CYC AD ALES 157 



and Cycadofilicales is due to heredity, so that the Cycadofihcales 

 represent only the further evolution of some of the Filicales. 



The strongest evidence for the derivation of the Cycadofilicales 

 from the Filicales, as we have remarked before, is furnished by the 

 seed. The Evolution of the Seed would make a good title for a book. 

 One would have to transport himself back to the days of fairies and 

 giants, of wonderful lamps and carpets, to believe that the seed 

 originated without any ancestry. 



The heterosporous Pteridophytes of today show unmistakably 

 their homosporous ancestry; and it is only reasonable to suppose 

 that heterosporous forms of ancient times arose from homosporous 

 in the same way, some of the sporogenous cells failing to reach full 

 development and giving up their substance to nourish the one or 

 more spores which thus reached a higher stage in evolution. Mega- 

 spores became larger and larger until a free nuclear period arose 

 within the spore, the gametophyte being retained within the spore, 

 just as the spore itself, later, became retained within the sporan- 

 gium. This, we believe, was the mode of origin of the seed habit. 

 Naturally, it might take place without any great accompanying 

 changes in the leaf. 



In Selaginella, a lycopod, but nevertheless showing what probably 

 took place in the heterosporous ferns, the megaspore is shed with its 

 contained female gametophyte in various stages of development. 

 Usually, the gametophyte has passed the free nuclear stage and en- 

 tered the stage of cell formation. In Selaginella apiis, a semi-aquatic 

 species, archegonia may be formed and fertilization may take place 

 before the megaspore falls out from the sporangium. In extreme 

 cases there is little dehiscence of the sporangium and the megaspore 

 remains inside, so that the shoot with its cotyledons and stem tip, 

 and the root, break through the sporangium. In this extreme case 

 the term "seed" is strictly applicable to Selaginella. Very probably, 

 such a situation was common in the Carboniferous, where so many 

 "seedlike" structures are found. 



The line between ferns and advanced members of the Cycadofili- 

 cales, like Trigonocarpon, was as sharp as the line between ferns and 

 seed plants of today. The place where it would be hard or impossible 

 to draw the line would be where the enlarging megaspores were just 



