmas are united to the face of the anthers, and along with them form a solid mass.
The ovule is attached to the placenta, and destined to become the seed. Ovules are most usually produced on the margins of the carpellary leaves, but are also formed over the whole surface of the leaf, as in Butomus. In other instances they rise The ovule. from the floral axis itself, either terminal, as in Polygonaceae and Piperaceae, or lateral, as in Primulaceae and Compositae. The ovule is usually contained in an ovary, and all plants in which the ovule is so enclosed are termed angiospermous; but in Coniferae and Cycadaceae it has no proper ovarian covering, and is called naked, these orders being denominated gymnospermous. In Cycas the altered leaf, upon the margin of which the ovule is produced, and the peltate scales, from which they are pendulous in Zamia, are regarded by all botanists as carpellary leaves. As for the Coniferae great discussion has arisen regarding the morphology of parts in many genera. The carpellary leaves are sometimes united in such a way as to leave an opening at the apex of the pistil, so that the ovules are exposed, as in mignonette. In Leontice thalictroides (Blue Cohosh), species of Ophiopogon, Peliosanthes and Stateria, the ovary ruptures immediately after flowering, and the ovules are exposed; and in species of Cuphea the placenta ultimately bursts through the ovary and corolla, and becomes erect, bearing the exposed ovules. The ovule is attached to the placenta either directly, when it is sessile, or by means of a prolongation funicle (fig. 110, f). This cord sometimes becomes much elongated after fertilization. The part by which the ovule is attached to the placenta or cord is its base or hilum, the opposite extremity being its apex. The latter is frequently turned round in such a way as to approach the base. The ovule is sometimes embedded in the placenta, as in Hydnora.
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| Fig. 106. | Fig. 107. | Fig. 108. | Fig. 109. |
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Figs. 106 and 107.—Successive stages in the development of an ovule. n, Nucellus; i, inner; o, outer integument in section; m, micropyle. | |||
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Fig. 108.—Orthotropous ovule of Polygonum in section, showing the embryo-sac s, in the nucellus n, the different ovular coverings, the base of the nucellus or chalaza ch, and the apex of the ovule with its micropyle m. | |||
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Fig. 109.—Vertical section of the ovule of the Austrian Pine (Pinus austriaca), showing the nucellus a, consisting of delicate cellular tissue containing deep in its substance an embryo-sac b. The micropyle m is very wide. | |||
The ovule appears at first as a small cellular projection from the placenta. The cells multiply until they assume a more or less enlarged ovate form constituting what has been called the nucellus (fig. 106, n), or central cellular mass of the ovule. This nucellus may remain naked, and alone form the ovule, as in some orders of parasitic plants such as Balanophoraceae, Santalaceae, &c.; but in most plants it becomes surrounded by certain coverings or integuments during its development. These appear first in the form of cellular rings at the base of the nucellus, which gradually spread over its surface (figs. 106, 107). In some cases only one covering is formed, especially amongst gamopetalous dicotyledons, as in Compositae, Campanulaceae, also in walnut, &c. But usually besides the single covering another is developed subsequently (fig. 106, o), which gradually extends over that first formed, and ultimately covers it completely, except at the apex. There are thus two integuments to the nucellus, an outer and an inner. The integuments do not completely invest the apex of the nucellus, but an opening termed the micropyle is left. The micropyle indicates the organic apex of the ovule. A single cell of the nucellus enlarges greatly to form the embryo-sac or megaspore (fig. 108, s). This embryo-sac increases in size, gradually supplanting the cellular tissue of the nucellus until it is surrounded only by a thin layer of it; or it may actually extend at the apex beyond it, as in Phaseolus and Alsine media; or it may pass into the micropyle, as in Santalum. In Gymnosperms it usually remains deep in the nucellus and surrounded by a thick mass of cellular tissue (fig. 109). For an account of the further development of the megaspore, and the formation of the egg-cell, from which after fertilization is formed the embryo, see Gymnosperms and Angiosperms.
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| Fig. 110. | Fig. 111. |
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Fig. 110.—Campylotropous ovule of wall-flower (Cheiranthus), showing the funicle f, which attaches the ovule to the placenta; p, the outer, s, the inner coat, n, the nucellus, ch, the chalaza. The ovule is curved upon itself, so that the micropyle is near the funicle. | |
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Fig. 111.—Anatropous ovule of Dandelion (Taraxacum), n, nucellus, which is inverted, so that the chalaza ch, is removed from the base or hilum h, while the micropyle f is near the base. The connexion between the base of the ovule and the base of the nucellus is kept up by means of the raphe r. | |
The point where the integuments are united to the base of the nucellus is called the chalaza (figs. 111, 112). This is often coloured, is of a denser texture than the surrounding tissue, and is traversed by fibro-vascular bundles, which pass from the placenta to nourish the ovule.
