EXOGENOUS STEM. BARK. 45 



a layer of tabular cells or periderm below. On the exterior of the epi- 

 phloeum is situated the epidermis, which has already been described 

 (f 47). 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. 



86. 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 separ- 

 ation 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 tunes they are separated by 

 rings of cellular tissue, and thus remain conspicuous. In the case of the 

 cellular portions of the bark, there are also successive additions, sometimes 

 to a great extent, but they do not usually exhibit any marked divisions. 



87. 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 con- 

 densed, the fibres are often separated so as to form meshes (as in the 

 lace-bark), its epidermis 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 the 

 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 epiphlreum 

 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 throws off 

 that which preceded it. The epidermis separates early, and no renewal 

 of it takes place. There is, however, an internal covering, which is 

 formed of various portions of the bark. To this covering the name 

 Periderm (ve^l, around, and Sc^a, skin) has been given by Mohl. 



88. From the mode in which the outer layers of bark separate, it 

 follows that inscriptions made on them, and not extending 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 

 interrupting the passage of their fluids. A spiral groove may thus be 

 formed on the surface of the stem, by the compression exercised by a 

 twining plant, such as honeysuckle. From what has been stated rela- 



