72 ELEMENTARY BIOLOGY. [VIII. 



antJier, for example, is strictly comparable to a sporangium. 

 The pollen grains answer to the male spores of those flower- 

 less plants in which the spores are of distinct sexes some 

 spores giving rise to prothallia which develope only anthe- 

 ridia, and others to prothallia which develope only arche- 

 gonia ; instead of the same prothallia producing the organs 

 of both sexes, as in Pteris. And the pollen tube corresponds 

 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 detached fertilizing 

 bodies or antherozooids, but exerts its fertilizing influence 

 without any such previous differentiation. The connecting 

 links between these two extreme modifications are furnished, 

 on the one hand, by the Conifers, 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 protoplasm of the male spores ( = pollen 

 grains) divides into cells which form no prothallus, but give 

 rise directly to antherozooids. 



On the other hand, the embryo sac is the equivalent 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 

 are presented by. the Conifers and the Club-mosses. For, 

 in the Conifers, the protoplasm of the embryo sac gives 

 rise to a solid prothallus-like endosperm, in which bodies 

 called corpuscula, which answer to the archegonia, are formed; 

 and in these the embryo cells arise ; while, in some of the 

 Club-n^osses, there are female spores distinct from the male 



