Elementary Botany

by George Francis Atkinson

841. In the pteridophytes water serves as the medium for conveying the sperm cell to the female organ.—In the ferns and their allies, as well as in the liverworts and mosses, surface water is a necessary medium through which the generative or sperm cell of the male organ, the spermatozoid, may reach the germ cell of the female organ. The sperm cell is here motile. This is true in a large number of cases in the algæ, which are mostly aquatic plants, while in other cases currents of water float the sperm cell to the female organ.

842. In the higher plants a modification of the prothallium is necessary.—As we pass to the gymnosperms and angiosperms, however, where the primitive phase (the gametophyte) of the plants has become dependent solely on the modern phase (the sporophyte) of the plant, surface water no longer serves as the medium through which a motile sperm cell reaches the egg-cell to fertilize it. The female prothallium, or macrospore, is, in nearly all cases, permanently enclosed within the sporangium, so that if there were motile sperm cells on the outside of the ovary, they could never reach the egg to fertilize it.

843. But a modification of the microspore, the pollen tube, enables the sperm cell to reach the egg-cell. The tube grows through the nucellus, or first through the tissues of the ovary, deriving nutriment therefrom.

844. But here an important consideration should not escape us. The pollen grains (microspores) must in nearly all cases first reach the pistil, in order that in the growth of this tube a channel may be formed through which the generative cell can make its way to the egg cell. The pollen passes from the anther locule, then, to the stigma of the ovary. This process is termed pollination.

Pollination.

845. Self pollination, or close pollination.—Perhaps very few of the admirers of the pretty blue violet have ever noticed that there are other flowers than those which appeal to us through the beautiful colors of the petals. How many have observed that the brightly colored flowers of the blue violet rarely “set fruit”? Underneath the soil or débris at the foot of the plant are smaller flowers on shorter, curved stalks, which do not open. When the anthers dehisce, they are lying close upon the stigma of the ovary, and the pollen is deposited directly upon the stigma of the same flower. This method of pollination is self pollination, or close pollination. These small, closed flowers of the violet have been termed “cleistogamous,” because they are pollinated while the flower is closed, and fertilization takes place as a result.

But self pollination takes place in the case of some open flowers. In some cases it takes place by chance, and in other cases by such movements of the stamens, or of the flower at the time of the dehiscence of the pollen, that it is quite certainly deposited upon the stigma of the same flower.

846. Wind pollination.—The pine is an example of wind-pollinated flowers. Since the pollen floats in the air or is carried by the “wind,” such flowers are anemophilous. Other anemophilous flowers are found in other conifers, in grasses, sedges, many of the ament-bearing trees, and other dicotyledons. Such plants produce an abundance of pollen and always in the form of “dust,” so that the particles readily separate and are borne on the wind.

847. Pollination by insects.—A large number of the plants which we have noted as being anemophilous are monœcious or diœcious, i.e. the stamens and pistils are borne in separate flowers. The two kinds of flowers thus formed, the male and the female, are borne either on the same individual (monœcious) or on different individuals (diœcious). In such cases cross pollination, i.e. the pollination of the pistil of one flower by pollen from another, is sure to take place, if it is pollinated at all. Even in monœcious plants cross pollination often takes place between flowers of different individuals, so that more widely different stocks are united in the fertilized egg, and the strain is kept more vigorous than if very close or identical strains were united.