Callusing and Repairing 



93 



place, of course, during the growing season only, and is 

 due to division and enlargement of cambium cells into wood, 

 cork, or bark cells. Since these cells preferably divide ver- 

 tically or lengthwise, and since the assimilated food materials 

 required in their growth are carried from the foliage down- 

 ward, the upper edges and the sides of the wound usually 

 close more rapidly than tlie 

 lower edges. 



For the same reasons, a 

 branch stub protruding from 

 the trunk or larger brancli 

 heals more slowl}-, for here 

 the cells must divide hori- 

 zontally or crosswise, which 

 they do with difficult}-; more- 

 over, the cells, being out of 

 the direct path between root 

 and foHage, have to deri\'e 

 their food materials circui- 

 touslv from a neishborincf 



Fig. 2T,. — Satisfactory growth of the 

 callus over a pruned branch. 



branch, and are apt to tind them less in quantity and less 

 readily available than if a direct supply from the foliage of 

 its own lost portion could have been had. Hence a vertical 

 wound, running up or down the trunk or branch, is much 

 less dangerous and more quickly covered than a much 

 smaller wound running around the bole or branch, and 

 similarly, the wound made by the loss of a branch at its 

 very base is more rapidly closed than when cut or broken 

 above the base and across the diameter. Branch stubs are, 

 therefore, apt to die back and to decay most readily, becau.se 

 longer exposed to the action of rot fungi without any ^■ital 

 process counteracting these fungi. 



In the case of small branchlels or twigs, wliich have been 



