Page:EB1911 - Volume 10.djvu/580



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

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.

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