NOSPERMS).
Pollination.—When the pollen-sacs 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 pollination (q.v.) is promoted in various ways, the whole form and structure of the flower being adapted to the process. In some plants (e.g., pellitory) the mere elasticity of the filament is sufficient ; in others (anemophilous) pollination is effected by the wind (e.g., grasses) and in such cases enormous quantities of pollen are produced; but the common agents of pollination are insects. To attract them to the flower the odoriferous secretions and gay colours are produced, 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 hermaph rodite flowers for self-fertilization to occur and the various forms of dichogamy, dimorphism and trimorphism prevent this.
The union in a syncarpous pistil is not always complete; it may take place by the ovaries alone (fig. 58), when the organ becomes a compound ovary; or by ovaries and styles; or by stigmas and the summits of these styles. Various intermediate states exist ; the union is usually most complete at the base.
The Placenta.—The ovules are attached to the placenta, through which the fibrovascular bundles pass. The placenta is usually formed on the edges of the carpellary leaf (marginal placentation). But often the placentas arise from the axis (axile placentation) and are not connected with the carpellary leaves. In marginal placentation, the placenta is borne on the inner or ventral suture, corresponding to the margin of the carpellary leaf, the outer or dorsal suture corresponding to the mid-rib. As the placenta is formed on each margin of the carpel, it is essentially double. When the pistil is simple, the inner margins unite and usually form a common placenta. When the pistil is apocarpous, there are generally separate placentas at each margin. In a syncarpous pistil, however, the carpels are so united that the edges of each of the contiguous ones, by their union, form a septum or dis sepiment, and the number of these septa consequently indicates the number of carpels in the pistil. When the septa extend to the axis, the ovary is divided into cells, being bilocular, trilocu lar, etc., according to the number, each cell corresponding to a single carpel. In these cases, the marginal placentas meet in the axis and unite to form a single central one. When the carpels of a syncarpous pistil do not f old inwards, the ovary is unilocular and the placentas are parietal (e.g. Viola). Often the margins of the carpels which f old 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 (e.g., Cucurbi taceae, fig. 63). Cases occur, however, in which the placentas are not connected with the walls of the ovary (free central placenta tion, figs. 64, 65) ; this may be due to the separation of the carpel lary leaves from the axis, as in Caryophyllaceae, in which there are often traces of the septa at the base of the ovary; or to the placenta being an axile formation produced by the elongation of the axis (e.g. Primulaceae).
Occasionally divisions take place in ovaries which are not formed by the edges of contiguous carpels. These spurious dis sepiments are often horizontal, only developed after fertilization. In Cruciferae, however, they are vertical and arise from the pro longation of the placentas.
The ovary is usually spherical or curved, sometimes smooth on its surface, at other times hairy and grooved. The grooves usually indicate the divisions between the carpels. When the ovary is on the centre of the receptacle, free from the outer whorls, so that its base is above the insertion of the stamens, it is superior (e.g. Primula, figs. 43, 67). When the margin of the receptacle is pro longed upwards, carrying with it the floral envelopes and staminal leaves, the basal portion of the ovary being formed by the recep tacle and the carpellary leaves alone closing the apex, the ovary is inferior (e.g., Fuchsia). In many Saxifragaceae these are inter mediate forms (half-inferior).
The Style.—The style proceeds from the summit of the carpel (fig. 67) and is traversed by a narrow canal, a continuation of the placenta, constituting conducting tissue, which ends in the stigma. In some cases, owing to more rapid growth of the dorsal side of the ovary, the style becomes lateral (fig. 66) ; this may be accentuated so that the style appears to arise from the base (basilar) ; but it still indicates the organic though not the ap parent, apex of the ovary. Several basilar styles may unite (e.g., Boraginaceae) to form a single gynobasic style. The style is usually cylindrical, filiform and simple; sometimes it is grooved on one side, or flat, thick, angular, compressed or even petaloid (e.g. Iris). It may bear hairs, which aid in the application of pollen to the stigma (collecting hairs, e.g. Campanula). The styles of a syncarpous pistil, when separate, alternate with the septa; when united, the style is simple (fig. 67). The style of a single carpel may be divided. The length of the style depends upon the relation which should subsist between the position of the stigma and that of the anthers to allow proper application of pollen.
The Stigma.—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 germinate. The stigmas alternate with the septa of a syncarpous pistil, i.e. corresponds to the back of the loculi ; but in some cases half the stigma of one carpel unites with half that of the next, the stigmas being thus opposite the septa (e.g. poppy). The divisions of the stigma usually mark the number of carpels in the pistil, but sometimes (e.g. Gramineae) the stigma of a single carpel divides. It may be terminal or lateral and may present sensitive laminae which close when touched (e.g. Mimulus). It may be globular, umbrella-like, ovoid (e.g. Fuchsia), radiating, as in the poppy, where the true stigmatic rays are attached to a shield-like (pel tate) body. The lobes of the stigma may be flat and pointed, fleshy and blunt, smooth, granular or feathery (e.g. many grasses, fig. 68). In Orchidaceae, the stigma is on the anterior surface of the column beneath the anthers.
