The sterile branches (B) are more slender than the spore-bearing ones, and the sheaths shorter. Surrounding the joints, apparently just below the sheaths, but really breaking through their bases, are circles of slender branches resembling the main branch, but more slender. The sterile branches grow to a height of forty to fifty centimetres, and from their bushy form the popular name of the plant, “horse-tail,” is taken. The surface of the plant is hard and rough, due to the presence of great quantities of flint in the epidermis,—a peculiarity common to all the species.
The stem is mainly composed of large, thin-walled cells, becoming smaller as they approach the epidermis. The outer cells of the ground tissue in the green branches contain chlorophyll, and the walls of some of them are thickened. The fibro-vascular bundles differ entirely from those of the ferns. Each bundle is nearly triangular in section (E), with the point inward, and the inner end occupied by a large air space. The tracheary tissue is only slightly developed, being represented by a few vessels[9] (tr.) at the outer angles of the bundle, and one or two smaller ones close to the air channel. The rest of the bundle is made up of nearly uniform, rather thin-walled, colorless cells, some of which, however, are larger, and have perforated cross-walls, representing the sieve tubes of the fern bundle. There is no individual bundle sheath, but the whole circle of bundles has a common outer sheath.
The epidermis is composed of elongated cells whose walls present a peculiar beaded appearance, due to the deposition of flint within them. The breathing pores are arranged in vertical lines, and resemble in general appearance those of the ferns, though differing in some minor details. Like the other epidermal cells the guard cells have heavy deposits of flint, which here are in the form of thick transverse bars.
The spore cases have thin walls whose cells, shortly before maturity, develop thickenings upon their walls, which have to do with the opening of the spore case. The spores (H, I) are round cells containing much chlorophyll and provided with four peculiar appendages called elaters. The elaters are extremely sensitive to changes in moisture, coiling up tightly when moistened (I), but quickly springing out again when dry (H). By dusting a few dry spores upon a slide, and putting it under the microscope without any water, the movement may be easily examined. Lightly breathing upon them will cause the elaters to contract, but in a moment, as soon as the moisture of the breath has evaporated, they will uncoil with a quick jerk, causing the spores to move about considerably.
The fresh spores begin to germinate within about twenty-four hours, and the early stages, which closely resemble those of the ferns, may be easily followed by sowing the spores in water. With care it is possible to get the mature prothallia, which should be treated as described for the fern prothallia. Under favorable conditions, the first antheridia are ripe in about five weeks; the archegonia, which are borne on separate plants, a few weeks later. The antheridia ([Fig. 72], J, an.) are larger than those of the ferns, and the spermatozoids (K) are thicker and with fewer coils, but otherwise much like fern spermatozoids.
The archegonia have a shorter neck than those of the ferns, and the neck is straight.
Both male and female prothallia are much branched and very irregular in shape.
There are a number of common species of Equisetum. Some of them, like the common scouring rush (E. hiemale), are unbranched, and the spores borne at the top of ordinary green branches; others have all the stems branching like the sterile stems of the field horse-tail, but produce a spore-bearing cone at the top of some of them.
Class III.—The Club Mosses (Lycopodinæ).
The last class of the pteridophytes includes the ground pines, club mosses, etc., and among cultivated plants numerous species of the smaller club mosses (Selaginella).
Two orders are generally recognized, although there is some doubt as to the relationship of the members of the second order. The first order, the larger club mosses (Lycopodiaceæ) is represented in the northern states by a single genus (Lycopodium), of which the common ground pine (L. dendroideum) ([Fig. 73]) is a familiar species. The plant grows in the evergreen forests of the northern United States as well as in the mountains further south, and in the larger northern cities is often sold in large quantities at the holidays for decorating. It sends up from a creeping, woody, subterranean stem, numerous smaller stems which branch extensively, and are thickly set with small moss-like leaves, the whole looking much like a little tree. At the ends of some of the branches are small cones (A, x, B) composed of closely overlapping, scale-like leaves, much as in a fir cone. Near the base, on the inner surface of each of these scales, is a kidney-shaped capsule (C, sp.) opening by a cleft along the upper edge and filled with a mass of fine yellow powder. These capsules are the spore cases.
The bases of the upright stems are almost bare, but become covered with leaves higher up. The leaves are in shape like those of a moss, but are thicker. The spore-bearing leaves are broader and when slightly magnified show a toothed margin.
The stem is traversed by a central fibro-vascular cylinder that separates easily from the surrounding tissue, owing to the rupture of the cells of the bundle sheath, this being particularly frequent in dried specimens. When slightly magnified the arrangement of the tissues may be seen ([Fig. 73], E). Within the epidermis is a mass of ground tissue of firm, woody texture surrounding the central oval or circular fibro-vascular cylinder. This shows a number of white bars (xylem) surrounded by a more delicate tissue (phloem).
On magnifying the section more strongly, the cells of the ground tissue (G) are seen to be oval in outline, with thick striated walls and small intercellular spaces. Examined in longitudinal sections they are long and pointed, belonging to the class of cells known as “fibres.”