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 the 
lower edges. 
For the same reasons, a 
branch stub protruding from 
the trunk or larger branch 
heals more slowly, for here 
the cells must divide hori- 
zontally or crosswise, which 
they do with difficulty; more- 
over, the cells, being out of 
the direct path between root 
and foliage, have to derive 
their food materials circui- Fic. 23.— Satisfactory growth of the 
tously from | 2 neighboring callus over a pruned branch. 
branch, and are apt to find 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, because 
longer.exposed to the action of rot fungi without any vital 
process counteracting these fungi. 
In the case of small branchlets or twigs, which have been 
fy 
a\ 
