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

by Mary Somerville

The roots of ferns, like those of forest trees, are delicate fibres, which descend either from a woody stem called a rhizome, from rhiza, a root, or from a caudex, so named from the Latin, caudex, a trunk. Thus the stems of ferns are of two distinct types. The rhizome generally grows horizontally, and creeps along the surface of the soil or rock, or tree trunk to which it is affixed, but it is sometimes subterranean. In this, the growing point is in advance of the fronds, which appear at intervals along the exposed side or sides, and after they have reached maturity drop off, leaving a clean scar or cicatrix. The caudex varies from the size of a small wire-like thread to the size of a tree trunk. It is sometimes elongated, as in the superfine wiry thread-like stems of Hymenophyllum, the scandent ivy-like rooting stems of Stenochlæna, and the subterranean, horizontal, widely-creeping stems of Pteris aquilina, all of which develop single fronds at intervals, but fronds which adhere permanently to the stem, and which do not fall off, leaving a scar, as do the fronds borne by the rhizome. The more frequent form of caudex, however, is that of a short stocky stem producing fronds on all sides from its apex: very frequently this is a globose or oblong mass growing erect, yet scarcely reaching above the surface of the soil; but sometimes it is more lengthened, showing a more or less elevated stem, or turning sideways, and taking a decumbent position, the young fronds, however, always rising from its apex. It is on the latter plan, by a continuous erect and upward growth, that the trunks of tree ferns, which rise sometimes sixty or eighty feet high, are formed. The common Lady Fern, Athyrium Filix-fœmina, furnishes an example of the ordinary herbaceous caudex; as also does the Male Fern, Lastrea Filix-mas. In the Hart’s-tongue, or Scolopendrium, the caudex is generally compact, and increases by the formation of new crowns or centres of growth around the older one, till the whole becomes an almost spherical mass of considerable size.

The stipes or leaf-stalk of a fern is often of considerable length, and in its upper part, called the rachis, which bears the leafy portion, is commonly more or less branched, as well as furrowed on its upper surface. The fronds, or parts analogous to leaves, are sometimes simple, as in the Hart’s-tongue, the rachis of which has a symmetrical limb or wing on each side, so as to form a long, even-margined, narrow, tongue-shaped frond. If the limb on each side of the rachis be deeply divided, as in the Polypodium vulgare, the frond is said to be wing-cleft, or pinnatifid, and the divisions are called segments or lobes. In most ferns, however, the frond is once, twice, or three times winged or pinnate, and in such cases the first divisions are called pinnæ, and the secondary or subsequent ones when present, pinnulæ or pinnules. The leafy portion, whether simple, pinnatifid, or pinnate, is always traversed by veins arranged on some definite plan, a most important circumstance, since the sori or fructification of the ferns is always produced in connection with the veins.

The fertile fronds, in certain groups, differ in form from the sterile, generally by the greater or less contraction of their parts. In most ferns the full-grown fronds are flat, that is, with all their parts lying in one plane; but, during their vernation, that is, when they first rise from the stem, they are circinate or curled inwards, like a crosier.

When a fern acquires a considerable stem, as in Tree Ferns, it consists of a central or medullary part, consisting of cellular tissue, and an external or cortical portion, formed of the consolidated bases of the fronds, in which may be seen, on cutting a section of a trunk, an irregular zone, formed of fibro-vascular bundles, scalariform ducts, and woody fibre. Prolongations from this zone pass into the leaf-stalks, and thence into the midrib of the leaf, whence they spread into its lateral branches, and ultimately appear in the leafy parts in the form of veins. It is the arrangement of these bundles of coloured woody tissue in the cellular tissue of the leaf-stalks of the herbaceous ferns, which give rise to those peculiar figures on their transverse sections, such as a star, the letter T, and the heraldic spread eagle, from which latter the common Brake Fern (Pteris) takes its botanical specific name of aquilina. [Fig. 54] shows an oblique section of the footstalk of a fern leaf with its bundles of scalariform ducts, as determined by Dr. Carpenter’s microscopic observations.

Fig. 54. Section of footstalk of Fern frond showing scalariform ducts.

The fructification of the ferns is arranged with the most perfect symmetry, usually on the under surface of the leaf, but sometimes at the margin, and assuming a great variety of forms and positions. It consists of sori, that is, groups of nearly globular spore cases or sporangia, sometimes sessile on the frond, sometimes with a footstalk. They are always situated on a vein or its branches, or at the extremity of the veins on the margin of the frond. In fact the sporangia originate in the cellular tissue immediately in contact with a vein, beneath the epidermis or skin of the leaf, which is forced up as the sporangia increase in size, in the form of a whitish membrane, which constitutes the indusium, or protecting cover of the sori. While the fruit is advancing to maturity, the indusium separates partly or wholly from the surrounding skin or epiderm, and subsequently either shrivels or falls off altogether. In some few species the opening is in the centre of the indusium, and then it surrounds the sori like a cup; in other ferns, the skin from both surfaces of the leaf extends beyond the margin, includes the sori between it, and fulfils the office of an indusium.

Fig. 55. Pinnule of Polypodium bearing sori.

The sori, as already noted, take a variety of forms, and are variously situated. Some are round and dot-like, some are oblong and straight, some are hippocrepiform or horse-shoe-shaped, while some are continuous in a line-like band. The indusium, when present, takes more or less exactly the form of the sorus. These peculiarities are so well marked that they are taken advantage of in the discrimination of genera.