The dicotyledonous stems are common to our trees and most plants, and may therefore be considered with advantage. The stem consists of the vascular tissue called “pith,” and we give an illustration of the cells magnified very considerably. The arrow indicates the outward direction (fig. 757).
Fig. 757.—Dicotyledonous stem.
We here perceive the vascular bundle proper surrounded by a very large-celled tissue, aa′bef. The almost square cells, aa′, form the epidermis on which follows the less dense cellular tissue of the bark. The latter surrounds a half-moon-shaped bundle of bast-cells, c, which are separated in the direction towards the interior, by a layer of cambium, dd′d″, from the bundles of vascular tissue, consisting of vessels and longitudinal cells. The latter tissues may be distinguished in the transverse section by the thicker walls, gg, and by their greater breadth, hh. It is further to be remarked that the cambium transparent tissue, dd″, appears on both sides of the bundles of vascular tissue, and extends to the next bundle, and thus presents an uninterrupted circle throughout the entire circumference of the stem.
Fig. 758.—Stem one year old.
On examining the section of a one-year-old dicotyledonous stem, magnified six times, as in fig. 758, we perceive several parts clearly distinguishable from each other, corresponding with the arrangement of the bundles of vascular tissue.
Enclosed by the epidermis, a, is a large-celled tissue, b f and m, in which a number of vascular bundles form a circle. In each of these we notice that the outer portion, consisting of bast-shell, c, is separated by the cambium, d, from the inner woody portion, e. The cambium forms a closed circle which penetrates through all the vascular bundles.
In the course of the further development of the stem, the parts, a b c, constitute the bark, the vascular bundles, e, the wood, and the cellular tissue, f, its pith. The tissues, m, penetrating between the vascular bundles, are called the medullary rays. The cambium is to be regarded as the most important part, since it is the source of new bundles of vascular tissue which year by year increase the circumference of the stem.
