74 STRUCTUKAL BOTANY 



may even say that the prothallus is the more inde- 

 pendent of the two, for while the young Fern-plant is 

 for a time dependent for its nutrition on the prothallus, 

 the latter is never dependent in any way on the Fern- 

 plant. At any rate we have in normal Ferns two per- 

 fectly definite generations, as distinct as possible from 

 each other ; one bearing exclusively the sexual, and the 

 other exclusively the asexual organs of reproduction, 

 and in the ordinary course of life these two generations 

 succeed each other in regular alternation. It was in fact 

 from the Ferns that the idea of alternation of generations 

 among plants first arose, though it had been recognised 

 in the animal kingdom long before. 



Of course the same phenomenon really occurs in 

 Selaginella and even in Flowering Plants, but in all these 

 it is much less conspicuous, because, as we ascend the 

 scale, the sexual generation becomes more and more 

 dependent on the asexual, so that at last the former is 

 reduced to a mere insignificant appendage of the latter, 

 and can scarcely be distinguished from it. 



We have just seen that, even in such Ferns as our 

 type, when the prothalli happen to be dioecious, the male 

 specimens often remain rudimentary. In Selaginella, 

 where the difference of sex is fixed, this has gone much 

 further ; the male prothallus is reduced to one little cell, 

 and is so insignificant as to be scarcely recognisable. 

 The female prothallus, which has much more work to do, 

 is much less reduced, but remains almost shut up in 

 the coats of the megaspore, and so does not obviously 

 suggest an independent individual. When we come to 

 the more ancient Flowering Plants the Gymnosperms 

 we find the male prothallus at an equally low level 

 with that of Selaginella, but much modified, in accord- 



