THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



Selaginella plant. 



591 



Strobilus 



Megasporangium 



^ \ 



Microsporangium X^ 



Megaspores 



Microspores 



•Female Prothallus' 



Antherozoids 



■Male Prothallus' 

 Fig. 600. — Life-cycle of Selaginella kraussiana. 



It will be seen, however, that there is still the same alternation of genera- 

 tions between a sporophyte and a gametophyte, although the gametophyte 

 is entirely dependent upon the sporophyte for its food. A great reduction 

 in the size of the gametophyte has also occurred, especially in the male, which 

 is represented by a single prothallial cell and a single antheridium. Since 

 the only function of the male prothallus is to supply a number of motile 

 antherozoids this reduction is understandable, moreover, since the only 

 important part of the antherozoid is its nucleus and its life is very short, 

 there is no necessity for the accumulation of a large amount of food reserve. 

 In the female gametophyte, however, the provision of a good reserve is 

 necessary for the nourishment of the embryo which will be supplied from it. 

 The female prothallus is therefore associated with a considerable amount of 

 nutriment stored in the protoplasm of the large megaspore. Its peculiar 

 development and its lack of chlorophyll remove it a long way from the 

 independent prothallus of Dryopteris. At the same time, since the structure 

 of the archegonium is similar to that in Fern types we must regard the two 

 structures as homologous. 



The heterosporous condition illustrated by Selaginella constitutes one of 

 the most important phases in the evolution of plants, and as we pass further 

 up the evolutionary series we shall find that there is a further reduction 

 in the number of the megaspores until only one megaspore is formed in each 

 megasporangium. This has occurred even in some species of Selaginella, 

 such as S. apoda, which has advanced almost to the rank of a Seed Plant, as 

 the megaspore is retained in the sporangium and is fertilized and develops 



