On Molecular and Microscopic Science, Volume 1 (of 2)

by Mary Somerville

Fig. 68. Trichomanes radicans.

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.

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.

The Schizæineæ are many of them climbing ferns, and for the most part tropical. Lygodium articulatum climbs trees in New Zealand to the height of fifty or one hundred feet, and its tough wiry stems are used for cordage. In one group of this order, represented by Lygodium, the sporangia are disposed at the back of the frond in imbricated marginal spikes, formed by a transformation of the ultimate pinnæ. In the other division, represented by Schizæa, the fruitful spikes are really on the under-side of the frond, but the frond being reversed they seem to be on the upper.

The Ceratopteridineæ, sometimes called Parkeriaceæ, are tropical aquatic ferns, whose sterile fronds, which are membranaceous, with a thick vascular footstalk, float on the surface of the water, and by the time these are nearly decayed the fertile fronds are perfected. The latter are more erect, repeatedly divided and forked, the divisions being linear. There is only a single genus, Ceratopteris, which has continuous sori occupying the longitudinal veins at the edge of the frond, and covered by the indusioid margin. The sporangia have a very broad incomplete ring, and connect the ringless ferns with the group which possess them. The spores are triangular, and marked with three sets of concentric ridges.

The order Osmundineæ contains the Osmunda regalis, considered to be the finest of all European ferns, which is common throughout Great Britain in wet spongy soils, and appears to be the only species of the group that is European. The fronds grow in tufts from a thick woody caudex, which branches and extends widely by the formation of lateral crowns, but, if impeded, it elongates and rises in an erect position to the height of two or even three feet above the soil. The barren fronds of the large luxuriant tufts are highly developed, and are from six to nine or even eleven feet high; the fertile fronds are shorter and fewer in number. The bipinnate character prevails throughout; the primary divisions are opposite, the secondary alternate, and the pinnules are oblong and opposite. The sculptured sporangia have a very rudimentary ring, and, in the fruit-bearing fronds, four or five of the lower pairs of pinnules have the leafy character, while the remainder develop clusters of sporangia in place of pinnules. The sporangia on the fruitful branches are at first pale green, but gradually become reddish brown, hence the name of Flowering fern. The Leptopteris section of the genus Todea, almost peculiar to New Zealand, has beautiful transparent fronds, with naked sporangia placed upon the veins, forming very much scattered sori.

The Marattiaceæ may be partly known by their huge globose rhizome projecting above the ground, and rough with the processes from which the leaf-stalks have fallen. This sends out a few large fibrous roots, and consists of cellular tissue abounding in starch, with small bundles of fibro-vascular tissue regularly distributed through it. The stipes have a pair of stipule-like organs at their base; and the sori are either oblong fronds of a double row of sporangia, which in some cases is concrete, or they are circular with the sporangia annularly concrete, or they are connate throughout the fertile portions. These peculiarities respectively distinguish the smaller groups of Marattineæ, Kaulfussineæ, and Danæineæ, while the group Marattiaceæ itself is distinguished from Ophioglossaceæ by having its sori dorsal, that is, set on the back, or under-surface, of flat leafy fronds. The leaves of Angiopteris evecta are used as a perfume in the Sandwich Islands, and its rhizome serves for food, as that of the Marattia salicina does in New Zealand.