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 PoIysticJmm, all, equally 

 with Cy stopfer is, once included under the old family name 

 oi Aspidiiim ; 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. 