When the ovule is so developed that the chalaza is at the hilum (next the placenta), and the micropyle is at the opposite extremity, there being a short funicle, the ovule is orthotropous. This form is well seen in Polygonaceae (fig. 112), Cistaceae, and most gymnosperms. In such an ovule a straight line drawn from the hilum to the micropyle passes along the axis of the ovule. Where, by more rapid growth on one side than on the other, the nucellus, together with the integuments, is curved upon itself, so that the micropyle approaches the hilum, and ultimately is placed close to it, while the chalaza is at the hilum, the ovule is campylotropous (fig. 110). Curved ovules are found in Cruciferae, and Caryophyllaceae. The inverted or anatropous ovule (fig. 111) is the commonest form amongst angiosperms. In this ovule the apex with the micropyle is turned towards the point of attachment of the funicle to the placenta, the chalaza being situated at the opposite extremity; and the funicle, which runs along the side usually next the placenta, coalesces with the ovule and constitutes the raphe (r), which often forms a ridge. The anatropous ovule arises from the placenta as a straight or only slightly curved cellular process, and as it grows, gradually becomes inverted, curving from the point of origin of the integuments (cf. figs. 106, 107). As the first integument grows round it, the amount of inversion increases, and the funicle becomes adherent to the side of the nucellus. Then if a second integument be formed it covers all the free part of the ovule, but does not form on the side to which the raphe is adherent. These may be taken as the three types of ovule; but there are various intermediate forms, such as semi-anatropous and others.
The position of the ovule relative to the ovary varies. When there is a single ovule, with its axis vertical, it may be attached to the placenta at the base of the ovary (basal placenta), and is then erect, as in Polygonaceae and Compositae; or it may be inserted a little above the base, on a parietal placenta, with its apex upwards, and then is ascending, as in Parietaria. It may hang from an apicilar placenta at the summit of the ovary, its apex being directed downwards, and is inverted or pendulous, as in Hippuris vulgaris; or from a parietal placenta near the summit, and then is suspended, as in Daphne Mezereum, Polygalaceae and Euphorbiaceae. Sometimes a long funicle arises from a basal placenta, reaches the summit of the ovary, and there bending over suspends the ovule, as in Armeria (sea-pink); at other times the hilum appears to be in the middle, and the ovule becomes horizontal. When there are two ovules in the same cell, they may be either collateral, that is, placed side by side (fig. 92), or the one may be erect and the other inverted, as in some species of Spiraea and Aesculus; or they may be placed one above another, each directed similarly, as is the case in ovaries containing a moderate or definite number of ovules. Thus, in the ovary of Leguminous plants (fig. 91), the ovules, o, are attached to the extended marginal placenta, one above the other, forming usually two parallel rows corresponding to each margin of the carpel. When the ovules are definite (i.e. are uniform, and can be counted), it is usual to find their attachment so constant as to afford good characters for classification. When the ovules are very numerous (indefinite), while at the same time the placenta is not much developed, their position exhibits great variation, some being directed upwards, others downwards, others transversely; and their form is altered by pressure into various polyhedral shapes. In such cases it frequently happens that some of the ovules are arrested in their development and become abortive.
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From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. | |
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Fig. 112.—Ovary of Polygonum Convolvulus in longitudinal section during fertilization. (✕ 48.) | |
| fs, | Stalk-like base of ovary. |
| fu, | Funicle. |
| cha, | Chalaza. |
| nu, | Nucellus. |
| mi, | Micropyle. |
| ii, | inner, ie, outer integument. |
| e, | Embryo-sac. |
| ek, | Nucleus of embryo-sac. |
| ei, | Egg-apparatus. |
| an, | Antipodal cells. |
| g, | Style. |
| n, | Stigma. |
| p, | Pollen-grains. |
| ps, | Pollen-tubes. |
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| Fig. 113.—Vertical section of the ovule of the Scotch Fir (Pinus sylvestris) in May of the second year, showing the enlarged embryo-sac b, full of endosperm cells, and pollen-tubes c, penetrating the summit of the nucellus after the pollen has entered the large micropyle. |
When the pistil has reached a certain stage in growth it becomes ready for fertilization. Pollination having been effected, and the pollen-grain having reached the stigma in angiosperms, or the summit of the nucellus in gymnosperms, it is detained there, and the viscid secretion from the Fertilization. glands of the stigma in the former case, or from the nucellus in the latter, induce the protrusion of the intine as a pollen-tube through the pores of the grain. The pollen-tube or tubes pass down the canal (fig. 112), through the conducting tissue of the style when present, and reach the interior of the ovary in angiosperms, and then pass to the micropyle of the ovule, one pollen-tube going to each ovule. Sometimes the micropyle lies close to the base of the style, and then the pollen-tube enters it at once, but frequently it has to pass some distance into the ovary, being guided in its direction by various contrivances, as hairs, grooves, &c. In gymnosperms the pollen-grain resting on the apex of the nucellus sends out its pollen-tubes, which at once penetrate the nucellus (fig. 113). In angiosperms when the pollen-tube reaches the micropyle it passes down into the canal, and this portion of it increases considerably in size. Ultimately the apex of the tube comes in contact with the tip of the embryo-sac and perforates it. The male cells in the end of the pollen-tube are then transmitted to the embryo-sac and fertilization is effected. Consequent upon this, after a longer or shorter period, those changes commence in the embryo-sac which result in the formation of the embryo plant, the ovule also undergoing changes which convert it into the seed, and fit it for a protective covering, and a store of nutriment for the embryo. Nor are the effects of fertilization confined to the ovule; they extend to other parts of the plant. The ovary enlarges, and, with the seeds enclosed, constitutes the fruit, frequently incorporated with which are other parts of the flower, as receptacle, calyx, &c. In gymnosperms the pollen-tubes, having penetrated a certain distance down the tissue of the nucellus, are usually arrested in growth for a longer or shorter period, sometimes nearly a year. Fruit and seed are discussed in a separate article—Fruit. (A. B. R.)