176 
HISTORY OF BRITISH FERNS. 
are well adapted for the purpose of minute investigation 
into the nature of their venation and fructification. Their 
texture alone almost suffices to tell a practised eye their 
family position ; but the tyro needs a more precise charac¬ 
teristic, and this is found in the structure of the scale or 
indusium which covers the sori. The sori of these plants 
are round, as in Lastrea and Polystichum , all, equally 
with Cystopteris , once included under the old family name 
of Aspidium ; but here, instead of being almost flat and 
circular, the cover is inflated or bulged out like a hood, 
or, while young, even flask-like or bladdery in appearance ; 
hence these plants are called Bladder Ferns. This indu¬ 
sium is attached at the back (towards the base of the 
pinnule) of the sorus by its broad base, covering the spore- 
cases while in a young state, but becoming ultimately 
reflexed at the point, which is more or less jagged or 
fringed. There are three native species, of one of which 
numerous distinct forms or varieties occur. 
The technical name comes from two Greek words, Jcystos 
and pteris, which respectively mean bladder and fern; so 
that in this case the English appellation is a literal trans¬ 
lation of the scientific name. 
