38 



they bleed only from these rings of pith ; 

 and I thi7ik I have remarked this in birch 

 trees. These rings of pith are probably 

 the main if not the sole conduits for the 

 upward sap, which from them and the 

 medullary rays exudes sideways, and^ 

 through the medium of the intercellular 

 passages, is possibly elaborated in the 

 vessels, tracheae, and cellular tissue ; but 

 probably all external paii:s of plants are 

 capable of transpiration or absorption, 

 according to circumstances, as well as of 

 inspiration or expiration. Happily, how- 

 ever, in a sketch of this sort, we are not 

 called on to unravel the mass of contra- 

 dictory theories 2i\\A facts of the microscopic 

 observers of the vessels, cellular tissue, 

 stomata, &c., of the woody layers, leaves, 

 cortical layers, and epidermis. This mass 

 of contradiction has been accumulating 

 and entangling itself from the time of 

 the admirable Grew and Henshaw, who 

 originated this species of anatomical re- 



