EXOGENOUS STEM — KAYS. 59 



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 epi- 

 phloeum 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. There is, however, an internal covering, 

 which is formed of various portions of the bark. To this covering 

 the name Periderm {''rigi, around, and bi^/ia, skin) has been given by 

 Mohl. 



From the mode ip which the outer layers of bark separate, it fol- 

 lows that inscriptions made on them, and not extending to the wood, 

 gradually fall off and disappear. A nail driven into these layers ulti- 

 mately 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. Thiis a spiral groove may 

 be formed on the surface of the stem by the compression exercised by 

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

 relative to the changes which take place in the bark, it will be under- 

 stood that it is often dilEcult to count its annual, layers, so as to esti- 

 mate the age of the tree by means of them. This may, however, be 

 done in some cases, as shown at fig. 117, where there are eight layers 

 of bark, e, corresponding to eight woody layers, h. 



Medullary Kays oe Plates. — While the bark and pith 

 become gradually separated by the intervention of vascular bundles, 

 the connection between them is kept up by means of processes called 

 medullary rays (figs. Ill, 112 r). These form the silver grain of 

 carpenters ; - they communicate with the pith and the cellular envelope 

 of the bark, and they consist of cellular tissue, which becomes com- 

 pressed and flattened so as to assume a muriform appearance (fig. 

 120 TO r). At first they occupy a large space (fig. Ill r); but as 

 the vascular bundles increase they become more and more narrow, 

 forming thin laminae or plates, which separate the woody layers. On 

 making a transverse or horizontal section of a woody stem, the medul- 

 lary rays present the aspect of narrow lines running from the centre 

 to the circumference (figs. 117, 118 r m); and in making a vertical 

 section of a similar stem through one of the rays, the appearance 

 represented in fig. 120 will be observed, where a meduUary ray, m r, 

 composed of flattened muriform cells, passes from the pith, p, to the 

 cellular envelope, e e, crossing the tracheae of the medullary sheath, t, 

 the ligneous tissue, I, the pitted vessels of the wood, h, and the fibres 

 of the liber, c /. The laminae do not by any means preserve an unin- 

 terrupted course from the apex to the base of the tree. They are 