The Ovule.—The ovule is usually produced on the margin of the carpellary leaf, but sometimes ovules arise all over the surface of the leaf, or from the floral axis, terminally (e.g. Polygonaceae) or laterally (e.g. Primulaceae). The ovule is usually contained in an ovary and is angiospermous; but in the Coniferae and Cycadaceae it has no proper ovarian covering and is naked or gymnospermous. It is attached to the placenta either directly (sessile) or by a funicle (figs. 71, 72, 73) ; this cord may become much elongated after fertilization. The ovule is attached to the placenta or funicle by its base or hilum, the opposite end being its apex.
The ovule first appears as a small projection from the placenta. The cells multiply and assume an enlarged ovate form consti tuting the nucellus. This nucellus may remain naked and alone form the ovule, as in some parasitic families; but in most plants it becomes surrounded by integuments, which appear first as rings at the base of the nucellus, which gradually spread over its surface. In some cases (e.g., Compositae), only one covering is formed, but usually another is developed subsequently, covering the first completely except at the apex, where neither integument invests the nucellus, but leaves an opening, the micropyle. A single cell of the nucellus enlarges to form the embryo-sac or megaspore, which gradually supplants the tissue of the nucellus until only a thin layer is left, and at the apex it may extend beyond it, or pass into the micropyle. In gymnosperms it usually remains deep in the nucellus. For further development, see ANGIOSPERMS, GYM NOSPERMS.
The point where the integuments are united to the base of the nucellus is the chalaza, which is often coloured, is denser than the surrounding tissue and is traversed by vascular bundles from the placenta. When the chalaza is at the hilum and the micropyle is opposite, there being a short funicle, the ovum is orthotropous (fig. 72). When by more rapid growth on one side than on the other, the nucellus and integuments are curved on themselves so that the micropyle approaches the hilum, the ovule is campylotro pous (e.g., Cruciferae, fig. 71). In an inverted or anatropous ovule (fig. 73) the commonest form in angiosperms, the apex with the micropyle is turned towards the point of attachment of the funicle to the placenta and the funicle coalesces with the ovule to form the raphe. The ovule thus curves from the point of origin of the integuments and if a second integument is formed, it does not extend to the side adherent to the raphe. Forms intermediate between these types occur. 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), when it is erect (e.g., Compositae) ; or it may be inserted a little above the base on a parietal placenta (ascending) ; or it may hang from an apical placenta at the summit of the ovary, its apex being directed downwards (inverted or pendulous) ; or from a parietal placenta near the summit (sus pended, e.g., Euphorbiaceae) . Sometimes a long funicle arises from a basal placenta, reaches the summit of the ovary and there, bend ing over, suspends the ovule (e.g., sea-pink) ; at other times the hilum appears to be in the middle and the ovule is horizontal. When there are two ovules in the same cell they may be side by side (collateral); or one erect and the other inverted; or one above the other, as is the case in ovaries containing a moderate number of ovules. When the ovules are definite (i.e., uniform and can be counted) their attachment usually forms a good character for classification. When very numerous (indefinite) and the placenta little developed, their position shows great variation and their form is altered by pressure into various polyhedral shapes.
Fertilization.—When the pistil has reached a certain stage in growth, it becomes ready for fertilization. The pollen-grain hav ing reached the stigma in angiosperms, or the summit of the nucellus in gymnosperms, it is detained there, and the viscid secretion from the glands of the stigma or from the nucellus in duce the protrusion of a pollen-tube through the pores of the grain. The pollen-tube (or tubes) passes down the canal (fig. 69), through the conducting tissue of the style, when present, and thence to the micropyle of the ovule, one pollen-tube going to each ovule. Frequently the tube has to pass some distance into the ovary, to reach the micropyle, being guided by hairs, grooves, etc. In gymnosperms the pollen-grain resting on the apex of the nucellus sends off its pollen-tubes which at once penetrate the nucellus (fig. 69). Ultimately the apex of the tube perforates the tip of the embryo-sac, the male cells are transmitted to the sac and fertilization is effected. Consequent upon this, after a longer or shorter period, those changes begin in the embryo-sac which result in the formation of the embryo plant, the ovule being converted into the seed, the ovary enlarging to form the fruit, often incorporated with which are other parts of the flower (receptacle, calyx, etc.). In gymnosperms the pollen-tubes, hav ing penetrated a certain distance down the tissue of the nucellus, are arrested in growth for a period, sometimes nearly a year. See ANGIOSPERMS : Flower; FRUIT; SEED.