12 
BRITISH FERNS. 
dorsal; others have it protruded from the margin of 
the frond, when it is called marginal. In all the dorsi- 
ferous ferns the spore-cases are connected together into 
groups called sort. The forms of these groups decide the 
genus of the fern, as round for the Polypody , narrow for 
the Asplenium , linear for the Blechnum, etc. In Hyme- 
nophyllum and Trichomanes, the fructification is mar- 
ginal, and the receptacle is formed of the point of the 
vein protruded beyond the frond. The genera Osmunda , 
Ophioglossum , and Botrychium belong to the exannulate 
division; they have the spore-cases collected upon the 
sides or surface of contracted fronds, and they are com- 
posed of two valves. When the spore-cases spring from 
beneath the outer membrane of the frond, the cuticle 
is pushed up into a kind of enwrapping membrane, 
called an indusium or involucre , and on this account 
certain groups are named indusiate or non-indusiate. 
The polypodies are without cover ; they are called non- 
indusiate; while all the families in which the involucre 
is present are called indusiate. The involucre may be 
described as “ a plane or cup-shaped membrane, pro- 
duced from the receptacle of each sorus; when pro- 
duced from the centre it is central , from one side lateral , 
and when attached all round the base it is calyciform , 
When central, it is usually orbicular , with its margin de- 
pressed and free all round ; when lateral, it is reniform , 
oval , oblong , or linear ; when calyciform, it is entire and 
globose, and opens after a time with a laciniated mar- 
gin.” Sometimes the margins of the fronds are 
changed in texture, and converted into a membranous 
involucre, with which the original involucre unites, and 
together they form a cyst ; or the whole of the sori of 
each segment are enclosed within a general involucre, 
