OF PRUNING TREES. 103 



by its own separate line of tubes ; or, which is the 

 same thing, each particular root of a tree has to 

 dpw nourishment from the soil to supply its own 

 particular branch ; and the communication between 

 those two points is maintained by a particular set 

 of vessels in the trunk of the tree. The watery 

 part of the sap, when it ascends into the leaves, is 

 for the most part given off by them in the form of 

 perspiration, and the sap which remains at this 

 point undergoes a change previous to its descent in 

 the form of proper woody matter, which change is 

 effected by the leaves inhaling carbonic acid and 

 other gases, which enter into the composition of 

 the returning sap ; and in this manner there is a 

 continual circulation of the sap in the tree — the 

 roots drawing in and supplying the whole with 

 moisture ; wliich, when it is raised to the leaves, un- 

 dergoes a chemical change, and is returned in the 

 form of proper woody matter. And now the prac- 

 tical deduction to be drawn from this is, that every 

 branch growing out of the main body of a tree, is 

 by nature meant to act as a laboratory, in which 

 woody matter is prepared and returned for the joint 

 supply of itself and the body of the tree ; and from 

 this we are bound to conclude, that when we cut 

 a branch from a tree, we take away from it the 

 means of supplying it with a certain proportion of 

 woody matter for its enlargement ; and this is, in- 

 deed, the case with pruning in all cases of the opera- 



