MODES OF REPRODUCTION IN PLANTS. 



165 



ous 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 Hhizocarpem, a small crypto- 

 gamic 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 pro- 

 duce 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; bb, leaves; 

 ff, 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 jDrothallus is larger, and only one end 

 of the spore bears a single archegonium. In Fig. 11, at A, is shown 



Fia. n. 



a vertical section of the archegonium end of a large spore ; to 10 are 

 parts of the ruptured spore-wall ; pp 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, s, 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 Rhizocarpeoe, 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 



