466 THE BEAN PLANT. [CHAP. 



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 may be compared to the first 

 filamentous 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 spermatozoids, but exerts its fertilizing influence 

 without any such previous differentiation, other than the 

 divisions of its nucleus 1 . 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 

 spermatozoids are developed ; and by Selaginella, in which 

 the protoplasm of the smaller spores (= pollen grains) 

 divides into cells which form a unicellular prothallus, and a 

 single antheridium in which the spermatozoids are produced. 

 On the other hand the embryo-sac is the equivalent of 

 the large spore which gives rise to a prothallus bearing 

 female organs. The ovum of the flowering plant corre- 

 sponds to the ovum contained in the archegonium of the 

 prothallus. There are other cells produced from the pro- 

 toplasm of the embryo-sac, which probably answer to the 

 cells of a prothallus. Here again the intermediate stages 

 are presented by the Conifers and Selaginella. 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 each of these an ovum is produced ; while, in Selagi- 

 nella the prothallus developed in the large spores does not 

 leave the cavity of the spore, but remains in it like an 

 endosperm. 



1 There is however an indication of cell -formation around two of the 

 daughter-nuclei. 



