OEGAXS OF XUTRITIOX. 



79 



■grain of cabinet-makers and carpenters, as it is to their presence 

 that many woods, such as the Plane and Sycamore, owe their 

 pecxiliar lustre. 



Fig. 166. Fig. 167. 



Fig. 166. Surface of the stem 

 of a Dicotyledonous tree 

 from which the barli has 



been removed. Fig. 16". 



Vertical section of a branch 

 of tlie common Maple, per- 

 pendicular to the medullary 

 rays. fl. Fibro- vascular 

 bundles. rm. Medullary 

 rays. 



4. The Bark or Cortical System. — The bark is situated on the 

 outside of the stem, surrounding the wood, to which it is orga- 

 nically connected by means of the medullary rays and cambium- 

 layer. When the stem is first formed the bark is composed, like 

 the pith, of parenchyma, but as soon as the wood begins to be 

 developed on the outside of the pith, a layer of liber-cells is also 

 deposited on the inside of the parenchyma of the bark, so that 

 the latter, when fully formed, consists of two distinct systems ; 

 namely, an internal or fibro-vascular, and an external or jparen- 



chymatous. Further the 

 parenchymatous system 

 exhibits also in all plants 

 which are destined to live 

 for any period, a separa- 

 tion into two portions ; 

 and the whole is covered 

 externally by the epider- 

 mis already described. 

 The fully developed bark 

 accordingly presents three 

 distinct layers, in addition 

 to the epidermis, which is 

 common to it and the 

 other external parts of 

 the plants. The three 

 layers proper to the bark 

 are called, proceeding 

 from within outwards, 1. 

 Liber, Inner Bark, or 

 Endopkloeion (Jigs. 168, c?, 

 and 163, /) ; 2. Cellular 



Fig. 168. 



Fig. 168. Transverse section of a portion of the 

 bark of a Dicotyledonous stem. a. Epidermis. 

 b. Epiphlceum. c. Mesophloeum, d. Endo- 

 phloeum. 



