Studies of Trees by Jacob Joshua Levison
page 79 of 203 (38%)
page 79 of 203 (38%)
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complex until we reach the tree, which is the largest and highest
form of all plants. The tree is a living organism composed of cells like any other living organism. It has many parts, every one of which has a definite purpose. The three principal parts are: the stem, the crown, and the root. The stem: If we examine the cross-section of a tree, Fig. 86, we will notice that it is made up of numerous rings arranged in sections of different color and structure. The central part is known as the _pith_. Around the pith comes a dark, close-grained series of rings known as the _heartwood_, and outside the heartwood comes a lighter layer, the _sapwood_. The _cambium layer_ surrounds the sapwood and the _bark_ covers all. The cambium layer is the most important tissue of the tree and, together with part of the sapwood, transports the water and food of the tree. It is for this reason that a tree may be hollow, without heart and sapwood, and still produce foliage and fruit. [Illustration: FIG. 86.--The Cross-Section of a Tree.] The crown: The crown varies in form in different species and is developed by the growth of new shoots from buds. The bud grows out to a certain length and forms the branch. Afterwards it thickens only and does not increase in length. New branches will then form from other buds on the same branch. This explains in part the characteristic branching of trees, Fig. 87. [Illustration: FIG. 87.--Characteristic Form and Branching of Trees. The trees in the photograph are pin oaks.] |
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