170 HISTORY or 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 pinnce, 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 skin 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- 



