The Water of Plants and Its Movements. 



57 



Exposure of the bark to undue heat or cold may de- 

 stroy the cambium, causing sunscdld (185). 



In periods of very rapid growth, when the cambium 

 cells are unusually active, large areas of bark, even ex- 

 tending clear around the stem and as 

 deep as the cam- 

 bium layer, may 

 sometimes be re- 

 moved from trees 

 without destroy- 

 ing their life, pro- 

 vided 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 new bark layer 

 forms over the exposed surface the same season. 



Several successive crops of bark are sometimes re- 

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

 case, the cambium layer is usually not injured. 



Section V. The Watee op Plants and Its 



Movements. 



73. Plants Contain Large Amounts of Water. We 



have seen that the cell-walls of living plants are con- 

 stantly saturated with water (62), and that the cells of 

 the growing parts are always more or less distended 

 with it. The proportion of water contained in living 



Fig, 



Healing 



wound formed by cutting 

 off a branch (A). 



Pig. 25. showing 

 callus at base of 

 willow cutting. 



■ Quercus suber. 



