The fronds of the Bristle fern, Trichomanes radicans, are somewhat deltoid, or of a more elongated form, flimsy, and beautifully reticulated when viewed with a microscope. They rise at intervals from a creeping rhizome, bristly, with narrow articulate scales, which often covers the most precipitous rocks on which it grows with a dark-coloured network. The rachis is branched and rebranched three or four times, and the whole being distinctly winged on both sides in the plane of the frond, the successive branches and branchlets running through the leafy part become the veins of those parts, so that a segment of the frond is merely a winged vein, the wings on the branches being however broader than those on the rachis. The veins, which divide alternately, are hard, woody, and wire-like, and, when barren, terminate before reaching the ends of the segments; but, when fertile, they extend beyond the segment, the tissue of which separates and distends in the form of a more or less elongated cup around the prolonged vein. At the base of that vein the sporangia form a small globular cluster; and, as they advance towards maturity, the vein extends in the form of a bristle, far beyond the mouth of the cup (fig. 68). The cup is winged in the more luxuriant form of the plant, in consequence of the double layer of tissue composing the segment to which it belongs not separating through its whole breadth. The cup-like cylinder being slightly compressed in the plane of the frond is indicative of its origin. [Illustration: Fig. 69. Hymenophyllum tunbridgense.] A slight difference in fructification separates the Hymenophyllum tunbridgense, or Film fern (fig. 69), from the Trichomanes, for both have a creeping rhizome, from which single fronds spring at short intervals. The leafy parts of the fronds are merely winged veins, and the wings consist of two layers of cellular tissue finely reticulated. The Hymenophyllum is distinguished from the Trichomanes by a two-valved cup spinously serrated, which completely conceals the fruit-bearing vein, while, in the Trichomanes, the cup is more cylindrical, smooth-edged, with the fertile vein projecting far beyond it (fig. 68). The beautiful little Film fern, with its semi-transparent fronds not more than two or three inches high, grows on shady wet rocks, among the moss on the branches and roots of old trees, and on the ground near lakes and rivulets. It is hardy enough to live in the Highlands of Scotland; but the H. unilaterale, a far less beautiful plant, has its limit in Unst, the most northern of the Shetland Islands. The fructification in the latter plant, though the same in position, is stalked instead of being sessile. The involucre is rounded and ovate, instead of being cup-shaped, with much swollen convex valves, meeting by their edges but not compressed towards the apex.