56 



Principles of Plant Culture. 



near its union with the stem. The wound, if not too 

 large, is "healed" by new growth from the adjacent, 

 uninjured cambium cells (Fig. 24). In planted cuttings, 



the uninjured cam- 



bium cells at the 



base form the callus 



(cal'-lus) by con- 



tinued division. 



(Fig. 25). 

 Exposure of the 



bark to undue heat 



or cold may destroy 



the cambium, caus- 



ing sunscald (186). 

 In periods of very 



off a branch (A). the CaniblUUl Cells willow cutting. 



are unusually active, large areas of bark, even extending 

 clear around the stem and as deep as the cambium layer, 

 may sometimes be removed from trees without destroy- 

 ing their life, provided the recently-formed wood layer is 

 not injured (71). 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 new bark 

 layer forms over the exposed surface the same season. 



Several successive crops of bark are sometimes removed 

 from the trunk of the cork oak,* but in this case, the 

 cambium layer is usually not injured. 



* Quercus suber. 



