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].
Fig. 60. Sorus and cup-shaped indusium of Deparia prolifera.
The group Peranemeæ, or Woodsieæ, is represented in the British Flora by two species of Woodsia, which are amongst the rarest and most curious of our ferns, and inhabit the crevices in our highest mountain tops. Their tufts of fronds are not more than two or three inches high, sometimes less, and the fronds themselves are lanceolate, and pinnate; in Woodsia ilvensis they have oblong and usually opposite deeply lobed pinnæ, whose under-surface is clothed more or less with jointed hairs and long attenuated scales. The sori in both species are circular, and situated at the extremity of the lateral veins. When young they are covered with an indusium, which opens at the centre, and forms a cup round the sori; afterwards it is divided into numerous jointed and usually incurved threads.
The Davallieæ are mostly tropical and subtropical ferns. Their sori are submarginal, placed upon the tip of a vein or veinlet, and enclosed within a tubular indusium, which is either short and approaching to cup-shaped, or more or less elongated. In this group the stem assumes the form of a rhizome, often creeping extensively, and the fronds are articulated, falling off with a clean scar when they perish. Davallia canariensis is cultivated in most conservatories, and is known as the Hare’s-foot fern, from the stoutish scaly rhizomes resembling the foot of a hare.
Somewhat allied to these are the Lindsæeæ, including Lindsæa and its allies; these have linear marginal sori, for the most part continuous, but sometimes regularly interrupted, in which state they approach some of the Davallieæ, Microlepia to wit. The indusium opens towards the margin of the frond, and the sori more or less connect the tips of the veins and veinlets.
Fig. 61. Scolopendrium vulgare.
The group of Asplenieæ is a very comprehensive one, including, besides the extensive typical genus Asplenium, the cognate subdivisions of Diplazieæ and Scolopendrieæ. The latter is represented by Scolopendrium vulgare, the Hart’s-tongue fern, in which the sori are linear, and situated in pairs on two parallel veins, so closely approximate that, though really double, they seem to form but one straight line; they are not covered with one indusium splitting down the middle, but each has its own indusium opening in opposite directions. On the under-surface of the long strap-shaped entire fronds of the common Hart’s-tongue, which is a type of the genus, the double sori with their opposite indusia form parallel equidistant straight lines, diverging on each side from the mid-vein of the frond, as in [fig. 61]. The fronds are cordate at the base; sometimes they are forked, and occasionally crisped and wavy, and the leaf-stem is shaggy with narrow membranaceous scales. The caudex is very compact and deep-rooted; it does not much elongate, but increases slowly by the formation of new crowns round the older, attaining considerable bulk. From this the fronds rise in circular tufts to the height of a foot or more, and attain from two to three feet, in favourable shady localities. At first the tufts are straight, but ultimately they radiate and bend outwards. This plant is exceedingly variable; hundreds of varieties have been found or raised in Great Britain. Though twin sori are characteristic of the Scolopendrieæ, they diverge somewhat in Camptosorus, a North American genus, sometimes called the Walking fern, from its habit of throwing forward a bud on a thread-like prolongation of the point of the frond, which becomes established as a new growing centre, and thus carries the plant onwards.