54 Principles of Plant Culture. 
the wound may, however, by division (15), form a cushion of 
new material that gradually extends itself over the injured 
part. A new cambium layer may thus be formed over the 
wound if it be not too large, so that growth of the stem 
may be resumed at this part. The same process occurs 
when a branch is cut off near its union with the stem. The 
wound, if not too large, is “‘ healed” by new growth from 
the adjacent, uninjured cambium cells. (Fig. 22). In 
Fic. 22. Healing of wound ~ Fic. 23. Showing callus 
formed by cutting off a branch at base of willow cutting. 
(A). 
planted cuttings, the uninjured cambium cells at the base, 
by continued division, form the callus (cal’-lus). (Fig. 23.) 
Exposure of the bark to undue heat or cold may destroy 
the cambium, causing sunscald (186). 
