176 Rhodora [SEPTEMBER 
end, the sorus may barely cross the veins so as to be slightly hooked, 
a “hamate” sorus, or it may develop a considerable posterior limb, 
becoming horse-shoe shaped, or “hippocrepiform.” In the latter 
case the two limbs may lie close to- 
gether, or the posterior limb may lie 
at a visible distance from the sub- 
tending vein. Moreover, the entire 
sorus may be several millimeters long, 
or may be so short as to appear 
nearly round. In typical Athyria, 
the indusium of the mature sorus is 
reflexed wherever there is sufficient 
room, and at the curved distal end, 
it is crowded up into a vertical posi- 
tion in the midst of the sporangia. 
It is easy to see that the condition 
in a very short athyrioid sorus is 
but a brief step from that which 
obtains in such a fern as Dryopteris 
Thelypteris (L.) A. Gray, in which 
an essentially round sorus is covered 
by a centrally placed indusium shaped like an umbrella, but discon- 
tinuous on the proximal side. Finally, in a very short sorus, only 
that part may be present which crosses the vein, and the indusium 
may then also merely cross the vein, usually somewhat obliquely and 
just behind the sorus, a condition very like that found in the genus 
Cystopteris. _ Cystopterid sori are also fairly common in some of the 
more primitive species of Dryopteris, e. g. D. Thelypteris and its allies. 
n this connection it is interesting to note that Bower! considers 
Cystopteris as a relatively primitive type from which the Aspidieae 
have probably developed, and it seems equally probable that Athyrium, 
and through it the other genera of Asplenieae have developed from 
a similar origin. 
It is readily seen, that with such variability of the sorus, it is not 
easy to define the limits of the genus Athyrium. In this connection 
there are two major problems, the limit between Athyrium and 
Dryopteris, and the question of the genus Diplazium. 


? Bower, F. O., The Origin of a Land Flora, 615 (1908). 
