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.
| From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. |
| 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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| Fig. 65.—Flower ofAralia in vertical section.c, Calyx; p, petal;e, stamen; s, stigmas.The calyx, petals andstamens spring fromabove the ovary (o) inwhich two chambersare shown each with apendulous ovule; d, discbetween the stamensand stigmas. | From Strasburger’s Lehrbuch der Botanik, by permissionof Gustav Fischer. |
| Fig. 66.—Flowers of Aristolochia Clematitiscut through longitudinally. I. Youngflower in which the stigma (N) is receptiveand the stamens (S) have not yet opened;II. Older flower with the stamens (S)opened, the stigma withered, and the hairson the corolla dried up. |
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.
| 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. Fig. 69.—Portion of wall of anther of Wallflower (Cheiranthus). ce, Exothecium; cf, endothecium; highly magnified. 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.