Fig. 67.—A, embryo of the ostrich fern just before breaking through the prothallium, × 50. st. apex of stem. l, first leaf. r, first root. ar. neck of the archegonium. B, young plant, still attached to the prothallium (pr.). C, underground stem of the maiden-hair fern (Adiantum), with one young leaf, and the base of an older one, × 1. D, three cross-sections of a leaf stalk: i, nearest the base; iii, nearest the blade of the leaf, showing the division of the fibro-vascular bundle, × 5. E, part of the blade of the leaf, × ½. F, a single spore-bearing leaflet, showing the edge folded over to cover the sporangia, × 1. G, part of the fibro-vascular bundle of the leaf stalk (cross-section), × 50. x, woody part of the bundle. y, bast. sh. bundle sheath. H, a small portion of the same bundle, × 150. I, stony tissue from the underground stem, × 150. J, sieve tube from the underground stem, × 300.
Up to this point, all the cells of the embryo are much alike, and the embryo, like that of the bryophytes, is completely surrounded by the enlarged base of the archegonium (compare [Fig. 67], A, with [Fig. 55]); but before the embryo breaks through the overlying cells a differentiation of the tissues begins. In the axis of each of the four divisions the cells divide lengthwise so as to form a cylindrical mass of narrow cells, not unlike those in the stem of a moss. Here, however, some of the cells undergo a further change; the walls thicken in places, and the cells lose their contents, forming a peculiar conducting tissue (tracheary tissue), found only in the two highest sub-kingdoms. The whole central cylinder is called a “fibro-vascular bundle,” and in its perfect form, at least, is found in no plants below the ferns, which are also the first to develop true roots.
The young root and leaf now rapidly elongate, and burst through the overlying cells, the former growing downward and becoming fastened in the ground, the latter growing upward through the notch in the front of the prothallium, and increasing rapidly in size ([Fig. 67], B). The leaf is more or less deeply cleft, and traversed by veins which are continuations of the fibro-vascular bundle of the stalk, and themselves fork once or twice. The surface of the leaf is covered with a well-developed epidermis, and the cells occupying the space between the veins contain numerous chloroplasts, so that the little plant is now quite independent of the prothallium, which has hitherto supported it. As soon as the fern is firmly established, the prothallium withers away.
Comparing this now with the development of the sporogonium in the bryophytes, it is evident that the young fern is the equivalent of the sporogonium or spore fruit of the former, being, like it, the direct product of the fertilized egg cell; and the prothallium represents the moss or liverwort, upon which are borne the sexual organs. In the fern, however, the sporogonium becomes entirely independent of the sexual plant, and does not produce spores until it has reached a large size, living many years. The sexual stage, on the other hand, is very much reduced, as we have seen, being so small as to be ordinarily completely overlooked; but its resemblance to the lower liverworts, like Riccia, or the horned liverworts, is obvious. The terms oöphyte (egg-bearing plant) and sporophyte (spore-bearing plant, or sporogonium) are sometimes used to distinguish between the sexual plant and the spore-bearing one produced from it.
The common maiden-hair fern (Adiantum pedatum) has been selected here for studying the structure of the full-grown sporophyte, but almost any other common fern will answer. The maiden-hair fern is common in rich woods, and may be at once recognized by the form of its leaves. These arise from a creeping, underground stem ([Fig. 67], C), which is covered with brownish scales, and each leaf consists of a slender stalk, reddish brown or nearly black in color, which divides into two equal branches at the top. Each of these main branches bears a row of smaller ones on the outside, and these have a row of delicate leaflets on each side ([Fig. 67], E). The stem of the plant is fastened to the ground by means of numerous stout roots. The youngest of these, near the growing point of the stem, are unbranched, but the older ones branch extensively (C).
On breaking the stem across, it is seen to be dark-colored, except in the centre, which is traversed by a woody cylinder (fibro-vascular bundle) of a lighter color. This is sometimes circular in sections, sometimes horse-shoe shaped. Where the stem branches, the bundle of the branch may be traced back to where it joins that of the main stem.
A thin cross-section of the stem shows, when magnified, three regions. First, an outer row of cells, often absent in the older portions; this is the epidermis. Second, within the epidermis are several rows of cells similar to the epidermal cells, but somewhat larger, and like them having dark-brown walls. These merge gradually into larger cells, with thicker golden brown walls ([Fig. 67], I). The latter, if sufficiently magnified, show distinct striation of the walls, which are often penetrated by deep narrow depressions or “pits.” This thick-walled tissue is called “stony tissue” (schlerenchyma). All the cells contain numerous granules, which the iodine test shows to be starch. All of this second region lying between the epidermis and the fibro-vascular bundle is known as the ground tissue. The third region (fibro-vascular) is, as we have seen without the microscope, circular or horse-shoe shaped. It is sharply separated from the ground tissue by a row of small cells, called the “bundle sheath.” The cross-section of the bundle of the leaf stalk resembles, almost exactly, that of the stem; and, as it is much easier to cut, it is to be preferred in studying the arrangement of the tissues of the bundle ([Fig. 67], G). Within the bundle sheath (sh.) there are two well-marked regions, a central band (x) of large empty cells, with somewhat angular outlines, and distinctly separated walls; and an outer portion (y) filling up the space between these central cells and the bundle sheath. The central tissue (x) is called the woody tissue (xylem); the outer, the bast (phloem). The latter is composed of smaller cells of variable form, and with softer walls than the wood cells.
A longitudinal section of either the stem or leaf stalk shows that all the cells are decidedly elongated, especially those of the fibro-vascular bundle. The xylem ([Fig. 68], C, x) is made up principally of large empty cells, with pointed ends, whose walls are marked with closely set, narrow, transverse pits, giving them the appearance of little ladders, whence they are called “scalariform,” or ladder-shaped markings. These empty cells are known as “tracheids,” and tissue composed of such empty cells, “tracheary tissue.” Besides the tracheids, there are a few small cells with oblique ends, and with some granular contents.
The phloem is composed of cells similar to the latter, but there may also be found, especially in the stem, other larger ones ([Fig. 67], J), whose walls are marked with shallow depressions, whose bottoms are finely pitted. These are the so-called “sieve tubes.”
For microscopical examination, either fresh or alcoholic material may be used, the sections being mounted in water. Potash will be found useful in rendering opaque sections transparent.
The leaves, when young, are coiled up ([Fig. 67], C), owing to growth in the earlier stages being greater on the lower than on the upper side. As the leaf unfolds, the stalk straightens, and the upper portion (blade) becomes flat.
The general structure of the leaf stalk may be understood by making a series of cross-sections at different heights, and examining them with a hand lens. The arrangement is essentially the same as in the stem. The epidermis and immediately underlying ground tissue are dark-colored, but the inner ground tissue is light-colored, and much softer than the corresponding part of the stem; and some of the outer cells show a greenish color, due to the presence of chlorophyll.