[ 53 3 



The innermost part of the bark is constituted by 

 the cortical layers, and their numbers vary with the age 

 of the tree. On cutting the bark of a tree of several 

 years standing, the productions of different periods 

 may be distinctly seen, though the layer of every 

 particular year can seldom be accurately defined. 



The cortical layers are composed of fibrous parts 

 which appear interwoven, and which are transverse 

 and longitudinal. The transverse are membranous 

 and porous, and the longitudinal are generally com- 

 posed of tubes. 



The functions of the parenchymatous and cortical 

 parts of the bark are of great importance. The tubes 

 of the fibrous parts appear to be the organs that re- 

 ceive the sap ; the cells seem destined for the elabora- 

 tion of its parts, and for the exposure of them to the 

 action of the atmosphere, and the new matter is annu- 

 ally produced in the spring, immediately on the inner 

 surface of the cortical layer of the last year. 



It has been shewn by the experiments of Mr. 

 Knight, and those made by other physiologists, that 

 the sap descending through the bark after being 

 modified in the leaves, is the principal cause of the 

 growth of the tree ; thus, if the bark is wounded, the 

 principal formation of new bark is on the upper edge 

 of the wound ; and when the wood has been removed, 

 the formation of new wood takes place immediately 

 beneath the bark : yet it would appear from the late 

 observations of M. Palisot de Beauvois, that the sap 

 may be transferred to the bark, so as to exert its nutri- 

 tive functions, independent of any general system of 



