CYCADOFILICALES 31 



need not be discussed here, since we are considering only the origin 

 of the seed, not its later history. 



The microsporangium and its microspores, taken by themselves, 

 do not prove, with much certainty, whether they belong to the ferns 

 or seed plants. They are conservative, and their line, even as we see 

 it in living seed plants, has not differed so greatly from the heteros- 

 porous fern condition, or even from the earlier homosporous condi- 

 tion. Living heterosporous Pteridophytes shed their microspores in 

 the uninucleate stage or, later, after a tissue has formed inside the 

 spore. Even in a living homosporous fern, Onoclea, there may be 

 several cell divisions within the spore, and the prothallia may under- 

 go considerable development before they escape from the sporan- 

 gium. The sporangium with small spores must be associated with 

 other structures before it can be known definitely whether it is the 

 sporangium of a fern or seed plant. 



The close resemblance between the sporangia of ferns and those 

 known to be the microsporangia of the Cycadofilicales is a strong 

 evidence of genetic relationship. 



It is doubtful whether any two genera of the living heterosporous 

 Pteridophytes are closely related to each other with the possible ex- 

 ception of Marsilia and Pilularia; but they are all heterosporous and 

 their heterospory has been achieved in the same way — by the dis- 

 organization of megaspores which have been absorbed by the grow- 

 ing megaspores. We should be slow to believe that Azolla has been 

 derived from Salvinia by direct descent, or that Salvinia has come 

 from Azolla; it is easier to believe that similar conditions and a gen- 

 eral tendency in evolution have produced heterospory independently 

 in the two genera. Heterospory seems to be the goal toward which 

 a general tendency of evolution is urging all the vascular plants. 



The Cycadofilicales resemble ferns so closely that they were mis- 

 taken for ferns until their seeds were discovered. Could it be possible 

 that transmigrants, coming upon the land at different periods, could 

 have developed into these two groups independently? Since the two 

 groups are associated geographically and edaphically, the conditions 

 which they had to meet must have been somewhat similar. The de- 

 mands of protection, absorption, conduction, and assimilation would 

 probably have resulted, in both cases, in the production of roots. 



