THE BLADDER FERNS. 211 
ovate, acute and pinnate, the pinne rather distant, 
broadest at base and themselves pinnatifid or again pin- 
nate. The pinnules are narrow, bluntish, lobed or 
toothed and usually decurrent on the rachis. There is, 
however, a wide range in the shape and cutting of the 
fronds. Our illustrations show two interesting forms of 
American specimens. A pinnule is also shown in the 
Key to the Genera. 
Not only is this species the first to put forth its fronds, 
but it is one of the earliest to fruit and, unlike other 
species which fruit early, fertile fronds may be found all 
summer. Nearly every frond bears sporangia. The sori 
are rather small and thickly scattered on the lobes of the 
pinnules. Owing to the early withering of the indusia, 
they usually appear as if naked. Only in the young- 
est sori can the indusium be seen to advantage. It is 
ovate, very thin, and taper pointed. From the ~hape of 
the arching indusium, this species was anciently cnown 
as the cup fern. 
The fronds of this species are very easily confused with 
those of ]oodsta obtusa, with which it often grows, and 
the difficulty in separating them is increased by the 
evanescent nature of the indusia in both species. In 
Cystopteris, however, a careful search will usually reveal 
enough shrivelled vestiges of the indusia to make identi- 
fication sure. In Woodsia, too, the pinne and pinnules 
are ordinarily broader and blunter. 
In America, the common bladder fern is found from 
British America to Georgia and Arizona. It is frequently 
called the brittle bladder fern, a translation of its specific 
name. The name of white-oak fern has also been given 
it, though for what reason does not appear. Several 
varieties have been named but none of them are very 
