300 STRUCTURAL BOTANY 



mode of fertilisation has kept back the oophyte, which 

 throughout the Pteridophyta is always a damp-loving 

 organism, and never completely adapts itself to terres- 

 trial life. The asexual plant, on the other hand, has 

 attained the greatest complexity, rivalling that of the 

 Flowering Plants. 



In all the three main groups both homosporous and 

 heterosporous forms have occurred, though among the 

 Horsetails heterospory is now extinct. 



Most Ferns, like our type, are homosporous, though 

 even here rudimentary purely male prothalli are not 

 uncommon. Heterospory in plants of the Fern alliance 

 is now limited to two small families. The origin 

 of heterospory may be compared to the origin of 

 sexual differentiation (see above, p. 296). Just as 

 nutritive functions came to be assigned specially to 

 the ovum, so here they are assigned specially to the 

 female prothallus, which has to feed the embryo during 

 its development. The male prothallus can safely be 

 treated by the plant on strictly economical principles, 

 for it has nothing more to do than produce a few 

 minute spermatozoids. We see this change beginning 

 both among Ferns and Horsetails; in the latter the 

 smaller, less vigorous prothalli, are exclusively male. 

 Then the difference extends further back. The insigni- 

 ficant male prothallus only needs a small spore to grow 

 from, while the female must be fed up from the first, 

 and so a large spore, full of reserve food, is set apart for 

 its formation. On the other hand, it is an advantage to 

 have plenty of males in order to ensure fertilisation. 

 Thus we get a large number of microspores forming 

 small male prothalli, and a small number of megaspores 

 forming large female prothalli. 



