Botany by Green J. Reynolds (Joseph Reynolds) 1848-1914

Author:Green, J. Reynolds (Joseph Reynolds), 1848-1914
Language: eng
Format: epub
Tags: Botany
Publisher: New York, D. Appleton and Company

FIG. 26. Diagram of sections of stem of dicotyledon at three ages. A, young condition, showing commencement of differentiation of the plerome and its vascular strands: a, strand; b, limits of the plerome; c, periblem; m.r., medullary ray; pi, pith. B, a little older stage: p, bast; x, wood; c, cambium; i.e., interfascicular cambium: (one of the strands has been shaded). C, older stage, after the commencement of secondary thickening: px, protoxylem or first-formed wood; x, secondary wood; ph, secondary bast. (After Sachs.)

curious patterns; with substitution of lignin for cellulose as the material of which they are composed; and with the disappearance of many of the transverse separating walls, causing a vertical row of cells to become a vessel. The cells to show the change first are those on the inside of the wedge-shaped strand—the protoxylem. In these the thickening of the walls is laid down in the shape of a spiral band, or a series of rings. These vessels remain of small diameter. The other wood cells and vessels are thickened more irregularly and are called reticulated ; in some cases when the thickening deposit leaves only very small thin spots they are known as pitted elements (Fig. 13, p. 35).

The bast of the strand begins to be differentiated on the side nearest the periphery, where the cells are called the protophloem. The vessels of the bast are sieve tubes (Fig. 27) as in the root. The other elements are mainly elongated cells with thin cellulose walls.

As the differentiation begins at the front and back of the bundle and advances in each direction the wood and bast are not very long in meeting. In plants that only live for a few weeks or months they come into actual contact, but to those whose lives are longer provision is made for further development by the last layer left between them remaining meristematic or capable of continuous dividing. This is the cambium layer of which we have spoken. It is only a single cell in thickness.

This arrangement of the supporting tissue is very strong and most economical. The hollow cylinder or tube is one of the strongest forms of support that a structure can possess. It has, too, a certain flexibility, for while the strands are gradually hardening they can bend freely without breaking. The young stem thus shows itself built for toughness and elasticity, so possessing a power of bending to wind and recovering as the force of the air passes it. The continuity of the

THE STRUCTURE OF THE SHOOT

69

vascular strands throughout the plant ensures the proper distribution of the water absorbed from the soil.

Certain other features of the framework of the plant next call for attention. If we examine the outer layer, called in the stem and leaf the epidermis, we find it as a continuous sheet over the whole, and in most cases a

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.