CYSTOPTERIS. 107 



branch generally bearing a sorus at about midway its length. 

 The sori are thus generally numerous, and rather irregularly 

 disposed; and it often occurs that they are so numerous as, 

 when fully grown, to become confluent into a mass of fructi- 

 fication covering the whole under surface of the frond. The 

 number of sori produced, and consequently the sparse or 

 crowded disposition of the fructification, is a matter alto- 

 gether dependent upon the circumstances of growth, and 

 hence exceedingly liable to vary even in the same plant, and 

 within the same year, as heat or cold, drought or moisture, 

 may preponderate. The sori, which are nearly circular, are 

 covered while young by a concave or hood-shaped indusium, 

 which is attached by its broad base on one side beneath the 

 sori, and has its apex ultimately free; this part usually be- 

 comes torn or split into narrow segments, and the whole soon 

 becomes pushed back or cast off by the growing spore- cases. 

 There are many forms or varieties of this species. Two 

 of them, called cynapifotia and antliriscifolia, do not seem 

 to need any distinct description. Another, called angustata, 

 is rather larger, generally, than the typical form, but differs 

 more by having the points of its pinnae and the apex of the 

 frond itself drawn out considerably into very narrow r points 

 than in any other circumstance readily pointed out. 



