Silvery Spleenwort 105 



A. thelypteroides, A. filix-jcemina, and A. cydosorum are 

 undoubtedly congeneric. In habit and general character they 

 are more nearly related to the group of Dryopteris of which Z>. 

 noveboracensis, D. thelypteris, and D. simulata are represent- 

 atives than to some species of Asplenium. A. thelypteroides is so 

 named from its resemblance to D. thelypteris, and D. simulata 

 from its simulation of a form of A. filix-fcemina. Shorten 

 athyrioid sori sufficiently and we have dryopterioid sori: some 

 of the short athyrioid sori often seen, on leaves of A. cydosorum 

 and A. filix-foemina particularly, are not distinguishable from 

 sori of Dryopteris. 



The evolution of Dryopteris from an ancestor with asplenioid 

 sori, and also the manner in which the change in the sori may 

 have come about, is thus suggested. For instance, let us suppose 

 that athyrioid sori were produced first in some Asplenium by 

 casual union of otherwise normal asplenioid sori, as they are 

 apparently produced in A. thelypteroides, A. filix-foemina, and A. 

 cydosorum; a tendency to produce such sori might be inherited 

 by the descendants of that A splenium and augmented until these 

 sori came to be produced not only on such veins as at first, but 

 on other veins as well, and finally to supplant the asplenioid 

 sori. 



