To the petals the beautifully-varied colours of plants are due, and though it is not possible to enter upon the subject here, we may conclude that the various beauties of the colours of flowers are owing to light and air acting upon the various “colouring matters” contained in the plant. Seeds planted in a dark cellar will spring up pale; admit light, and they will become green, for light thus acts upon the chlorophyl. But the flowers of the plant are not so dependent upon light, as can easily be proved. Many interesting experiments have been made upon flowers by acids and gases.[36]
The Stamens are the next in order for our consideration. They are found within the petals (or the calyx if no petals be present). Stamens vary very much in different plants both in number and general features—but, as in the case of petals, they keep, as a rule, to certain numbers and doubles of them. The stamen consists of two portions—a lower, thread-like part called the filament, and an oblong bag or head, termed the anther. This contains a powdery matter called pollen, and is the essential part of the stamen. The filament, which corresponds to the petiole of the leaf, may be absent, in which case the anther is called sessile. A lily will show the stamens perfectly, the anther being prominent in many other plants also, such as daffodils and fuchsias.
The stamens are very important organs with regard to the classification of plants—for number, length, and position, whether free or united, are all characteristic features. The length of filaments is always the same in the same kind of plant, and therefore is a very palpable test.
The anther contains the pollen, a powdery matter, usually yellow-coloured, but sometimes also red, brown, violet, or green-coloured. Pollen-grains vary from 1/20 to 1/300 of a line in diameter. Under a powerful microscope they appear as ellipsoidal, or sometimes spherical, triangular, polyhedral vesicles, filled with a granular semi-fluid matter. To effect fecundation, the pollen-grains must come into contact with a certain part of the plant which is intended to receive them, and which is called the ovule, and is found in the fourth or innermost verticil of the flower, the pistil. Of the further development of the ovule, we shall have occasion to speak in the paragraph treating of the seed.
At the proper time the anther opens and discharges its contents, the pollen-grains, some of which reach the place of their destination. The position of the stamens to the pistil is usually such that the latter can readily receive the pollen-grains. In many plants, however, the stamens are too short to reach the pistils; or the two essential organs of reproduction are in separate flowers, or even on different plants. In such cases, the conveyance of the pollen from the anthers to the pistils is effected by the agency of the wind, or by that of insects, and more particularly by the bee. If the anthers are removed from the flower previously to their opening, no fruit is produced.
Varieties of flowers and fruits are produced artificially, by shaking the pollen of one plant upon the flowers of another, deprived of the stamens. Many esteemed sorts of stock-gilliflowers and pinks have been produced in this way.
The pistil constitutes the fourth and innermost whorl, and occupies accordingly the centre of the flower and the apex of the axis, whose growth is terminated with the production of the fruit.
Fig. 776.—Pistil.
The pistil also is formed by one or several modified leaves, called carpels, in this part of the flower, and which exhibit a more marked resemblance in colour and structure to the ordinary leaves than the stamens and petals do. The formation of the pistil from the leaf may be considered to proceed in this manner: that the edges of the leaf are folded inwards and unite, whilst the mid-rib is prolonged upwards (fig. 776A). The place where the margins of the folded leaves are united is called suture or seam (ventral suture, in contradistinction to the mid-rib, which is called the dorsal suture); and it is here that the seed-buds or ovules are developed.