Page:A primer of forestry, with illustrations of the principal forest trees of Western Australia.djvu/15

9 that is, forests that have had little or no assistance from man in the way of promoting tree-growth, it is seldom that a forest consists wholly of one variety of trees. There may be, and in most cases there actually is, one type of tree that greatly outnumbers all the others; this type is said to form the dominant species, and it is customary to use its name as generally descriptive of the whole forest. For instance, we speak of jarrah forests, but in every jarrah forest other kinds of trees are found, although the jarrah vastly outnumbers all these. In forests* planted by man very often only one class of tree is to be found, for such plantations are usually made with the definite object of producing one class of timber only.

All trees have some common features. All of them have roots, and a trunk or trunks, and branches and leaves or substitutes for leaves.

A tree is a woody growth springing from the ground, with, as a rule, one straight bole or trunk only. Jarrah, karri, and many other Western Australian trees are examples in point. Others again have only a very short bole which divides at or near the ground into several large branches, each having all the appearance and functions of a trunk. York Gum is an example of this class. There are still others which send up a large number of boles direct from the roots, giving the tree a shrubby appearance: such trees do not attain the size of the types already mentioned. Many species of trees growing in the Eastern Goldfields and the drier regions of Australia adopt this habit of growth, and the mallee is the typical example of this class. The three kinds named may be regarded as the standard or general forms which trees take, but there are other forest growths usually called "trees" which differ so very much in appearance from the standard types that it seems scarcely right to apply the term "tree" to them. The "black-boy," the "grass tree," and Zamia palm are cases in point. From the point of view of the botanist, however, these odd forest growths are very similar to trees, for they have roots and boles and branches and parts which perform the same functions as do the leaves in trees of the ordinary character.

A tree is made up of three parts. Firstly, the roots, which extend into tho ground to a depth and width of spread that depends very much on the kind o! tree, and also upon the kind of soil in which the tree is growing. The roots are the great feeders of trees; they take up from the soil water and certain mineral substances that are essential to tree life. Secondly, the bole or trunk or stem, which supports the branches and crown and is the channel through which water and food are supplied to these. Thirdly, the branches and crown, with their network of foliage (leaves and buds). It is in the green portions of the foliage only that all the food taken up by the tree from the soil and air is worked over and transformed and made ready to assist in the growth of the whole tree. The crown of a tree has quite as much to do with its growth and healthy life as the other parts. If the crown be totally destroyed and not allowed to develop again, the tree will die just as surely as if its roots had been destroyed. In the crown the most Important processes in the reproduction of the tree and in the digestion of its food take place. For the reason that it is possible to control its size and shape, the crown is of much importance to those engaged in forestry work. When the forester is growing a crop of trees he can by skilful attention to the crown produce the class of tree that best suits the object he has in view.

The Food of a Tree.—A tree has two storehouses from which it gets its food—the soil and the air. If the rootlets or root filaments—for many of them are no thicker than a thread—be examined, it will be noticed that they are thickly covered with minute hairs. These hairs have an important function, for it is they that take up water from the ground. At the same time, they absorb various substances, almost wholly mineral in character, which water holds in solution. These other substances are the earthy constituents of the tree, and when timber is burnt we have left them once more in the ashes. The water containing the mineral