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. [Illustration: 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, C_a_. Beginning from the outside, we have: 1. Cork cells, X, formed from the cork cambium, C.C_a_: the cells developed on the inside of the latter, C_l_, 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, C_a_: this phloem consists of hard bast fibers, _hb_, sieve tubes, S, and cells, _c_, and is added to internally by the cambium, C_a_, each year. It is also traversed by medullary rays, M_r_, which are continuations of those in the wood. The dotted line, [psi], in the cortical parenchyma indicates where the new cork cambium will be developed: when this is formed, all the tissues (e.g. _pa_, C_l_) 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.