102 BOTANY [NUTRITIVE ORGAKS. phlceum with the mesophlooiuu underncntli. The cellular envelope, or mesophloeum, lies immediately on the outside of the liber. It consists of polyhedral, often prismatical cells, elongated vertically to the surface, usually having chlorophyll, or green colouring matter, in their interior, but sometimes being colourless, and containing raphides. They are dis tinguished from those of the epiphlceum by their form and direction, by their thicker walls, their green colour, and the intercellular spaces which occur among them. This cover ing is usually less developed than the outer suberous layer, but sometimes, as in the Larch and common Fir, it be comes very thick, and separates like the epiphlceum. In the cellular envelope laticiferous vessels occur. The epi phlceum is the outer covering of the bark, consisting of cells which usually assume a cubical or flattened tabular form. The cells have no chlorophyll in their interior, are placed close together, and are elongated in a hori zontal direction ; and thus they are distinguished from the cells of mesophloeum. In the progress of growth they become often of a brown colour. This covering may be composed of a single layer of tabular cells ; but in some trees it consists of numerous layers, forming the substance called cork, which is well seen in Quercus Suber, the Cork- oak ; hence the name suberous, or corky layer, which is given to it. The form of its cells varies in some instances, being cubical at one part, and more compressed or tabular at another, thus giving rise to the appearance of separate layers. On the exterior of the epiphlceum is situated the epidermis, which has already been described. It is formed of a layer of cells, which in woody stems serve only a temporary purpose, becoming ultimately transformed in various ways. The bark, in its increase, follows an order exactly the reverse of that which occurs in the woody layers. The layers of liber owe their increase to the cambium cells, which, by their constant reproduction, mark the separation between the xylem and phloem portions of the stem. These layers are often so compressed and united together as to be counted with difficulty, while at other times they are separated by rings of cellular tissue, and thus remain con spicuous. As the additions are made to the woody layers on the outside, and to the bark on the inside, there is a constant distension going on, by which the bark becomes compressed, its layers of liber are condensed, the fibres are often separated (fig. 76) so as to form meshes, its epider mis is thrown off, and the epiphlceum is either detached along with it, or, when thick, is ruptured in various ways, so as to give rise to the rugged appearance presented by such trees as Elm and Cork-oak. In some instances the bark is very distensible, and its outer cellular covering is not much developed, so that the surface remains smooth, as in the Beech. The outer suberous layer sometimes separates with the epidermis, in thin plates or scales ; in the Birch these have a white and silvery aspect. There is thus a continual destruction and separation of different portions of the bark. The cellular envelope and liber may remain while the epiphlceum separates, or they also may be gradually pushed off the parts which were at first internal becoming external. In the case of some Australian trees both the cellular and fibrous portions are detached in the form of thin flakes, and occasionally each annual layer of liber pushes off that which preceded it. The epidermis separates early, and no renewal of it takes place. It is, however, replaced by the cork layer, which then covers the outer part of the stem. To this covering the name 2^eriderm is given. From the mode in which the outer layers of bark separate, it follows that inscriptions made on them, and not extend ing to the wood, gradually fall off and disappear. A nail driven into these layers ultimately falls out. In consequence of the continued distension of an exogenous stem, it is found that woody twining plants cause injury, by interrupt ing the passage of the fluids. Tims a spiral groove may be formed on the surface of the stem by the compression exercised by a twining plant, such as Bush-rope (Bauhinia, fig. 77) or Honeysuckle. From what has been stated relative to the changes which take place in the bark, it will be understood that it is often difficult to count its annual layers, so as to estimate the age of the tree by means of them. This may, however, be done in some cases, as shown at fig. 75, where there are eight layers of bark e, and eight woody layers b. Medullary Rays or Plates. While the bark and pith become gradually separated by the intervention of vascular bundles, the con nection between them is kept up by means of processes called medullary rays (figs. 78 and 79). These form the silver grain in wood, so conspicuous in the maple; they communicate with the pith and the cellular envelope of the bark, and they consist of cellular tissue, which becomes compressed and flattened so as to assume a murifonn appearance. At first they occupy a large space (fig. 72, st); but as the vascular bundles increase they become more and more narrow, forming thin laminae or plates, which separate the woody bundles. On making a transverse or horizontal section of a woody stem, the medullary rays present the aspect of narrow lines running from the centre to the circumference (fig. 74); and on making a vertical section of a similar stem through one of the rays, the appearance represented in fig. 78 will be observed, where a medullary ray mr, composed of flattened murifonn cells, passes from the pith p to the cellular Medullai rays. Stem of an .Exo genous tree, 8urrounded by a woody clim ber calledBush- rope. . TJi m r FIG. 78. Vertical section of a young Dicotyledonous stem, parallel to the medul lary rays. FIG. 79. Vertical section of the same, tangential to the medullary rays. envelope ce, crossing the trachea of the medullary sheath t, the ligneous tissue I, the pitted vessels of the wood I, and the fibres of the liber cf. The laminae do not by any means preserve an uninterrupted course from the apex to the base of the tree. They are broken up by the interven tion of woody fibres, as seen in a vertical section of a woody stem (fig. 79), tangentially to the medullary rays mr, mr, mr, which arc separated by interlacing fibres II. The primary medullary rays extend completely from the pith to the bark; but in the secondary wood and secondary cortex new rays are formed which, therefore, extend only through a portion of the stem. These are secondary medul lary rays. All may increase by division of the merismatic cells of the cambium. Medullary rays are conspicuous in the Cork-Oak, Hazel, Beech, Ivy, Clematis, Vine. They
are not so well marked in the Lime, Chestnut, Birch, Yew.