Page:Popular Science Monthly Volume 39.djvu/354

340 wall of the mother-cell, thus leaving the grains separate in the anther. In entomophilous plants, on the contrary, the pollen is often viscid, or else studded with points, so that it may in some way adhere to the legs, bodies, or probosces of insect visitors.

While, as a rule, the pollen grains are free in the anther cells, there are two families, those of the Orchids and the Milkweeds, in which the grains are developed in a peculiar manner. In these families, but especially in the former, the grains cohere to one another by means of a viscid matter, and thus form one mass, technically known as a pollinium (Fig. 6). The cohesion is brought about by the walls of the mother-cells remaining, and so binding the grains more or less completely together. The cohesion is but slight, for, on the application of a pollinium to the viscid surface of a stigma, several grains are left on it, and thus one pollen mass will serve to fertilize several flowers. One of the most striking features of these pollinia is that the stem or caudicle undergoes on exposure to the air an act of depression (Fig. 7), so that while it stands erect on its first withdrawal from the anther cell, in a short time a contraction in the substance of the caudicle is observed, and then the pollinium becomes horizontal.

While in ordinary cases pollen is yellow, there are instances in which this feature varies. For example, in one form of flower of loose-strife (Lythrum salicaria) it is green. In the willow-herb (Epilobium angustifolium) it is blue. In the tulip it is black, and in mullein (Verbascum) it is red.

The seeds of flowering plants are produced by the action of the pollen on the stigma. Though both bulb and seed bear in themselves the potentiality of the future plant, the two are very different. This difference can be well stated by saying that the bulb perpetuates the individual, and the seed the species. When the anther is nearly ripe, its inner wall becomes thinner and thinner, either along certain lines or in particular spots. The process continues until by pressure the wall is ruptured at these places and the pollen escapes. When the grains are placed upon the viscid stigma, the moisture absorbed by endosmose causes the contents to swell, and the intine bursts through the thin places in the extine and protrudes in the form of a tube, which penetrates the stigma to the ovary (Fig. 8). While the tube is at first only a projection of the intine, it afterward becomes a growth, for it is many times larger than could be contained in the grain. It is, therefore, nourished by the conducting tissue of