CH. Vl] BARK. 81 



cylinder, and since the cambium-ring is formed in the 

 vascular cylinder, the cortex is obviously outside the 

 cambium, and therefore all secondary tissue formed by 

 the cambium towards the outside must at first be covered 

 by the primary cortex. 



Secondly, the growth of the wood has an influence on 

 the bark. If the bark were to cease to grow while the 

 cambium continued to make new layers of wood, it is 

 obvious that the bark would be too small for the branch 

 and would burst by pressure from inside. Although this 

 is an imaginary state of things, it is worthy of note, 

 because in spite of the fact that the bark does grow, it is 

 nevertheless stretched by the growing wood, and this 

 helps to produce a distortion and compression of the 

 elements which are characteristic of the bark. 



Thirdly, the structure of the bark depends partly on 

 the growth of certain tissues which have no connection 

 with the cambium, but which originate in a meristematic 

 layer in the primary cortex. It will be convenient to 

 describe this tissue the corky layer before dealing with 

 the secondary phloem. 



In a young oak twig the epidermis is seen as a 

 limiting membrane, a pavement of a single layer of cells. 

 The outer wall of each cell c, fig. 38 (p. 83), has (as is 

 usual in epidermic cells) a special character. It is not 

 only thicker than the other walls but strikingly different 

 in its chemical nature; it is no longer pure cellulose, 

 but is cuticularised. 



The layer forming the cuticularised outer walls of 

 the epidermic cells is known, as cuticle. It resembles 



D. E. B. 6 



