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. thelypteroides. 
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. jilix-jamina, 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. 
