HOW TREES GROW 19 



For some years all the wood throughout the trunk of our 

 young tree helps to conduct sap up and down and because of 

 this function it is called sapwood. But gradually there forms in 

 the inner portion a cylinder of harder wood usually dark in 

 color. The walls of the cells of this wood have been hardened 

 and the living elements have died. This inner core is known as 

 heartwood and is darker because of organic materials in the 

 wood cell walls. No longer is it a living part of the tree. It no 

 longer serves as a passage for sap, and its use now is to give 

 strength and support to the trunk. Yet from man's standpoint, 

 this heartwood is the most valued part of the tree. For from 

 it comes the durable, strong lumber for which man has count- 

 less needs. Should the heartwood remain soft, as it does in the 

 willow, it rots easily and the tree may become hollow and 

 weakened, and its wood of little service. 



This formation of heartwood begins at varying ages of a 

 tree's life fifteen years in the oak and forty in the ash. But as 

 the tree grows older the inner rings of sapwood turn to heart- 

 wood and a new layer of sapwood is formed each year next to 

 the bark. So as the tree ages, its trunk grows thicker, and the 

 heartwood increases in thickness, but the thickness of the 

 sapwood remains about the same. 



In addition to the formation of wood each year, the tree 

 forms a layer of bark. This bark often becomes quite thick, 

 sometimes a foot or more through, as in the sequoias. Some- 

 times, as in the sycamore, it scales off as rapidly as it is formed 

 and remains like a thin envelope about the tree. Were it not 

 for this tendency of the bark to peel, we could tell a tree's age 

 by counting the bark layers just as readily as by counting the 

 annual layers of wood. 



