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during the first year. After that they grow by the same process as the trunk and branches. It is obvious, therefore, that it is only by means of the youngest twigs that a tree increases in height, and in the spread of its branches. After the first year the length of young twigs is fixed; younger twigs stretch from new buds and the older twigs continue to grow by the deposition of wood-cells as in the old parts of the tree. Between the wood and the bark there is a very thin layer of living cells called the cambium. This cambium is nourished by the food sap and grows in thickness, its inner layers producing wood-substance and its outer layers producing bark, and yet between the two there always remains a thin layer, of cambium. The production of cells inwards which turn into wood is greater than that outwards of cells which turn into bark, and for this reason the layer of wood formed annually is thicker than the layer of bark. In this way every year a new layer of wood is added on the outside of the older wood, and a new layer of bark is added on the inside of the older bark. The outcome of this latter process is that the bark, as its older parts become stretched to breaking point, cracks or peels. It will be noticed that the process of bark production is even more complicated than that of wood. What is usually called bark is made up of two parts. All that is outside of the latest formed layer is dead matter, because no water or food supply can get to it from the interior of the tree. The purpose of this dead matter is to protect the true bark and the wood from damage from the outside.
The Structure of Wood.—The structure of wood is the main element which determines its class and its value. Trees differ from each other owing mainly to the differences in the size, shape and form of the cells which make up the fibre of wood. These differences give a particular character to each kind of wood, and are also the means by which it may be readily and surely identified. The structure of wood explains why some kinds are harder and stronger than others, why some are cross-grained and some straight-grained, why some have a tendency to crack or split, and why some wood, when cut, has a more or less beautiful pattern, and some has scarcely any pattern, or, indeed, none at all. Jarrah and banksia, for example, when cut are quite different in appearance, and this is due entirely to the difference in the structure of their wood.
All wood is composed of very small tubes and cells. In the latest formed wood the tubes act as pipes through which the water flows up the tree. In this part the wood is usually soft and light in colour, and is known as the "sapwood." Inside the bark lies the sapwood, and, as has been said, it is usually light in colour. This is particularly noticeable in almost all the eucalypts in the West Australian bush, jarrah and karri being notable examples. It varies in width from half an inch to several inches, according to the age of the tree and the species. Inside the sapwood and continuing to the centre of the tree is what is known as the "heartwood." The heartwood has practically no other function than that of mechanical support. On the other hand, the sapwood is a living part of the tree, serving largely to store up the starch and other food and to conduct the unelaborated sap from the roots up to the leaves. The extreme outer portion of the sapwood is the cambium already referred to.
On cutting across the trunk of a tree we may see the annual rings, but these rings are not equally visible in every tree. In some kinds of trees they stand out quite clearly and may be counted easily. In other kinds of trees they are very difficult to detect, and in some they are non-existent. In most of the eucalypt trees the rings are faint or non-existent. In jarrah, for instance, it is quite impossible to detect them.
In looking for the reason for this difference, we note that trees which come from a country having a severe winter usually show far more distinct rings than those which grow in countries which have a mild winter. Broad-leafed trees in cold countries usually shed their leaves in winter and consequently photosynthesis can-
