CH. II, 



COURSE OF THE SAP. 



61 



a man knows that he has lost his way, he will at least 

 go carefully ; he will be On the look-out, and be the 

 more likely to find it. 



It is certain that s^reat chemical changes, or elabo- But 



^ elaboration 



ration of the sap, must take place in the root or stem ^akef place 

 befoi-e it reaches the leaf, since sap of very different sap^'reaches 

 quahties is drawn from the stems of different trees. 

 Witness the sugar from the maple and birch, the resin 

 from the fir, &c. &c., which are found in the heai^t- 

 wood ; also, the alteration of the heart-wood in 

 density, and the change of sap-wood into heart-wood, 

 argue elaboration in the stem, and deposit from the 

 upward sap. 



It is also certain that great chemical changes, or and also 



^ ^ ' after it 



elaboration of the sap, must take place in the bark, or ^^^^^ 

 elsewhere, after it has left the leaf; since wood of very 

 different qualities is deposited on the stem of the same 

 grafted tree. 



Of the fact which I have supposed probable, 

 namely, the lateral communication of the sap through- 

 out the wood to the bark, I will give proof immedi- 

 ately, while considering the office of the pith (or 

 medulla, ' marrow '), and its rays or silver grain 

 (medullary rays), which many have supposed to be the 

 means of this lateral communication between the pith 

 and the bark. But I will first state those theories, in Received 



theories of 



reference to the growth in girthing, which have been f^thing'' 

 most generally accepted. 



Grew said that the new layer of wood is formed 



