58 EXOGENOUS STEM — BAEK. 



and the intercellular spaces which occur among them. This covering 

 is usually less developed than the outer suherous layer, but sometimes, as 

 in the Larch and common Fir, it becomes very thick, and separates like 

 the epiphloeum. In the cellular envelope latioiferous vessels occur. 



The Epiphloeum is the outer covering of the bark, consisting of 

 cells which usually assume a cubical or flattened tabular form (fig. 

 116 Us, p). The cells have no chlorophyll in their interior, are 

 placed close together, and are elongated in a horizontal direction ; and 

 thus they are distinguished from the cells of mesophloeum. In the 

 progress of growth they become often of a brown colour. This cover- 

 ing 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 Saber, the Cork-oak (fig. 118 y) ; hence 

 the name suierous, 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. After a certain period (sometimes eight or nine 

 years), the corky portion becomes inactive, and is thrown off in the form 

 of thickish plates, leaving a layer of tabular cells or periderm below. 

 On the exterior of the epiphloeum 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 

 dry, and being thrown off in the form of plates or shreds. 



The bark, in its increase, follows an order exactly the reverse of 

 that which occurs in the woody layers. Its three portions increase 

 by additions to their inside. The layers of liber owe their increase 

 to the cambium cells, which, by their constant reproduction, mark the 

 separation between the vascular bundles of the wood and the fibres 

 of the endophloeum. 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 conspicuous. 

 In the case of the cellularjportions of the bark there are also succes- 

 sive additions, sometimes to a great exent, but they do not usually 

 fehibit any marked divisions. 



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. 119) so as to form meshes (as in the 

 lace-bark), its epidermis is thrown off, and the epiphloeum is either de- 

 tached 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 

 the Elm and Cork-oak. In some instances the bark is very disten- 

 sible, 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. 



