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
