242 PHYSIOLOGY OF TREES. 



It would be right, perhaps, to preface the 

 following observations by giving a view of all the 

 different opinions promulgated by botanical phy- 

 siologists, from Grew down to Poiteau : but as 

 this would embrace a great bulk of obsolete 

 matter, it is as well omitted. I shall therefore 

 proceed to describe, as plainly as I can, the dif- 

 ferent parts and organization of a tree, availing 

 myself of every discovery of science which is 

 agreeable to and has been confirmed by practice, 

 and of every result of practical agency which is 

 sanctioned by the principles of science. 



Scon after the first appearance of the jjlwnula^ 

 or rising shoot from the seed, we find, on making 

 a transverse section of the stem, that it consists of 

 a central pith enclosed in a cylinder of fibrous 

 matter, perpendicularly arranged, this having on 

 its exterior a thin separate film of bark; the 

 whole being saturated with sap peculiar to the 

 plant. 



Of the pith. — The pith, or medulla, is so called, 

 because it resembles the marrow of a bone. It 

 is only a temporary part of the stem ; compara- 



