Page:Popular Science Monthly Volume 25.djvu/175

Rh horse-tail develops from the fertilized germ-cell, and constitutes the spore-bearing asexual generation. In some species a colorless or brownish stem comes up in early spring, which bears the spores on whorls of modified leaves, and afterward perishes. Later in the spring the green stems arise. This shows a greater differentiation in the asexual generation.

Next above the horse-tails come the Rhizocarpeœ, a small cryptogamic group of water-plants, inhabiting ditches, streams, etc. Thus far, in our upward course, we have found only one kind of spore. Here there are two sorts, the large and the small. The former produce archegonia, and are therefore essentially female, while the smaller spores are male, and produce antherozoids. These spores are formed in spore-cases, termed sporocarps. Fig. 10 shows a plant of Marsilia salvatrix, reduced one half; K is the terminal bud; b b, leaves; f f, sporocarps. In these last the spores of both sizes are produced. The contents of the small or male spores divide and develop into a number of antherozoids, which afterward escape through a rupture in the spore-wall. A small portion of the spore does not take part in this formation of antherozoids, and may be considered the prothallus. In the large or female spore the prothallus is larger, and only one end of the spore bears a single archegonium. In Fig. 11, at A, is shown



a vertical section of the archegonium end of a large spore; w w are parts of the ruptured spore-wall; p p is the prothallus, and g the germ-cell. At B is a male spore of the same species, with its wall ruptured, and the corkscrew-like antherozoids, 5, escaping. The second generation soon develops from the fertilized germ-cells, and produces the mature plant. It is seen that the sexual generation in the Marsilia group is reduced to two kinds of spores, with their rudimentary prothallia. In another branch of the Rhizocarpeœ, while in most features the life-history is as just described, there is a further differentiation in the sporocarps. The male and female spores are produced in separate sporocarps. Fig. 12 shows a section through three spore-cases, two