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. 



