When the bark of an Exogenous tree is examined, it will be found to consist of four parts or layers, which to a greater or less extent can be made out in every tree. These layers have been technically called the Epidermis, the Epiphlmton, the Mesophltrum, and the Endophlreum.
The Epidermis is but a continuation of that layer of condensed cellular tissue which is found on the external surface of every part or organ of the plant. It varies in thickness as well as compactness in almost every tree. It is frequently split up by tho growth of the layers which lie beneath it, and with the next layer is separated from the stem in large pieces, as is the case in the common Birch (Betula alba). Like the epidermis on the leaves, it possesses stomates, which in the case of plants, as the Caetacem, seem to possess the power of performing the functions of the same organs on the leaf. The epi dermis is variously coloured as well as affected by the colour of the layer immediately beneath it.
The Epiphlceurn is the outermost layer of bark ; it is composed of cellular tissue, and when cut through presents under the microscope a tabular appearance indicative of pressure above and below. This layer is very variable in its development. Sometimes it grows to a very remarkable extant, as is seen in the Cork Oak (Owes, Sober), in which the layer of the bark used for making corks, ac., is the endophheurn. On this aecount it has been called the Suberoua Layer. It occurs in other plants besides the Queers. Saber, and constitutes a very pretty variety of the common elm, in which, in consequence of the growth of the suberous layer, the stem becomes quite altered in character.
The lies immediately beneath the epiphkeum ; it con sists of a layer of polyhedral cellular theme, and in the cells of which it is composed the colouring matter of the bark is deposited. It is in these cells that chlorophyle is deposited in the stems of most young plants, which give to them their green and fresh appearance. Even when the epidermis has Resumed another colour the epiphlteum often appears quite green, as is the case in the common Elder (Sambecus nigra).
The mesoplikeum, with the opiphlicum above it, is often split by the growth of the endophkeum beneath, giving to trees the rough broken surface which they often present. The meaopliketun and epiphlteurn are occasionally thrown off from the same cause as the epi dermis. The under layers grow rapidly, and the union between them
and the upper layers being maintained by no organic matter, the latter is thrown oft This takes place in the Plane (Platanus), in which large masses of the epiphkeum are constantly flaking off. New cellular layers in this case are formed below, and it is supposed that this process ex plains the reason of the tolerance which the planes exhibit of a London atmosphere. In other trees the function of the bark is interfered with by the particles of carbon and perhaps gases affecting the function of the bark, whilst the plane constantly renewing the outer layers of its bark, is not liable to this interruption. Whether this explanation be the true one or not, it is certainly very remarkable, that of all trees the plane flourishes beet in the squares of the metropolis.
The EndophImum, or Liber, is the inner layer of the bark, and con slats of woody fibre as well as cells, that is, of vascular as well as cellular tissue. The vascular tissue grows here in the form of bundles, as it does in the wood itself. Its fibrous character is made manifest during the growth of many trees. In the vine it Is thrown off with the layers above it by the growth of the wood underneath. In the Lace-Bark Tree (Layette Lintearia), the growth of the wood beneath the bark causes such an arrangement of the fibres that they are separated from one another, but making junctions where they cross, they form a natural kind of net-work, which has been employed as a substitute for artificial net-work in the construction of ornamental clothing. The liter of the bark of plants is much less dense than the wood; hence, where pliable materials are required, it is often made use of, as in the construction of mats from the bark of a species of Tilia, and the use of the bark of various trees in different parts of the world as a substitute for cordage.
The bark is nourished in the mune manner as the wood of the tree, by sap carried into the stem from the roots below. The cellular and vascular tissue of which it is comported, as long as they live, are capable of producing new cells, by which it new and increasing growth is ever supplied.
(Schleiden, Priedplea of Scientific Botenty ; Lindley, Introduction to Botany ; Balfour, .Manual of Botany.)