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.—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
