170 HISTORY OE BRITISH FERNS. 



are supported on shaggy stipes of about a third of their 

 entire length. 



The fronds have a strong midrib or costa, extending 

 throughout their whole length, from which are produced 

 forked veins, the branches of which (venules) lie parallel, 

 and proceed direct towards the margin, terminating just 

 within the edge in a club-shaped apex. The veins are usually 

 forked twice, but they are not constant to any exact num- 

 ber of divisions. The sori, which are oblong patches of 

 unequal length, lying in the direction of the veins at short 

 intervals along the upper two-thirds of the length of the 

 frond, are each composed of two proximate lines of fructi- 

 fication laterally united ; each of these lines, however, consist- 

 ing of a complete sorus, so that the two united are properly 

 called a twin sorus. This is the mark of the genus Scolo- 

 pendrium. This twin sorus is always produced between two 

 fascicles of veins ; that is, the lowermost venule produced by 

 one vein, and the uppermost venule produced by the vein 

 next below these two venules lying, of course, contiguous, 

 each become the receptacle upon which a line of spore-cases 

 is produced. The indusia which cover these two lines of 

 spore-cases have their attachment respectively on the upper 

 and lower sides of their venules, the other edges overlapping 



