THE EARTH'S BOUNTY 



between it and the sapwood is very slight. 

 Since water from the roots rises only in the 

 sapwood, it is easy to kill trees with heartwood 

 by girdling them, provided all the sapwood is 

 cut through. But in those which have no 

 heartwood the tubes of the older layers of 

 wood can still convey water to the crown, and 

 when such trees are girdled it is often several 

 years before they die." 



In a young forest, the rapid growth of tol- 

 erant species not only retards and overtops 

 their intolerant cotemporaries, but in time, as 

 their own branches spread and come in close 

 proximity, they lash and slash each other every 

 time a breeze sways the trees. Thus the young 

 shoots at the extremities are broken off, which 

 checks growth in that direction, and stimulates 

 it in the trunk and upper branches. Over- 

 shadowed by the increasing topgrowth, which 

 robs them of the power to assimilate food, the 

 lower branches gradually die, and are eventu- 

 ally severed from the trunk, either by a storm 

 or by their own weight. Then the annual de- 



310 



