CONCLUSION 301 



In Selaginella we have an extreme case of heterospory, 

 with an enormous difference between microspores and 

 megaspores There is, however, another change going on 

 concurrently with that just sketched. Both prothalli 

 have become the mere bearers of sexual organs, and are 

 losing the character of distinct organisms. Hence the 

 female prothallus, though so highly developed relatively 

 to the male, is itself reduced, as compared with the 

 prothallus of a homosporous Cryptogam. 



The homologies between Selayinella and a gymno- 

 spermous Flowering Plant, such as Picea, are quite clear, 

 and have been fully demonstrated above (p. 31). It is 

 doubtful, however, whether there is any near affinity 

 between Selaginella and the Gymnosperms, which pro- 

 bably had quite a different origin. In any case the 

 homologies hold good, whatever the particular hetero- 

 sporous family or families may have been from which 

 the Gymnosperms sprang. 



In the latter Class the female oophyte has lost its 

 independence altogether, and never leaves the megaspore, 

 which itself remains shut up in the sporangium. Ferti- 

 lisation leads to the formation of a seed the charac- 

 teristic structure of Flowering Plants, consisting of 

 sporangium, prothallus, and embryo, united to form one 

 body, and fed entirely at the expense of the parent 

 sporophyte. 



Until quite recently there appeared to be a sharp 

 distinction between Cryptogams and Phanerogams, in the 

 method of fertilisation, for active spermatozoids were 

 supposed to be peculiar to the former, the generative 

 cell in Phanerogams being carried passively to the ovum 

 by the growth of the pollen-tube. The researches of 

 the Japanese botanists Hirase and Ikeno have broken 



