VI.
If we turn our attention for a moment to the illustrations in the first article, it will be remembered that our typical log of timber was clothed in a sort of jacket termed the cortex, the outer parts of which constitute what is generally known as the bark. This cortical covering is separated from the wood proper by the cambium, and I pointed out that the cells produced by divisions on the outside of the cambium cylinder are employed to add to the cortex.
[2] Continued from Supplement, No. 644, page 10281.
Now this cortical jacket is a very complicated structure, since it not only consists of numerous elements, differing in different trees, but it also undergoes some very curious changes as the plant grows up into a tree. It is beyond the purpose of these articles to enter in detail into these anatomical matters, however; and I must refer the reader to special text books for them, simply contenting myself here with general truths which will serve to render clearer certain statements which are to follow.
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.
In the first place, it is clear that the continuous sheet of cork (Co) must first be extended, and finally ruptured, by the pressure exerted from within. It is true, this layer is very elastic and extensible, and impervious to water or nearly so—in fact, it is a thin layer or skin, with properties like those of a bottle cork—but even it must give way as the cylinder goes on expanding, and it cracks and peels off. This would expose the delicate tissues below, if it were not for the fact that another layer of cork has by this time begun to form below the one which is ruptured: a cork-forming layer arises along the line φ and busily produces another sheet of this protective tissue in a plane more or less exactly parallel with the one which is becoming cracked. This new cork-forming tissue behaves as before: the outer cells become cork, the inner ones add to the green succulent parenchyma cells (pa). As years go on, and this layer in its turn splits and peels, others are formed further inward, and if it is remembered that a layer of cork is particularly impervious to water and air, it is easy to understand that each successive sheet of cork cuts off all the tissues on its exterior from participation in the life processes of the plant: consequently we have a gradually increasing bark proper, formed of the accumulated cork layers and other dead tissues.
A great number of interesting points, important in their proper connections, must be passed over here. Some of these refer to the anatomy of the various "barks"—the word "bark" being commonly used in commerce to mean the whole of the cortical jacket—the places of origin of the cork layer, and the way in which the true bark peels off: those further interested here may compare the plane, the birch, the Scotch pine, and the elm, for instance, with the oak. Other facts have reference to the chemical and other substances found in the cells of the cortex, and which make "barks" of value commercially. I need only quote the alkaloids in cinchona, the fibers in the malveceæ, the tannin in the oaks, the coloring matter in Garcinia (gamboge), the gutta percha from Isonandra, the ethereal oil of cinnamon, as a few examples in this connection, since our immediate subject does not admit of a detailed treatment of these extremely interesting matters.
The above brief account may suffice to give a general idea of what the cortical jacket covering our timber is, and how it comes about that in the normal case the thickening of the cylinder is rendered possible without exposing the cambium and other delicate tissues: it may also serve to show why bark is so various in composition and other characters. But it is also clear that this jacket of coherent bark, bound together by the elastic sheets of cork, must exert considerable pressure as it reacts on the softer, living, succulent parts of the cortex, trapped as they are between the rigid wood
cylinder and the bark; and it is easy to convince ourselves that such is the case. By simply cutting a longitudinal slit through the cortex, down to near the cambium, but taking care not to injure the latter, the following results may be obtained. First, the bark gapes, the raw edges of the wound separating and exposing the tissues below; next in course of time the raw edges are seen to be healed over with cork—produced by the conversion of the outer cells into cork cells. As time passes, provided no external interference occurs, the now rounded and somewhat swollen cork-covered edges of the wound will be found closing up again; and sooner or later, depending chiefly on the extent of the wound and the vigor of the tree, the growing lips of the wound will come together and unite completely.
But examination will show that although such a slit wound is so easily healed over, it has had an effect on the wood. Supposing it has required three years to heal over, it will be found that the new annual rings of wood are a little thicker just below the slit; this is simply because the slit had released the pressure on the cambium. The converse has also been proved to be true—i.e., by increasing the pressure on the cambium by means of iron bands, the annual rings below the bands are thinner and denser than elsewhere.
But we have also seen that the cambium is not the only living tissue below the bark: the cortical parenchyma (pa) and the cells (c) of the inner cortex (technically the phloem) are all living and capable of growth and division, as was described above. The release from pressure affects them also; in fact, the "callus," or cushion of tissue which starts from the lips of the wound and closes it over, simply consists of the rapidly growing and dividing cells of this cortex, i.e., the release from pressure enables them to more than catch up the enlarging layer of cortex around the wound.
An elegant and simple instance of this accelerated growth of the cortex and cambium when released from the pressure of other tissues is exhibited in the healing over of the cut ends of a branch, a subject to be dealt with later on; and the whole practice of propagation by slips or cuttings, the renewal of the "bark" of cinchonas, and other economic processes, depend on these matters.
In anticipation of some points to be explained only if these phenomena are understood, I may simply remark here that, obviously, if some parasite attacks the growing lips of the "callus" as it is trying to cover up the wound, or if the cambium is injured below, the pathological disturbances thus introduced will modify the result: the importance of this will appear when we come to examine certain disturbances which depend upon the attacks of fungi which settle on these wounds before they are properly healed over. In concluding this brief sketch of a large subject, it may be noted that, generally speaking, what has been stated of branches, etc., is also true of roots; and it is easy to see how the nibbling or gnawing of small animals, the pecking of birds, abrasions, and numerous other things, are so many causes of such wounds in the forest.
(To be continued.)
SIBLEY COLLEGE LECTURES.—1887-88.
BY THE CORNELL UNIVERSITY NON-RESIDENT LECTURERS IN MECHANICAL ENGINEERING.
III.—The Evolution of the Modern Mill.[3]
By C. J. H. Woodbury, Boston, Mass.