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 light ; 

 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 Gork^ 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 Wliite and Pa- 4 '"©s'st^ES^Ss^^®''*®^^^ 

 per Birch, thin layers, of a very . ^rr^ ^^S/-^^ ^^^^^^ 'i^rg^ 



durable nature, are formed for ^m'^^W^^^S^^ 

 a great number of years ; each ^^^".S^.^^';:^^^^^^ 



layer of tabular and firmly cohe- 



rent cells (I'lg. 200, cr) aUernates ^s^'SS'Sg.^ 



with a thinner stratum of delicate, ^°° 



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



fine powder when disturbed, mid 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 jjlainly 

 distinguishable into layers, corresponding with or more numerous 

 than the annual layers of the wood : often, there is scai'cely 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 Avith 

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

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



* The best account of tlie liber that has yet been given is that by Mohl, 

 in tlie Botanische Zeitiiwj, Vol. 13, p 873 (1855), of which a French translation is 

 published in the Annales des Sciences Naturelles, scr. 4, Vol. 5, p. 141, et seq. 

 (1856). 



riG 200. Transverse section of a minute portion of WTjite Birch bark, the corky layer 

 highly magnified : a, the firm, tabular cells, b, delicate thin-walled cells which separate the 

 papery plates. (After Link.) 



