166 HISTORY OF BRITISH ERNS. 



represented in Plate XVII. The general form is ovate, a 

 little elongated; that of its pinnae (the secondary pinnae) 

 narrow lanceolate. These latter are placed rather closely 

 together, and are again divided into a series of pinnules. 

 Two forms will be met with, one apparently equally common 

 with the other : in one the pinnules are undivided, and at- 

 tached to the rachis by their base without the intervention 

 of any stalk, and these bear a line of spore-cases along each 

 margin ; in the other the pinnules are larger, more elongated, 

 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 ex- 

 plaining the form represented in Plate XYIL, which shows 

 the least divided form of the plant. Each pinnule, as is 

 there shown, has a distinct midvein, producing alternate 

 lateral venules, 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 



