. Beginners botany. Botany. outside of the plant cylinder. It is evident, then, that the covering of bark must expand in order to alloiv of the expan- sion of the woody cylinder ivithin it. The tis- sues, therefore, must be under constant pressure or tension. It has been determined that the pressure within a growing trunk is often as much as fifty pounds to the square inch. The lower part of the limb in Fig. 6i shows that the outer layers of bark (which are long since dead, and serve only as protective tissue) have reached the limit of their expanding capacity and have begun to split. The pupi
. Beginners botany. Botany. outside of the plant cylinder. It is evident, then, that the covering of bark must expand in order to alloiv of the expan- sion of the woody cylinder ivithin it. The tis- sues, therefore, must be under constant pressure or tension. It has been determined that the pressure within a growing trunk is often as much as fifty pounds to the square inch. The lower part of the limb in Fig. 6i shows that the outer layers of bark (which are long since dead, and serve only as protective tissue) have reached the limit of their expanding capacity and have begun to split. The pupil will now be interested in the bark on the body of an old elm tree (Fig. 62); and he should be able to suggest one reason why stems remain cylindri- cal, and why the old bark becomes marked with furrows, scales, and plates. Most woody plants increase in diameter by the addition of an annual layer or "ring'' on the outside of the woody cylinder, underneath the bark. The monocotyledo- nous plants comprise very few trees and shrubs in temperate climates (the palms, yuccas, and other tree-like plants are of this class), and they do not increase greatly in diameter and they rarely branch to any extent. Bark-bound Trees.—If, for any rea- son, the bark should become so dense and strong that the trunk cannot ex- pand, the tree is said to be '' bark-bound.'' Such condition is not rare in orchard trees that have been neglected. When good tillage is given to such trees, they Fig. 61.— Cracking OF THE Bark on an Elm Fig. 62. — Piece of Bark from an Old Elm Trunk,. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bailey, L. H. (Liberty Hyde), 1858-1954. Toronto : Macmillan
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectbotany, bookyear1921
