EXOGENOUS STKUCTUEE. 



121 



216. The Cellular Envelope, or Green Layer (Fig. 191, 51), also called, 

 from its intermediate position, the Mesophlceum. This is com- 

 posed of loose parenchyma, with thin walls, much like the green 

 pulp of leaves, and containing an equal abundance of chlorophyll. 

 It is the only part of the bark that retains a green color. In woody 

 stems this is soon covered with 



217. The Corky Envelope, or Epiphlcettm (Fig. 191, I), which 

 gives to the twigs of trees and shrubs the hue peculiar to each spe- 



' cies, generally some shade of ash-color or brown, or occasionally of 

 much more vivid tints. It is this tissue, which, taking an unusual 

 development, forms the cork of the Cork-Oak, and those corky ex- 

 pansions of the bark which are so conspicuous on the branches of 



"vr 



-\r 





e ce ge b I 



Ik 



m 



m 



FIG. 194. Portion of a transverse section, and 195, a corresponding vertical section, magni- 

 fied, reaching from, the pith, Py to the epidermis, e, of a stem of Negundo, a year old : E, the 

 bark ; TT, the wood ; and C, the cambium-layer, as found in February. The references are in 

 the text above ; except jnr, portion of a medullary ray, seen on the vertical section, where it 

 runs into the pith ; dd, dotted ducts : cZ, the inner part of the cambium-layer, which begins 

 the new layer of wood. In this tree, we find a thick layer of parenchyma (/) inside of the baat- 

 tissue, and therefore belonging to the liber. No bast-tissue is formed in it the second year. 



11 



