106 POPULAB EBBOES. 



for some time after being stripped of its bark is 

 certainly conclusive evidence that sap passes up- 

 ward in the wood, while the no less familiar 

 example of the edges of a wound growing more 

 upon the upper than upon the lower side is regarded 

 as equally conclusive that there is a downward 

 current between the wood and bark. 



The truth is, that there is no downward current 

 anywhere at any time, and that the upward move- 

 ment of the sap takes pla'ce in the cambium and 

 living part of the bark as well as in the outer 

 layers of the wood. It is also true that, except 

 possibly for brief periods, there is never any well 

 defined upward current, the so-called sap vessels 

 being under ordinary circumstances filled with air. 

 In the season of active growth, when the upward 

 movement of sap is really most rapid, no free sap 

 capable of flowing is found in the plant. The 

 moisture in trees at such times all passes upward 

 through the living cells of the cambium, and in 

 and upon the walls of the cells and vessels of the 

 newer layers of the wood and bark. How, then, 

 since the only general movement is upward, 

 does the elaborated sap reach the lower portion of 

 the plant, even the extremities of the longest roots, 

 and perform its part in promoting growth ? 



In the first place, there is no "elaborated sap" 

 in the ordinary sense of the term as distinct from 



