102 
HISTORY OF BRITISH FERNS. ' 
what is called bipinnate, though less decidedly so than 
occurs in some other species, for here those pinnules only 
which are nearest to the main rachis are quite separated 
from each other. The pinnae are narrow and tapering, 
with a few of the lowest pinnules distinct, the rest united 
at the base. These pinnules are of an obtusely oblon^ 
form, and serrated on the margin. The fructification 
of this plant is generally very copious, and is usually 
confined to the lower half of the pinnules, where it is 
crowded. 
This is one of the best of the British species to study 
with the view of understanding the fructification of Ferns; 
for here the indusium, a very important organ, is seen to 
be remarkably prominent in fronds which have about 
reached their full development. In that state the indu¬ 
sium is as yet closed over the clusters of spore-cases, and 
will be seen to consist of a lead-coloured, tumid, kidney¬ 
shaped, conspicuous scale, which, at the proper time, 
becomes elevated on one side, to allow the dispersion of 
the spores. This may readily be noticed by watching the 
progress of the fronds just as they reach their full growth. 
If they are gathered in that state for preservation in 
the herbarium, they burst open more or less, in the pro- 
