74 ELEMENTARY BIOLOGY. [CHAP. 



The difference between a flowering plant, such as the 

 Bean, and a flowerless plant, such as the Fern, at first sight 

 appears very striking, but it' has been proved that the two 

 are but the extreme terms of one series of modifications. 

 The anther, for example, is strictly comparable to a sporan- 

 gium. The pollen grains answer to the male spores of those 

 flowerless plants in which the spores are of distinct sexes 

 some spores giving rise to prothallia which develope only 

 antheridia, and others to prothallia which develope only 

 archegonia; instead of the same prothallia producing the 

 organs of both sexes, as in Pteris. And the pollen tube cor- 

 responds with the first hypha-like process of the spore. But, 

 in the flowering plants, the protoplasm of the pollen tube 

 does not undergo division and conversion into a prothallus, 

 from which antheridia are developed, giving rise to de- 

 tached fertilizing bodies or antherozooids, but exerts its 

 fertilizing influence without any such previous differentia- 

 tion. The connecting links between these two extreme 

 modifications are furnished, on the one hand, by the Coni- 

 fers, in which the protoplasm of the pollen tube becomes 

 divided into cells, from which, however, no antherozooids 

 are developed; and the Club-mosses, in which the proto- 

 plasm of the male spores (= pollen grains) divides into cells 

 which form no prothallus, but give rise directly to anthero- 

 zooids. 



On the other hand, the embryo sac is the equi\*alent of a 

 female spore: the endosperm cells, which are produced from 

 part of its protoplasm, answer to the cells of a prothallus; 

 while the embryo cell of the flowering plant corresponds 

 with the embryo cell contained in the archegonium of the 

 prothallus. In the development of the female spore of the 

 flowering plant, therefore, the free prothallus and the arche- 

 gonia are suppressed. Here, again, the intermediate stages 



