Fig. 20.—A piece of the cambium and cortical jacket of a young oak, at the end of the first year. It may be regarded as consisting of three parts, in addition to the cambium, Ca. Beginning from the outside, we have: 1. Cork cells, X, formed from the cork cambium, C.Ca: the cells developed on the inside of the latter, Cl, are termed collenchyma, and go to add to the cortex. 2. The cortex proper, consisting of parenchyma cells, pa, some of which contain crystals. 3. The inner or secondary cortex (termed phloem or bast), developed chiefly by the activity of the cambium, Ca: this phloem consists of hard bast fibers, hb, sieve tubes, S, and cells, c, and is added to internally by the cambium, Ca, each year. It is also traversed by medullary rays, Mr, which are continuations of those in the wood. The dotted line, ψ, in the cortical parenchyma indicates where the new cork cambium will be developed: when this is formed, all the tissues (e.g. pa, Cl) lying on the outside of the new cork will die, and constitute (together with the cork) the true bark.
It is possible to make two generalizations, which apply not only to the illustration (Fig. 20) here selected, but also to most of our timber trees. In the first place, the cortical jacket, taken as a whole, consists not of rigid lignified elements, such as the tracheids and fibers of the wood, but of thin-walled, soft, elastic elements of various kinds, which are easily compressed or displaced, and for the most part easily killed or injured—I say for the most part easily injured, because, as we shall see immediately, a reservation must be made in favor of the outermost tissue, or cork and bark proper, which is by no means so easily destroyed, and acts as a protection to the rest.
The second generalization is, that since the cambium adds new elements to the cortex on the inside of the latter, and since the cambium cylinder as a whole is traveling radially outward—i.e., further from the pith—each year, as follows from its mode of adding the new annual rings of wood on to the exterior of the older ones, it is clear that the cortical jacket as a whole must suffer distention from within, and tend to become too small for the enlarging cylinder of rigid wood and growing cambium combined. Indeed, it is not difficult to see that unless certain provisions are made for keeping up the continuity of the cortical tissues, they must give way under the pressure from within. As we shall see, such a catastrophe is in part prevented by a very peculiar and efficient process.
Before we can understand this, however, we must
take a glance at the structural characters of the whole of this jacket (Fig. 20). While the branch or stem is still young, it may be conveniently considered as consisting of three chief parts.
(1) On the outside is a thin layer of flat, tabular cork cells (Fig. 20, Co), which increase in number by the activity of certain layers of cells along a plane parallel to the surface of the stem or branch. These cells (C.Ca) behave very much like the proper cambium, only the cells divided off from them do not undergo the profound changes suffered by those which are to become elements of the wood and inner cortex. The cells formed on the outside of the line C.Ca in fact simply become cork cells; while those formed on the inside of the line C.Ca become living cells (Cl) very like those I am now going to describe.
(2) Inside this cork-forming layer is a mass of soft, thin-walled "juicy" cells, pa, which are all living, and most of which contain granules of chlorophyl, and thus give the green color to the young cortex—a color which becomes toned down to various shades of olive, gray, brown, etc., as the layers of cork increase with the age of the part. It is because the corky layers are becoming thicker that the twig passes from green to gray or brown as it grows older. Now, these green living cells of the cortex are very important for our purpose, because, since they contain much food material and soft juicy contents of just the kind to nourish a parasitic fungus, we shall find that, whenever they are exposed by injury, etc., they constitute an important place of weakness—nay, more, various fungi are adapted in most peculiar ways to get at them. Since these cells are for the most part living, and capable of dividing, also, we have to consider the part they play in increasing the extent of the cortex.
(3) The third of the partly natural, partly arbitrary portions into which we are dividing the cortical jacket is found between the green, succulent cells (pa) of the cortex proper (which we have just been considering) and the proper cambium, Ca, and it may be regarded as entirely formed directly from the cambium cells. These latter, developed in smaller numbers on the outside, toward the cortex, than on the inside, toward the wood, undergo somewhat similar changes in shape to those which go to add to the wood, but they show the important differences that their walls remain unlignified, and for the most part very thin and yielding, and retain their living contents. For the rest, we may neglect details and refer to the illustration for further particulars. The tissue in question is marked by S, c, hb in the figure, and is called phloem or bast.
A word or two as to the functions of the cortex, though the subject properly demands much longer discussion. It may be looked upon as especially the part through which the valuable substances formed in the leaves are passing in various directions to be used where they are wanted. When we reflect that these substances are the foods from which everything in the tree—new cambium, new roots, buds, flowers, and fruit, etc.—are to be constructed, it becomes clear that if any enemy settles in the cortex and robs it of these substances, it reduces not only the general powers of the tree, but also—and this is the point which especially interests us now—its timber-producing capacity. In the same way, anything which cuts or injures the continuity of the cortical layers results in diverting the nutritive substances into other channels. A very large class of phenomena can be explained if these points are understood, which would be mysterious, or at least obscure, otherwise.
Having now sketched the condition of this cortical jacket when the branch or stem is still young, it will be easy to see broadly what occurs as it thickens with age.