462 FILICES. [part ii. 



which unroll their leaves, when they rise from 



the stem, and which have their sori either on 

 the back or on the margin of the frond. The 

 thecse are on stalks, and they are furnished with 

 a ribbed, elastic, articulated but incomplete ring, 

 which seems to serve as a sort of hinge when 

 they burst. This elastic ring is a continuation 

 of the stalk of the theca, which always bursts on 

 the opposite side. The following are the princi- 

 pal genera in this division : Polypody {Polypo- 

 dium)^ sori without any indusium ; Shield Fern 

 (^Aspidium), Bladder Fern (Cistopteris) , and 

 Spleenwort {Aspleniiun) ^ all of which have their 

 fronds pinnate or pinnatifid ; Maiden Hair 

 (Adiantum), Hart's-tongue (Scolopendrium), the 

 frond of which is simple and shaped like a tongue, 

 and the sori oblong ; and Brake (Pteris), the 

 leaves of wdiich are pinnatifid, with the sori 

 placed round the margin so as to form a con- 

 tinuous line, and the edge of the leaf turned over 

 them. The rhizoma of the Brake is eaten in 

 many countries, and the fronds, when burnt, 

 yield alkali, which is used in making both soap 

 and glass. 



The second division Osmundacese comprises 

 those Ferns which apparently have flowers ; the 

 flowers, however, being merely sori, with the 

 leaves on which they grew shrivelled up round 

 them. The most remarltable of these is the 



