What are known as the vegetative organs of a tree, as opposed to the organs specially concerned with reproduction, may be divided into leaves, stem and roots. The functions of the roots to take the last mentioned organs first are, firstly, to hold the tree firmly upright by anchoring it in the soil; and secondly, to absorb certain substances contained in the soil which are essential for the nourishment of the plant Among the most important of these substances, in addition to water, are various compounds of nitrogen, phosphorus and potash.
Before these substances can be utilised as food by the different parts of the plant, it is necessary for them to be altered and in most cases combined with the still more important substance carbon, which is only obtained through the leaves. One of the most important functions of the leaves therefore is to absorb carbon from the atmosphere in the form of carbonic acid gas. The functions of the leaves however are by no means confined to absorption. In addition we may compare the leaves to so many minute kitchens or chemical laboratories, wherein the different ingredients of the food of the tree are prepared and compounded into a form in which they can be utilized and readily digested by the cells that make up the roots, stem and other organs. The energy required for these transformations is derived from the rays of the sun; and in order that the necessary chemical changes may be properly carried out, it is necessary for the leaves to be spread out in a position where they are well exposed to air and sunshine.
We may next pass on to consider the functions of the stem or trunk of the tree. The first of these is to support the leaves in a position well exposed to light and air; the second is to provide a channel for conducting the necessary mineral substances from the roots to the leaves in a state of very dilute solution, and also for conducting the elaborated food supply downwards from the leaves to the roots. The liquid which thus circulates through the different organs of the tree is popularly known as the sap. A third function carried out by the trunk or stem of most plants is the storage of reserve food materials, which are accumulated in special cells, often in the form of starch.
As it is from the trunk of the tree that rubber is derived in the vast majority of cases, it is necessary to enter rather more fully into the structure and functions of this region. The trunk of a tree is well known to consist of two main portions the wood and the bark, including under the latter term the layers described by botanists as the cortex. The cortex may be roughly defined as the softer internal part of the bark which adjoins the wood. If the bark is stripped from the wood, separation takes place at an extremely soft and delicate layer of tissue known as the cambium. The cambium, as we shall see later, is the seat of growth in thickness for both wood and bark.
Channels for the conduction of sap occur both in the wood and in the bark, and two entirely different streams of sap are associated with these two regions. An upward current of sap occurs in the outer part of the wood through a series of minute vessels, in which the mineral substances absorbed by the roots are carried to the leaves in a state of very weak solution. The perfected food materials are carried down through definite channels, known as phloem tubes, in the inner part of the bark, by a downward stream of sap which is entirely independent of the upward stream. In species which produce latex there is also present in the inner bark a special system of minute vessels or tubes, which contain an emulsion of rubber and other substances. This system is closed, and is entirely separate from both the above-mentioned sets of channels, having nothing to do with either of them, although in position it is closely associated with the phloem tubes which carry the downward sap current The upward and downward streams of sap are found in all trees, but latex tubes occur in only a comparatively small number of species.
A most important organ of the tree is the cambium. It is in this layer of very delicate cells that the growth and formation of new tissues are continually going forward. On the inner side of the cambium fresh layers, Consisting of newly constructed vessels, fibres and cells, are constantly being added to the wood. On the outer side of the cambium, and consequently on the inner side of the bark, similar new additions are constantly being made to the latter. These additions to the bark provide for the increased strain on the capacity of the conducting vessels, consequent on the general growth of the tree; and replace the losses occasioned by ordinary wear and tear, or in the case of cultivated rubber trees by the tapping knife. In addition to the new channels for the descent of sap. these additions include in certain cases new vessels for the storage of latex. We shall see. however, that in the majority of latex-producing plants, the growth of the laticiferous system is independent of the cambium.