The _red mulberry_ (_Morus rubra_) is apt to be confused with the white mulberry, but differs in the following characters: The leaves of the red mulberry are rough on the upper side and downy on the under side, whereas the leaves of the white mulberry are smooth and shiny. The buds in the red are larger and more shiny than those of the white. The _Osage orange_ (_Toxylon pomiferum_) is similar to the mulberry in the light, golden color of its bark, but differs from it in possessing conspicuous spines along the twigs and branches and a more ridged bark. CHAPTER IV THE STRUCTURE AND REQUIREMENTS OF TREES To be able fully to appreciate trees, their mode of life, their enemies and their care, one must know something of their structure and life requirements. Structure of trees: Among the lower forms of plants there is very little distinction between the various parts--no differentiation into root, stem, or crown. Often the lower forms of animal and vegetable life are so similar that one cannot discriminate between them. But as we ascend in the scale, the various plant forms become more and more 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.]