The bark is constantly undergoing increase in thickness owing to the activity of the cambium. As the phloem tubes grow older their functions are given up, and their place is taken by the younger elements within. A considerable part of the older phloem and medullary rays becomes converted into stone cells, which presumably serve a protective function. Active latex vessels are distributed through a large part of the area occupied by stone cells. Ultimately these also lose their functions and dry up. Finally their remains are cut off by layers of cork, and the rubber which they contain is probably lost with the dry bark which rubs off from the surface of the tree.
When the outer part of the living bark is removed in the process of paring, the cambium and young bark remaining are stimulated in a healthy tree to still more active growth, and the tapped area is said to undergo renewal There is evidence to show that the effect of this stimulus is not confined to the area actually tapped, but that the bark of neighbouring untapped areas is also stimulated to more rapid growth, provided the rate of tapping is not excessive. The time required by the bark for renewing such a thickness as will permit of a repetition of the paring process, varies according to circumstances. Factors which affect the rate of renewal are the age of the tree, its relative size, strength and state of nutrition, and any external circumstances which modify the nutritive processes, such as climate, soil, elevation and rainfall. Renewal is also affected by the method of tapping adopted, and by the extent to which the tree has been tapped. For although moderate tapping stimulates renewal, excessive tapping hinders this process, and the more heavily a tree is tapped after a certain rate has been reached the longer will be the period which must elapse before tapping can be repeated. It follows that there must be a certain optimum rate of tapping for any given tree. Experience alone can determine this rate. A sound judgment upon this point is perhaps the most important item in the equipment of the successful rubber planter. Some knowledge of the experimental work which has been done by others, will, however, be found useful in setting out to acquire the necessary experience.