Fig. 93.—A, base of a plant of shepherd’s-purse (Capsella bursa-pastoris), × ½. r, the main root. B, upper part of the inflorescence, × 1. C, two leaves: i, from the upper part; ii, from the base of the plant, × 1. D, a flower, × 3. E, the same, with sepals and petals removed, × 3. F, petal. G, sepal. H, stamen, × 10. f, filament. an. anther. I, a fruit with one of the valves removed to show the seeds, × 4. J, longitudinal section of a seed, × 8. K, the embryo removed from the seed, × 8. l, the first leaves (cotyledons). st. the stem ending in the root. L, cross-section of the stem, × 20. fb. fibro-vascular bundle. M, a similar section of the main root, × 15. N, diagram of the flower.
The structure of the tissues, and the peculiarities of the flower and fruit, will be better understood by a somewhat careful examination of a typical dicotyledon, and a comparison with this of examples of the principal orders and families.
One of the commonest of weeds, and at the same time one of the most convenient plants for studying the characteristics of the dicotyledons, is the common shepherd’s-purse (Capsella bursa-pastoris) (Figs. [93]–[95]).
The plant grows abundantly in waste places, and is in flower nearly the year round, sometimes being found in flower in midwinter, after a week or two of warm weather. It is, however, in best condition for study in the spring and early summer. The plant may at once be recognized by the heart-shaped pods and small, white, four-petaled flowers. The plant begins to flower when very small, but continues to grow until it forms a much-branching plant, half a metre or more in height. On pulling up the plant, a large tap-root ([Fig. 93], A, r) is seen, continuous with the main stem above ground. The first root of the seedling plant continues here as the main root of the plant, as was the case with the gymnosperms, but not with the monocotyledons. From this tap-root other small ones branch off, and these divide repeatedly, forming a complex root system. The main root is very tough and hard, owing to the formation of woody tissue in it. A cross-section slightly magnified ([Fig. 93], M), shows a round, opaque, white, central area (x), the wood, surrounded by a more transparent, irregular ring (ph.), the phloem or bast; and outside of this is the ground tissue and epidermis.
The lower leaves are crowded into a rosette, and are larger than those higher up, from which they differ also in having a stalk (petiole), while the upper leaves are sessile. The outline of the leaves varies much in different plants and in different parts of the same plant, being sometimes almost entire, sometimes divided into lobes almost to the midrib, and between these extremes all gradations are found. The larger leaves are traversed by a strong midrib projecting strongly on the lower side of the leaf, and from this the smaller veins branch. The upper leaves have frequently two smaller veins starting from the base of the leaf, and nearly parallel with the midrib (C i). The surface of the leaves is somewhat roughened with hairs, some of which, if slightly magnified, look like little white stars.
Magnifying slightly a thin cross-section of the stem, it shows a central, ground tissue (pith), whose cells are large enough to be seen even when very slightly enlarged. Surrounding this is a ring of fibro-vascular bundles (L, fb.), appearing white and opaque, and connected by a more transparent tissue. Outside of the ring of fibro-vascular bundles is the green ground tissue and epidermis. Comparing this with the section of the seedling pine stem, a resemblance is at once evident, and this arrangement was also noticed in the stem of the horse-tail.
Branches are given off from the main stem, arising at the point where the leaves join the stem (axils of the leaves), and these may in turn branch. All the branches terminate finally in an elongated inflorescence, and the separate flowers are attached to the main axis of the inflorescence by short stalks. This form of inflorescence is known technically as a “raceme.” Each flower is really a short branch from which the floral leaves arise in precisely the same way as the foliage leaves do from the ordinary branches. There are five sets of floral leaves: I. four outer perigone leaves (sepals) (F), small, green, pointed leaves traversed by three simple veins, and together forming the calyx; II. four larger, white, inner perigone leaves (petals) (G), broad and slightly notched at the end, and tapering to the point of attachment. The petals collectively are known as the “corolla.” The veins of the petals fork once; III. and IV. two sets of stamens (E), the outer containing two short, and the inner, four longer ones arranged in pairs. Each stamen has a slender filament (H, f) and a two-lobed anther (an.). The innermost set consists of two carpels united into a compound pistil. The ovary is oblong, slightly flattened so as to be oval in section, and divided into two chambers. The style is very short and tipped by a round, flattened stigma.
The raceme continues to grow for a long time, forming new flowers at the end, so that all stages of flowers and fruit may often be found in the same inflorescence.