156 A TEXTBOOK OF GENERAL BOTANY 

Healing of wounds. When a cut is made into the wood of a 
dicotyledon or a conifer, the cambium cells adjoining the wound 
area proliferate and produce what is known as a callus. This 
grows over the wound and covers it (Fig. 155). A phellogen 
layer is produced in the outer part of the callus, while a cam- 
bium, which is continuous with that in the stem, is formed in 
the interior. As the edges of the callus grow together over the 
wound the edges of the cambium also coalesce. ‘ 
Removal of a strip of bark and cambium all the way around 
the trunk of a tree is called girdling. If the strip removed is 











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Fig. 155. Stages in healing of wound caused by removal of branch 
sufficiently wide, the tree will be killed, because this operation. 
destroys the food-conducting tissues which supply the roots. 
Some plants will recover from girdling if the strip removed is 
not too wide, as the wound may heal by the production of a 
callus and the subsequent formation of new conducting tissues. 
When a tree is girdled, the part of the trunk above the wound 
grows much faster and produces a much larger callus than that 
below it (Fig. 154), as the part above the wound is abundantly 
supplied with food material, while the part below is not. 
Pruning. In trimming a tree, branches should be cut off even 
with the trunk, and no portion of a dead or amputated branch 
should be left if the wound is to heal properly. Stumps of 
branches are not in the direct line of the movement of food 
materials and usually die quickly if they do not send out new 
