THE HOMOSPOROUS LEPTOSPORANGIAT^ 



393 



group they give evidence of being the most speciaHsed members 

 of the order to which they belong, and comprise a very large 

 majority of the species. Most of them agree closely in their 

 structure, which has been given in detail, and will not be re- 



FiG. 230. — A, Pinnule of Aspidium spinulosum, showing the sori {s) with kidney- 

 shaped indusium, X2^; B, cross-section of a pinna from a young sporophyll of 

 Onoclea struthiopteris ; s, sorus, X25. 



peated here. With very few exceptions the structure of the 

 prothallium and sexual organs is like that of Onoclea, but one 

 or two variations may be mentioned. In Vittaria (Britton and 

 Taylor (2)), is found a type of prothallium recalling that of 



..-sp 



Fig. 231. — A, Polypodium falcatum. Pinna with sori, sp; natural size. B, Pteris 



aquilina. C, Asplenium filix-foemina, X3' 



Hymenophylhim, both in its large size and extensive branching. 

 Its earlier stages show the ordinary development, but it later 

 branches extensively, and, like Hymenophylhim, numerous 

 groups of archegonia are formed upon one prothallium. Bod- 



