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].
Fig. 87.—Characteristic Form and Branching of Trees. The trees in the photograph are pin oaks.
The leaves are the stomach and lungs of the tree. Their broad blades are a device to catch the sunlight which is needed in the process of digesting the food of the tree. The leaves are arranged on the twigs in such a way as to catch the most sunlight. The leaves take up the carbonic acid gas from the air, decompose it under the influence of light and combine it with the minerals and water brought up by the roots from the soil. The resulting chemical combinations are the sugars and starches used, by the cambium layer in building up the body of the tree. A green pigment, chlorophyll, in the leaf is the medium by which, with the aid of sunlight, the sugars are manufactured.
Fig. 88.—Roots of a Hemlock Tree in their Search for Water.