THE general appearance of plants belonging to the class of Ferns is so well known that it need scarcely be described, especially since the introduction of the glass plant-cases, by means of which the air can be kept so damp that ferns are now grown in the very heart of our cities. Their bright green and finely cleft leaves (PI. II. figs. 9 & 16) or fronds (frons, a leaf) as the leaf-like organs of the lower plants are called, arising in tufts from the stems, give them the elegant appearance we are called upon to admire, whenever they are met with. The brownish spots or stripes seen upon the back or under surface of the fronds, and consisting of the fructification, form also a simple character by which they may generally be distinguished; although in a few of them the fructification is placed upon a distinct stalk. The stem or rhi´zome (????a, a root) of a fern is mostly situated just beneath or at the surface of the ground, and is commonly mistaken for the real root, which is buried in the earth. It is brownish outside, and covered with scurfy scales or ramen´ta (ramentum, a shaving). These scales are interesting microscopic objects, from the distinctness with which they exhibit the cellular network.

A section of the rhizome exhibits the fibro-vascular tissue arranged differently from that in the stems of either Exogens or Endogens. It forms curiously curved longitudinal plates, a very abundant component of which is the scalar´iform (scala, a ladder) duct (Pl. II. fig. 34). The walls of the scalariform ducts are angular, and the secondary deposit is arranged in the form of transverse bars, somewhat resembling the steps of a ladder; which structure is best seen in a transverse or slightly oblique section. The fronds are usually cleft nearly down to the main vein or midrib (fig. 9), or pinnatifÍd (pinna, a feather, findo, to cleave); sometimes the segments are similarly cleft, so that the fronds are bipinnatifid. The manner in which the veins usually branch is also peculiar, each branch separating from the point of division at an acute angle with the original direction of the vein (fig. 20), so as to be forked. It is also worthy of notice, that the young frond is rolled up into a flat spiral, or is cir´cinate (circino, to go round), before it opens.

We will now examine one or two species more minutely.

PolypodiaceÆ. PolypÓdium vulga´re (Pl. II. fig. 9, one-third of the natural size), a member of this family, is very common on old trunks of trees, on banks, &c. The frond is deeply pinnatifid, the segments being oblong, blunt (obtuse), scalloped (crenate) at the edges (PI. II. fig. 10), and becoming gradually shorter towards the apex of the frond.

On the back of the fronds are the little orange-coloured groups of capsules (Pl. II. figs. 9 & 10); these are called sor´i (s????, a heap). The capsules or thÉcÆ (????, a case), a magnified group of which is represented in fig. 11, consist each of an aggregation of cells, fixed to a stalk (fig. 12); and along the back of the capsule is a close row of thicker cells, forming an elastic ring, the an´nulus. When the seed-like bodies or spores (sp???, a seed) are ripe, the annulus becomes straightened from its elastic power, and tears the capsule open, so that the spores are set free and scattered.

Aspid´ium filix mas (Pl. II. fig. 16, reduced to one third or fourth of the natural size), is the most common British fern. In this the fronds are pinnate, i. e. the segments (pinnÆ) corresponding to those of Polypodium are cleft entirely to the main stalk, the pinnÆ (fig. 17) being pinnatifid; and the segments of the pinnÆ or the pinnules (fig. 18) are oblong, obtuse, and saw-edged (serrate). In all botanical works the fronds are incorrectly said to be bipinnate.

The sori are brown, kidney-shaped (fig. 18), and differ from those of Polypodium in being covered by a thin membrane (indÚsium), which is fixed to the frond at the notch.

Scolopen´drium vulgÁre (Pl. II. fig. 19), the Hart’s-tongue Fern, is common in hedges and on moist banks. Its fronds are simple or undivided, strap-shaped, heart-shaped at the base, and narrowed to a point at the apex. The sori (fig. 20) are brown, narrow, longish (linear), and transverse or slightly oblique. The indusium is cleft down the middle, so as to form a longitudinal fissure or suture.

Reproduction.—The spores of the ferns (Pl. II. fig. 12 a) resemble in appearance the seeds of flowering plants on a small scale. They are usually brown, covered on the surface with little tubercles or other markings, and when kept on a slide in a moist atmosphere, as over a saucer of water covered with a bell-glass, they germinate. When this takes place, one or more short, brownish, hair-like radicles emerge from one part of the surface (Pl. II. fig. 13), and a process containing chlorophyll issues from an adjacent spot. As growth proceeds, the latter by cell-division gives rise to a flattened two-lobed leafy cotyledon-like body or prothal´lium (p??, before, ?a????, leaf), with numerous rootlets springing from the base of the lobes. The prothallium is of a peculiar dull-green colour, different from that of the young frond which is subsequently formed. This arises from the absence of stomata and intercellular passages containing air; for the air in these passages of leaves and petals contributes greatly to the production of the brightness of their colours. Moreover, the cells of the prothallium resemble those of the parenchyma of a leaf, the epidermis with its wavy-margined cells being absent.

When the prothallium has attained its full development, minute scattered protrusions from its cells occur on the margin or under surface, resembling short and blunt hairs; and each of these becomes partitioned off to form a new cell, within which a number of crowded smaller cells are produced. These organs are called antherid´ia (anther, and e?d??, resemblance); and within each of the crowded smaller cells is contained a very minute, colourless, coiled fibre, furnished with still finer filaments, called cil´ia (cilium, an eyelash); the ciliated fibres being termed spermatozo´a (sp??a, seed, ????, animal). At a later period, other organs are found also on the back of the prothallium. These are larger than the antheridia, and are composed of several cells, arranged around a central canal which leads to an embryo-cell situated at its base (Pl. II. fig. 2). These organs are the archegÓnia (????, beginning, ?????, offspring). When the antheridia are ripe, they discharge the spermatozoa, which are enabled to swim about by means of their cilia in water (rain), and entering the canal, reach the embryo-cell, which thus becomes fertilized. When fertilized, the embryo-cells produce the little fronds which afterwards grow into the mature plants.

Hence the spores of ferns differ strikingly from the seeds of the higher plants in not containing the embryo radicle and cotyledons already formed, these being produced during or after germination; also in the fertilizing organs, viz. the antheridia or representatives of the anthers, and the archegonia or the representatives of the pistils, being produced from the cells of the prothallium.

The more minute of these structures are too difficult of observation and preparation for any one unaccustomed to microscopic manipulation, so that they have not been figured in detail; the figures given will, however, serve to guide the observer in their recognition.

Preservation.—The ferns may be easily preserved in the entire state, by laying them flat between sheets of coarse unsized paper, and subjecting them to moderate pressure in a screw-press; the paper should be changed, or dried before a fire every two or three days, and the pressure repeated until the specimens become dry and rigid. They may then be mounted on sheets of paper, being fastened either with thread passed round the stalk or portions of the frond with a needle, and tied in a knot behind, or with strips of paper gummed at the ends.

The minute structures may be preserved either in the dry state or in glycerine.