PTERIDOI'HYTES 179 



ferns, and that Salviniaceae have the transverse wall of the older 

 eusporangiate Marattiaceae. 



Conclusions. — It is evident that the water ferns are a very specialized 

 aquatic group, probably derived from the leptosporangiate ferns. More- 

 over, the features of the sporangium, in development and output, in- 

 dicate an origin from one of the higher leptosporangiate families. The 



Figs. 416, 417. — Female gametophyte of Marsilea: 416, the tissue protruding from 

 the heavy megaspore wall developing a single large archegonium; the top of the large 

 nutritive cell with its single nucleus seen; 417, the mature archegonium. — After 

 Campbell. 



annulus characters, which might have determined the point, are lacking. 

 It is interesting if heterospory has developed in these aquatic conditions ; 

 and it is noteworthy that the development of the female gametophyte is 

 very different from that of Selaginella and Isoetes, which resemble 

 gymnosperms in this regard. The fern connections of Marsileaceae are 

 clearer than those of Salviniaceae, which in habit are further removed 

 from the terrestrial leptosporangiates. 



