SECT. Ym. DICKSONIE^E. 349 



Acrophorus. The British species of Cystopteris are 

 small, fragile ferns, growing on walls and rocks in Alpine 

 and Subalpine districts. Their fronds are for the most 

 part erect, lanceolate or deltoid, and bipinnate or tripin- 

 nate. They are generically distinguished by having each 

 round sorus covered by a hood-shaped indusium, which 

 is hollow and attached by its base, and opens towards 

 the apex of the segment, its free margin being elongated 

 and fringed ; in this respect it bears some resemblance 

 to Woodsia, in which, however, the indusium projects from 

 beneath the spore-cases equally on all sides, becoming 

 incurved in a cup-like form. The Cystopteris fragilis, or 

 Brittle Bladder fern, flourishes in most shady mountain- 

 ous and rocky districts, but is also found in the lowlands. 

 It has a decumbent caudex, which extends slowly , branch- 

 ing and forming new crowns around the old one, often to 

 the number of several during the summer and autumn. 

 The fronds rise in tufts from these crowns in April, ra- 

 pidly attain their maturity, and die away in succession 

 as their place is supplied by others, till the frost comes 

 and destroys them all. The tufts vary in height from 

 two or three inches to a foot or more. The fronds differ 

 much in form and division even in the same crown, but 

 both they and the pinnae are usually lanceolate; the 

 pinnules are ovoid or oblong, always deeply pinnatifid, 

 and having the segments sharply toothed or serrated. 



The Dicksonia group, Dicksonieae, contains some of 

 the finest tree ferns. The Dicksonia antarctica some- 

 times attains a uniform girth of twelve feet throughout 

 its height of forty feet. In the stem of this fern, the 

 vascular bundles are symmetrically disposed round the 

 axis so as to form a closed cylinder. The New Zea- 

 landers slice the fibrous coating of the trunk, and use it 

 for constructing their houses. D. squarrosa reaches the 

 farthest south of the tree ferns. The D. lanata some- 

 times forms a distinct stem but not always, for tree 



