148 Our Field and Forest Trees, 



death. No sap or moisture can get through it. 

 Any part of the stem or branch which is outside 

 the cork skin is quite cut off from its source of 

 supplies, and so it gradually starves, withers, and 

 dies. All the outer bark of a tree is such dead 

 substance. 



Inside the trunk, earth-water is creeping up to 

 the leaves, and sap is coming down to feed all 

 that part of the wood which is inside the cork 

 barrier. But no sap can get through It to the dry, 

 cracking outer bark. 



There is one particular set of cells In every 

 growing tree whose business it is to make cork. 

 Botanists call these cells the " cork cambium." It 

 gets actively to work in spring, when the leaves 

 are young. 



The trees of the wood differ greatly in the out- 

 put of their cork factories. 



The cork elm and the sweet gum make so much 

 cork that it forms curious ridges on the branches 

 and twigs. The beeches, on the contrary, make 

 but a thin sheet each spring, just enough to re-llne 

 last season's ragged robe of bark, and so keep 

 themselves well covered. And the trees differ not 

 only in the output of their cork factories but in 

 its situation. 



In seedling trees of all sorts, the cork cambium 

 lies very close to the outside of the young trunk. 

 The beech carries on its cork-making each spring 

 near the surface of trunk and bough, and so do 



