104 Silvery Spleenwort 



the unopposed parts, if any, on any vein that did not stand in 

 such a relation to one of the midveins as to lead one to expect 

 a single sorus on it: judging from the positions of the single 

 sori on the other veins of the specimens. The material exam- 

 ined comprise the collections of this fern in the New York Botan- 

 ical Garden, two specimens typical of the highly developed and 

 of the less well developed leaves kindly sent me by Mr. Gilbert, 

 and a large number of specimens collected in Vermont. 



Union in formation of otherwise normal asplenioid sori is 

 thus, apparently, the explanation of the occurrence of athyrioid 

 sori in this fern. It is evident that athyrioid sori are liable to 

 occur in any Asplenium when, for any reason, two single sori 

 are borne in juxtaposition on opposite sides of the same vein. 

 That the union does not always occur when opportunity offers 

 is seen by the presence of the diplazioid sori in A. thelypter aides- 



Either of the two sori that make up the diplazioid or the 

 athyrioid sori in this fern may or may not extend down the 

 vein as far as the midvein toward which it opens. If, in the 

 athyrioid sorus, the two extend an equal distance, we have a 

 hipprocrepiform sorus, if an unequal, a hamate sorus, or one of 

 the two may be so short as to be barely distinguishable. Since 

 in the athyrioid sorus both are united at the outer end, one can- 

 not extend at that end farther on the vein than the other, but 

 one can and often does in the diplazioid. 



By some botanists A. thelypteroides, A. filix-fcemina, and A. 

 cyclosorum, the three representatives of Athyrium in the United 

 States, are included in the genus Asplenium. All three ferns 

 have a greater or less number of athyrioid sori on their leaves, 

 but it appears from all the specimens that I have seen that the 

 occurrence of the athyrioid sori is due to the same cause in all. 



