The section of the fibro-vascular bundle differs at different heights. Near the base of the stalk (Fig. D i) it is horseshoe-shaped; but, if examined higher up, it is found to divide (II, III), one part going to each of the main branches of the leaf. These secondary bundles divide further, forming the veins of the leaflets.
The leaflets (E, F) are one-sided, the principal vein running close to the lower edge, and the others branching from it, and forking as they approach the upper margin, which is deeply lobed, the lobes being again divided into teeth. The leaflets are very thin and delicate, with extremely smooth surface, which sheds water perfectly. If the plant is a large one, some of the leaves will probably bear spores. The spore-bearing leaves are at once distinguished by having the middle of each lobe of the leaflets folded over upon the lower side (F). On lifting one of these flaps, numerous little rounded bodies (spore cases) are seen, whitish when young, but becoming brown as they ripen. If a leaf with ripe spore cases is placed upon a piece of paper, as it dries the spores are discharged, covering the paper with the spores, which look like fine brown powder.
Fig. 68.—A, vertical section of the leaf of the maiden-hair fern, which has cut across a vein (f.b.), × 150. B, surface view of the epidermis from the lower surface of a leaf. f, vein. p, breathing pore, × 150. C, longitudinal section of the fibro-vascular bundle of the leaf stalk, showing tracheids with ladder-shaped markings, × 150. D, longitudinal section through the tip of a root, × 150. a, apical cell. Pl. young fibro-vascular bundle. Pb. young ground tissue. E, cross-section of the root, through the region of the apical cell (a), × 150. F, cross-section through a full-grown root, × 25. r, root hairs. G, the fibro-vascular bundle of the same, × 150.
A microscopical examination of the leaf stalk shows the tissues to be almost exactly like those of the stem, except the inner ground tissue, whose cells are thin-walled and colorless (soft tissue or “parenchyma”) instead of stony tissue. The structure of the blade of the leaf, however, shows a number of peculiarities. Stripping off a little of the epidermis with a needle, or shaving off a thin slice with a razor, it may be examined in water, removing the air if necessary with alcohol. It is composed of a single layer of cells, of very irregular outline, except where it overlies a vein ([Fig. 68], B, f). Here the cells are long and narrow, with heavy walls. The epidermal cells contain numerous chloroplasts, and on the under surface of the leaf breathing pores (stomata, sing. stoma), not unlike those on the capsules of some of the bryophytes. Each breathing pore consists of two special crescent-shaped epidermal cells (guard cells), enclosing a central opening or pore communicating with an air space below. They arise from cells of the young epidermis that divide by a longitudinal wall, that separates in the middle, leaving the space between.
Fig. 69.—A, mother cell of the sporangium of the maiden-hair fern, × 300. B, young sporangium, surface view, × 150: i, from the side; ii, from above. C–E, successive stages in the development of the sporangium seen in optical section, × 150. F, nearly ripe sporangium, × 50: i, from in front; ii, from the side. an. ring. st. point of opening. G, group of four spores, × 150. H, a single spore, × 300.
By holding a leaflet between two pieces of pith, and using a very sharp razor, cross-sections can be made. Such a section is shown in [Fig. 68], A. The epidermis (e) bounds the upper and lower surfaces, and if a vein (f.b.) is cut across its structure is found to be like that of the fibro-vascular bundle of the leaf stalk, but much simplified.
The ground tissue of the leaf is composed of very loose, thin-walled cells, containing numerous chloroplasts. Between them are large and numerous intercellular spaces, filled with air, and communicating with the breathing pores. These are the principal assimilating cells of the plant; i.e. they are principally concerned in the absorption and decomposition of carbonic acid from the atmosphere, and the manufacture of starch.
The spore cases, or sporangia ([Fig. 69]), are at first little papillæ (A), arising from the epidermal cells, from which they are early cut off by a cross-wall. In the upper cell several walls next arise, forming a short stalk, composed of three rows of cells, and an upper nearly spherical cell—the sporangium proper. The latter now divides by four walls (B, C, i–iv), into a central tetrahedral cell, and four outer ones. The central cell, whose contents are much denser than the outer ones, divides again by walls parallel to those first formed, so that the young sporangium now consists of a central cell, surrounded by two outer layers of cells. From the central cell a group of cells is formed by further divisions (D), which finally become entirely separated from each other. The outer cells of the spore case divide only by walls, at right angles to their outer surface, so that the wall is never more than two cells thick. Later, the inner of these two layers becomes disorganized, so that the central mass of cells floats free in the cavity of the sporangium, which is now surrounded by but a single layer of cells (E).
Each of the central cells divides into four spores, precisely as in the bryophytes. The young spores (G, H) are nearly colorless and are tetrahedral (like a three-sided pyramid) in form. As they ripen, chlorophyll is formed in them, and some oil. The wall becomes differentiated into three layers, the outer opaque and brown, the two inner more delicate and colorless.
Running around the outside of the ripe spore case is a single row of cells (an.), differing from the others in shape, and having their inner walls thickened. Near the bottom, two (sometimes four) of these cells are wider than the others, and their walls are more strongly thickened. It is at this place (st.) that the spore case opens. When the ripe sporangium becomes dry, the ring of thickened cells (an.) contracts more strongly than the others, and acts like a spring pulling the sporangium open and shaking out the spores, which germinate readily under favorable conditions, and form after a time the sexual plants (prothallia).