ON VEGETABLE PHYSIOLOGY. 61 



the cellular tissue, and the cortical vessels. The 

 former, or cellular tissue, consists of a soft sub- 

 stance formed into cells like the honey-comb of 

 bees, and filled most frequently with a green, 

 though sometimes with a brown, red, or yellow 

 fluid; the whole being arranged with the most 

 perfect symmetry. This interesting part of the 

 bark is not only essential to the stem and branch- 

 es, but it forms also a most important portion of 

 the organism of the leaves, flowers, fruit and roots; 

 and is the instrument by which the returning sap 

 is elaborated and converted into nourishment, 

 and into the various secretions peculiar to the 

 plant. 



Below the cellular tissue, though often blended 

 with it, and immediately above the wood, are 

 situated the cortical vessels, by some naturalists, 

 from their resemblance to the leaf of a book, 

 denominated the liber, by others, the interior por- 

 tion of the true bark. They consist of a number 

 of layers, the outer stratum of which is coarse and 

 loose in its texture, forming a very irregular kind 

 of net work, while the inner layers are soft, smooth, 

 and flexible; both being combined with longitu- 

 dinal vessels which separate and approach each 

 other alternately. 



The cortical vessels, with the cellular tissue, 

 take a very important lead in the economy of 

 the plant; since the former are destined to re- 

 ceive the returning sap from the leaf, and to con- 



