PHYSIOLOGY OF TREES. ' 259 



the scion and stock '*. A wound through the 

 bark heals by protrusions of new wood and 

 bark downwards faster than by new processes 

 upwards ; and, in the same case, the sap distils, 

 it is said, from above more copiously than from 

 the lower lip of the wound. When a branch or 

 young shoot is bound by a ligature, it swells 

 more above than below the band. All these cir- 

 cumstances shew clearly that there is a down- 

 ward motion of some component of the plant, 

 otherwise the above effects could not take place. 

 These effects, however obscure the cause, cannot 

 be accounted for by attributing them to the 

 descent of the sap. 



The next hypothesis to account for the annual 

 enlargement of the stems of trees is one which 

 admits the change of sap from a crude to a 

 mature state, and also its ascent, descent, and 

 lateral transfusion in the body of the plant ; but, 

 instead of attributing the formation of the new 



* If the matured sap be impelled upwards in the spring, as 

 part of it is said to be, along with that received from the roots, 

 why are not grafts tainted with its organisable properties ? 



s2 



