170 
HISTORY OF BRITISH FERNS. 
either undivided, and attached to the rachis by their base 
without the intervention of any stalk, bearing a line of 
spore-cases along each margin; or, are larger, more elon¬ 
gated, and deeply pinnatifid or sinuate, the margins of 
these lobes bearing the lines of spore-cases. The apices of 
the primary and secondary pinnae, and of the pinnatifid 
pinnules, become less and less divided, until at last the 
extreme points form an entire lobe, more or less elongated. 
In its venation there is some variety, dependent on the 
differences of structure and development which we have 
already pointed out. We shall be most intelligible by 
explaining the form represented in Plate XVII., which 
shows the least divided form of the plant. Each pin¬ 
nule, as is there shown, has a distinct midvein, producing 
alternate lateral veins, which become twice forked, and 
extend to the margin, where they meet a longitudinal 
marginal vein which forms the receptacle. The indusium 
consists of a bleached, membranous, fringed expansion of 
the upper skip or epidermis of the fronds, which reflexes 
so as to cover the spore-cases; but there is here another 
membrane which lies beneath the spore-cases, and is no 
doubt a similar expansion of the skin of the under-surface. 
The two very dissimilar forms of this plant we have pro- 
