The genus Cystopteris, which has species in both hemispheres, is the type of the Cystopterideæ, a small group which approaches Aspidieæ through Nephrolepis and 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 tripinnate. 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 mountainous and rocky districts, but is also found in the lowlands. It has a decumbent caudex, which extends slowly, branching 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, rapidly 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 pinnæ are usually lanceolate; the pinnules are ovoid or oblong, always deeply pinnatifid, and having the segments sharply toothed or serrated. The Dicksonia group, Dicksonieæ, contains some of the finest tree ferns. The Dicksonia antarctica sometimes 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 Zealanders 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 sometimes forms a distinct stem but not always, for tree ferns vary much with regard to the dimensions and elevation of their stem. The caudex of Cibotium Barometz is covered with long tawny hair; each has its own peculiarity. The species of Dicksonieæ belong principally to the tropics and southern isles. The ferns of this group have globose sori, which are submarginal, seated at the tip of a vein or veinlet. The indusium is lateral, persistent, and bivalved; the lower valve is formed by the true indusium, the upper by the altered tooth of the frond folded back. In Dennstædtia the indusium is cup-shaped, and curiously deflexed; while in Deparia, another section of the group, the cup-shaped indusium containing the sorus is extrorse marginal, as shown in fig. 60. [Illustration: Fig. 60. Sorus and cup-shaped indusium of Deparia prolifera.]