Class I.—Gymnosperms (Gymnospermæ).
The most familiar of these plants are the common evergreen trees (conifers), pines, spruces, cedars, etc. A careful study of one of these will give a good idea of the most important characteristics of the class, and one of the best for this purpose is the Scotch pine (Pinus sylvestris), which, though a native of Europe, is not infrequently met with in cultivation in America. If this species cannot be had by the student, other pines, or indeed almost any other conifer, will answer. The Scotch pine is a tree of moderate size, symmetrical in growth when young, with a central main shaft, and circles of branches at regular intervals; but as it grows older its growth becomes irregular, and the crown is divided into several main branches.[10] The trunk and branches are covered with a rough, scaly bark of a reddish brown color, where it is exposed by the scaling off of the outer layers. Covering the younger branches, but becoming thinner on the older ones, are numerous needle-shaped leaves. These are in pairs, and the base of each pair is surrounded by several dry, blackish scales. Each pair of leaves is really attached to a very short side branch, but this is so short as to make the leaves appear to grow directly from the main branch. Each leaf is about ten centimetres in length and two millimetres broad. Where the leaves are in contact they are flattened, but the outer side is rounded, so that a cross-section is nearly semicircular in outline. With a lens it is seen that there are five longitudinal lines upon the surface of the leaf, and careful examination shows rows of small dots corresponding to these. These dots are the breathing pores. If a cross-section is even slightly magnified it shows three distinct parts,—a whitish outer border, a bright green zone, and a central oval, colorless area, in which, with a little care, may be seen the sections of two fibro-vascular bundles. In the green zone are sometimes to be seen colorless spots, sections of resin ducts, containing the resin so characteristic of the tissues of the conifers.
The general structure of the stem may be understood by making a series of cross-sections through branches of different ages. In all, three regions are distinguishable; viz., an outer region (bark or cortex) ([Fig. 76], A, c), composed in part of green cells, and, if the section has been made with a sharp knife, showing a circle of little openings, from each of which oozes a clear drop of resin. These are large resin ducts (r). The centre is occupied by a soft white tissue (pith), and the space between the pith and bark is filled by a mass of woody tissue. Traversing the wood are numerous radiating lines, some of which run from the bark to the pith, others only part way. These are called the medullary rays. While in sections from branches of any age these three regions are recognizable, their relative size varies extremely. In a section of a twig of the present year the bark and pith make up a considerable part of the section; but as older branches are examined, we find a rapid increase in the quantity of wood, while the thickness of the bark increases but slowly, and the pith scarcely at all. In the wood, too, each year’s growth is marked by a distinct ring (A i, ii). As the branches grow in diameter the outer bark becomes split and irregular, and portions die, becoming brown and hard.
The tree has a very perfect root system, but different from that of any pteridophytes. The first root of the embryo persists as the main or “tap” root of the full-grown tree, and from it branch off the secondary roots, which in turn give rise to others.
The sporangia are borne on special scale-like leaves, and arranged very much as in certain pteridophytes, notably the club mosses; but instead of large and small spores being produced near together, the two kinds are borne on special branches, or even on distinct trees (e.g. red cedar). In the Scotch pine the microspores are ripe about the end of May. The leaves bearing them are aggregated in small cones (“flowers”), crowded about the base of a growing shoot terminating the branches ([Fig. 77], A ♂). The individual leaves (sporophylls) are nearly triangular in shape, and attached by the smaller end. On the lower side of each are borne two sporangia (pollen sacs) (C, sp.), opening by a longitudinal slit, and filled with innumerable yellow microspores (pollen spores), which fall out as a shower of yellow dust if the branch is shaken.
The macrosporangia (ovules) are borne on similar leaves, known as carpels, and, like the pollen sacs, borne in pairs, but on the upper side of the sporophyll instead of the lower. The female flowers appear when the pollen is ripe. The leaves of which they are composed are thicker than those of the male flowers, and of a pinkish color. At the base on the upper side are borne the two ovules (macrosporangia) ([Fig. 77], E, o), and running through the centre is a ridge that ends in a little spine or point.
The ovule-bearing leaf has on the back a scale with fringed edge (F, sc.), quite conspicuous when the flower is young, but scarcely to be detected in the older cone. From the female flower is developed the cone ([Fig. 75], A), but the process is a slow one, occupying two years. Shortly after the pollen is shed, the female flowers, which are at first upright, bend downward, and assume a brownish color, growing considerably in size for a short time, and then ceasing to grow for several months.
Fig. 75.—Scotch pine (Pinus sylvestris). A, a ripe cone, × ½. B, a year-old cone, × 1. C, longitudinal section of B. D, a single scale of B, showing the sporangia (ovules) (o), × 2. E, a scale from a ripe cone, with the seeds (s), × ½. F, longitudinal section of a ripe seed, × 3. em. the embryo. G, a germinating seed, × 2. r, the primary root. H, longitudinal section through G, showing the first leaves of the young plant still surrounded by the endosperm, × 4. I, an older plant with the leaves (l) withdrawing from the seed coats, × 4. J, upper part of a young plant, showing the circle of primary leaves (cotyledons), × 1. K, section of the same, × 2. b, the terminal bud. L, cross-section of the stem of the young plant, × 25. fb. a fibro-vascular bundle. M, cross-section of the root, × 25. x, wood. ph. bast, of the fibro-vascular bundle.