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 fila- 

 mentous 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 division of 

 its nucleus. The connecting links between these two ex- 

 treme modifications are furnished, on the one hand, by the 

 Conifers, in which the protoplasm of the pollen tube be- 

 comes divided into cells, from which, however, no sperma- 

 tozoids are developed; and by Selaginella, in which the 

 protoplasm of the smaller spores ( = pollen grains) divides 

 into cells which form no prothallus, but give rise directly to 

 spermatozoids. 



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 cor- 

 responds 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. 



