Scaly Spleenwort or "Rusty Back."
Passing now to the production of the spores, and so completing the cycle of a fern's existence,—these appear in cases which spring in some instances from leafless veins or central ribs, but mostly from the veins as they usually occur, and at the back or, in the bristle-fern and filmy-ferns, at the margin of the fronds. The cases grow in clusters which are termed sori, each of which is generally protected by a covering, though in the genus of the polypodies this is entirely absent, the clusters being fully exposed to the diversities of wind and weather. In the protected kinds, the cover assumes various forms. The filmy-ferns have it as a tiny cup, enclosing the spore-cases. In the bladder-fern it is like a fairy helmet. The shield-ferns, as their name implies, produce it as a little shield, fastened by its centre. In the buckler-ferns it is kidney-shaped, in the spleenworts long and narrow, and so on. Some kinds can scarcely be credited with the formation of a real cover, but their sori are protected by the turned-down margins of the fronds. In a few sorts, separate fronds are provided for the production of the spores, and these mostly differ in shape from the ordinary or barren fronds.
The spore-cases are generally almost microscopic, flask-like in shape, and encircled by an elastic ring of peculiar structure, which passes either from top to bottom like a parallel of longitude, or round the sides like the equator round the earth. The exact nature of this band,—whether its elasticity be due to the mechanical arrangement of its cells, which are narrower on the inner than on the outer side, and apparently filled with solid matter, or to a quality of its substance,—I am unable to determine.
Wilson's Filmy-Fern.
Tunbridge Filmy-Fern.