Page:Popular Science Monthly Volume 25.djvu/180

170. The male prothallus is reduced and rudimentary, and one cell of the pollen-grain, representing the antheridium, produces a tube instead of antherozoids. The endosperm is the female prothallus, and in it the germ-cell develops. The asexual generation is the plant that grows from the fertilized germ. In the angiosperms the sexual generation is reduced to its simplest form, namely, a single cell for the male part, and one or a few cells for the female.

We thus see that the alternation of generation, viewed in the light of its presentation among mosses and ferns, practically disappears in the higher flowering plants. The sexual generation is so reduced and merged with the asexual that the two seem to become one, and, were it not that the gradual simplifying of this generation may be traced, it would not be thought to exist.

If we recapitulate, in the reverse order, it is easy to evolve a conspicuous, independent plant from the single-celled pollen-grain, and a similar self-supporting plant from the simple embryo sac. The first step back is to the gymnosperms, where the ovules are not in ovaries, and the embryo sac has a rudimentary prothallus in the endosperm. The pollen-grain is made up of more then one cell. From this group we pass to the Selaginellæ, in which we have the female spore, with its rudimentary prothallus, and the smaller male spore, having one cell for the prothallus, the remaining ones forming the antherozoids. Descending to the Rhizocarpeæ both the spores increase in size and complexity. Further back we come to the higher orders of ferns, with but one kind of spore, the small prothallus, bearing both sexual organs. The lower orders have this sex-bearing generation much more developed. In the mosses and Hepaticæ the sexual generation surpasses the asexual in size and complexity.

The relative size of the two generations might be represented to the eye by drawing a rectangle with a diagonal. Fig. 18. One triangle



would indicate the size and complexity of the sexual generation, while the other represented the asexual generation. The sexual generation diminishes from the Hepaticæ to the angiosperms, while the asexual generation increases.

The engravings here employed are from treatises on botany by Sach, Prantl, and Bessey, to whom the writer is also indebted for many of the facts brought together.