PLANT PHYSIOLOGY AS RELATED TO PRUNING 



17 



In the first case the sap will continue to ascend for a 

 considerable time after the stem has been girdled ; in the 

 second the stem will increase considerably in girth, the 

 process requiring several weeks or months, dependent 

 upon the kind of plant, the size of the stem and other 

 factors. The following instances will prove interesting 

 at this oint. 

 e deb ez. 



FIG. 13— THREE SECTIONS OF OAK BRANCH SHOWING VARIOUS TISSUES 

 Upper surface, cross section; front face (from / to middle) loncirudinal section; 

 extreme left, tangential section showing ends of medullary rays; left center, cross 

 section showing medullary ray; a, cortex; ^, bast; c. cambiuin; d, spring-formed 

 wood; e, autumn-formed wood of previous year; /, lenticel. 



21. Teaacity of life in trees.— The fact that the course of the 

 crude sap in exogenous plants is upward through the sap-wood is 

 well illustrated by the tenacity of life in trees that have been de- 

 prived of their bark completely around their trunks to considerable 

 heights. For instance, trees are often girdled by mice and rabbits 

 to a height of several inches or even a foot, so that it is thought 

 necessary to resort to bridge or repair grafting (128) to help the 

 healing process. More remarkable, however, is the instance re- 

 corded by I. Simpson in the Gardeners' Chronicle — the peeling of 

 standing oaks to the height of 20 to 40 feet and the subsequent leaf- 

 ing out of these trees and the continuance of these leaves even to 

 the end of the season. 



