HOW TREES GROW 25 



bium cells of the trunk and yearling branches. 

 They can lengthen as well as thicken. So the 

 vigorous shoot goes forward by leaps and 

 bounds until warned by Nature to prepare for 

 winter's cold. 



The heart wood, or pith, of a large tree has 

 lost all capacity for growth. Its avenues for 

 water are closed, and it is of little use to the 

 tree beyond the support given by its strength 

 and staunchness. Thus trees sometimes live 

 and flourish for years after their trunks are 

 mere hollow shells. But what happens if the 

 cambium is destroyed? The return current, 

 the food supply, cannot descend from the 

 leaves and the roots starve. The leaves die 

 and fall, for they are dependent on the roots, 

 and so the tree perishes. "Girdling," or peel- 

 ing a circle of bark and cambium clear around 

 the tree, usually means sure death to the oaks, 

 maples, and other broad-leaved trees. If by 

 chance any such survives, its trunk thickens 

 above the wound, because the food supply gath- 

 ers here in an over-abundance, while waiting 

 for the chasm to be bridged. Some trees 

 "bleed" to death if a place of any size is 

 bruised. On the other hand, the evergreen, be- 

 cause of the gummy, resinous quality of its sap, 

 can withstand severe marring and girdling. 



