THE BARK. 127 



• 



it grows at all. The green layer does not increase at all after the 

 first year ; the opaque corky layer soon excludes it from the hght ; 

 and it gradually perishes, never to be renewed. The corky layer 

 commonly increases for a few years only, by the formation of new 

 tabular cells : occasionally it takes a remarkable development, form- 

 ing the substance called Gorlc, as in the Cork Oak. A similar growth 

 occurs on the bark of several 

 species of Elm, of our Liquid- 

 ambar or Sweet-Gum, &c., pro- 

 ducing thick corky plates on the 

 branches. In the White and Pa- 

 per Birch, thin layers, of a very 

 durable nature, are formed for 

 a great number of years ; each 

 layer of tabular and firmly cohe- 

 rent cells (Fig. 200, a) alternates 

 with a thinner stratum of delicate, 

 somewhat cubical and less compact cells (5), which break up into a 

 fine powder when disturbed, and allow the thin, paper-like plates to 

 exfoliate. 



227. The liber, or inner bark (215), continues to grow through- 

 out the life of the tree, by an annual addition from the cambium- 

 layer applied to its inner surface. Sometimes the growth is plainly 

 distinguishable into layers, corresponding with or more numerous 

 than the annual layers of the wood : often, there is scarcely any 

 trace of such layers to be discerned. In composition and appearance 

 the liber varies greatly in different plants,* especially in trees and 

 shrubs. That of Bass-wood or Linden, and of other plants with 

 a similar fibrous bark, may be taken as best representing the liber. 

 Here it consists of strata of very thick-walled cells alternating with 

 thin-walled cells. The thick-walled cells are bast-ceUs (55, Fig. 49, 

 53), are much elongated vertically, and form the fibrous portion of 



* The best account of the libcr that has yet been given is that by Mohl, 

 in the Botanische Zeitunij, Yol. 13, p. 873 (1855), of which a French translation is 

 published in the Annates des Sciences Naturelles, ser. 4, Vol. 5, p. 141, et seq. 

 (1856). 



FIG. 200. Transverse section of a minute portion of White Birch bark,- the corky layer 

 highly magnified : a, the firm, tabular cells, 6, delicate ttiiii-wallcd cells which separate the 

 papery plates. (After Link.) 



