Page:Popular Science Monthly Volume 33.djvu/756

736 parts out of its own material. Old age, as we say, comes on. And this is specially true of the higher and more complex organisms. The tree no longer puts forth new leaves; the plant no longer sends out fresh branches. Its individual vigor appears to be used up. Unless, then, some fresh stimulus can be supplied it from without, the plant must die, and the species thus must suffer extinction.

At this point, therefore, Nature steps in with a special remedy—the special remedy of cross-fertilization. The earliest and simplest form of this device is seen in certain algæ or pond-weeds, mere long green hairs that wave about like tresses in the water, and consist each of endless rows of cells growing out in single file like the beads of a necklace one from the other. But every now and then two of these algæ "conjugate," as biologists put it—that is to say, a cell of one bends over and unites with a cell of the other, the cell-contents (or protoplasm and chlorophyl) of one cell breaking through to join the cell-contents of its neighbor. The union thus effected seems to supply a fresh stimulus to growth: the two matters coalesce and combine, and a new and more vigorous alga springs up as the final result of this combination.

Now, in the higher plants we get exactly the same sort of combination, only far more complex in its mechanism and results. If we take any annual plant, like the pea, and look when and where the flowers are produced, we shall see that they come as soon as the plant has attained its full growth, and when the purely vegetative reproductive impulse is beginning to fail. As a rule, too, the flowers come at the end of the branches, and in many—indeed, in most—plants they form a terminal spike or bunch at the summit of the stem, as in the familiar instances of the hyacinth, the buttercup, the sunflower, or the grasses. In other words, as soon as the vegetative growth is beginning to slacken, the need is felt for "fresh blood," for the special stimulus or fillip to further exertion given by union with another individual.

For the purpose of bringing about the desired union, all the higher plants are supplied with special organs known as stamens and pistils. The pistils produce the embryo seed, which is, in fact, a tiny separate plant, whose development is arrested at a very early stage, unless fresh material from a neighboring stamen is supplied to supplement it. The stamens produce the pollen-grains, which are, in fact, free cells containing a large quantity of very vitalized matter capable of fertilizing and vivifying the embryo seed. When a grain of pollen is placed by any agency whatsoever—wind, an insect, or a camel's-hair brush, as the case may be—on a neighboring stigma, it sends out a pollen-tube which penetrates the ovary and at last enters into and coalesces with the embryo seed itself. The fresh material thus added to the embryo seems