CH. VIII.l THE DISEASES OF PLANTS. 301 



not give the full amount of the parent tree s power, 

 because the exhaustion incident to amputation neces- 

 sarily diminishes the vigour, and every minute the 

 power in the branch is reduced, because it is so 

 much the nearer to death. 



Now we know that a much less pressure than any 

 of those above mentioned would be capable of burst- 

 ingf the delicate membranes of anv of their exterior 

 descending sap vessels ; and it is in such outer ducts 

 that the injuiy first occurs. ^Yhen one exterior ves- 

 sel is ruptured, that next beneath it, having the sup- 

 porting pressure removed, is enabled to follow the 

 same course at the same locality ; and in proportion 

 to the length of the time that the sap continues in 

 excess, is the depth to which the mischief extends, 

 and the quantity of sap extravasated. 



If the extravasation proceeds from this cause, there 

 is but one course of treatment to be pursued : sever 

 one of the main roots, to afford the tree immediate 

 relief, and reduce the staple of the soil, by remo\dng 

 some of it, and admixing less fertile earthy compo- 

 nents, as sand or chalk. This must be done gra- 

 dually, for the fibrous roots that are suited for the 

 collection of food from a fertile soil, are not at once 

 adapted for the introsusception of that from a less 

 abundant pasturage. 



Care must be taken not to apply the above reme- 

 dies, before it is clearly ascertained that the cause is 



