56 



Principles of Plant Culture. 



near its union witli the stem. The wonncl, if not too 

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

 uninjured cambium cells (Pig. 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 " 



Fie 



Healing 



wound formed by cutting 



ofl" a branch (A). the 



rapid growth, when fig. 25. Showing 



callu.s at base of 



carubmm cells wiiiow cutting. 



are unusually active, laige areas of bark, even extending 

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

 may sometimes be reuKjved 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. 



Se\'eral successi\e crops of bark are sometimes removed 

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

 cambium layer is usually not injured. 



= Quereits suhcr. 



