THE GROWING PLANT 



49 



-— B 



A-- 



FiG. 24. — Healing of 

 wound formed by 

 cutting off a branch 

 (A). 



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 place. 



The same process occurs when a branch 



is cut off near its union with the 



stem. If the wound is not too large, 



callus healing follows soon by new 



growth from the uninjured cambium 



and other adjacent cells (Fig. 24). 



The younger the uninjured tissues 



are, the more rapid is the healing. 



In planted cuttings, the uninjured 



cambium cells at the base form the 



callus by continued division (Fig. 25). 



Exposure of the bark to undue heat or cold may destroy 

 the living tissues including the cam- 

 bium, causing sunscald (185). 



In periods of very rapid growth, when 

 the cambium cells are unusually active, 

 large areas of bark, even extending clear 

 around the stem and as deep as the cam- 

 bium layer, may sometimes be removed 

 from trees without destroying their life, 

 provided the recently-formed wood layer 

 is not injured (70). In this case, the 

 outer cells of the thin layer of cambium 

 that remains on the surface of the wood 

 promptly change to bark cells, hence a 

 callus at base of new bark layer forms over the exposed 

 willow cutting. surface the same season. 

 Several successive crops of bark are sometimes re- 

 moved from the trunk of the cork oak {Quemis Suber), 



but in this case the cambium layer is usually not injured. 



E 



Fig. 25. 



The 



