This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Even in winter, when the trees are leafless and comparatively at rest, the thin-walled cells of the cambium are small and filled with protoplasm; it really constitutes the only live portion of the tree at this period. It forms a thin, cylindrical jacket to the trunk of the tree, and gradually tapering cylinders to each branch and twig, till continuous with the small core in each live bud on the tree. The cambium descends to the roots in like manner. When the temperature rises in spring the cells are excited into rapid growth, and, with abundant supplies of stored food close to hand, they soon reach full size, divide, grow, and multiply rapidly. The cells on the inner side develop new fibro-vascular bundles, side by side, in a continuous ring all round last year's wood. Those on the outside of the cambium form new hard and soft bast. Thus the wood increases in thickness by the deposit of a new ring on the outside of its mass, while the bark thickens by the deposit of a new ring on the inside of last year's one.
The existence of the cambium explains the art of budding a Rose, grafting a scion or shoot of one Apple tree on to another, inarching a young Vine on the rod of an old one, and the grafting of shoots of a Clematis, Tree Paeony, and Wistaria on to the roots of another for the purpose of increasing their numbers. The object in each case is to get the only live portion of the scion of the tree, shrub, or climber into contact with the cambium of the stem and root, respectively, used as stocks. The cambium of the one coalesces or joins with that of the other, and forms a new layer of wood over the old. If the grafted portion of an Apple or other tree were examined after one hundred years, the old cut surfaces would still be present, for mature or ripened wood, being dead, never unites. The whole of the wood of a tree, after it is fully ripened, is dead, though it may exist for one thousand years or more, protected by the bark, and be of service to the tree.
Fig. 21. - Section of Dicotyledonous Stem, showing central pith, three zones of wood, and bark on the outside (diagrammatic).