138 HISTORY OF BRITISH FERNS. 



The pinnae are jointed to tlie racliis, and when old are 

 readily displaced, so that eventually the black rachis is 

 left denuded among the tuft of fronds. A distinct midvein 

 passes through each pinna, giving off on each side a series 

 of veins bearing venules, the anterior of these producing 

 the linear sorus just within the margin of the pinnce. 

 The sori, which in the young state are covered by thin 

 indusia having a somewhat crenulated free margin, very 

 frequently in a later stage become confluent, and cover the 

 whole of the under-surface. 



A very rare and very curious variety of this species, 

 named incisum, has the pinna? deeply pinnatifid, with 

 linear notched segments. Another, equally rare and still 

 more beautiful, has the ends of the fronds tasselled ; this 

 is called cristatum. There are some forms with the fronds 

 two or three times forked, the pinnae depauperated, and in 

 one instance deeply lobed. 



The species occurs rather plentifully, growing on rocks, 

 old walls, and ruins, and less frequently on hedgerow 

 banks. It is pretty generally distributed throughout the 

 United Kingdom and Ireland ; and also occurs through- 

 out Europe, and in each of the other divisions of the 

 globe. 



