The Growth of the Tree 39 



By repeating this annual program, the tree continues 

 to grow for an indefinite period. Some species are natu- 

 rally shorter-lived than others. The gray birch, for example, 

 rarely lives more than thirty or forty years, while the big 

 trees of the Pacific Coast are some of them three or four 

 thousand years old. There does not appear to be any 

 definite limit to the age of trees, but when growth becomes 

 slow and their vitality low they become subject to the 

 attacks of insects and fungi that cause death and destroy 

 the wood after they die. 



The thrifty tree may be picked out by its general ap- 

 pearance and the shape of its crown. So long as growth 

 is rapid and healthy, the crown retains a conical shape. 

 This is true of both conifers and hardwoods. When the 

 tree has attained its height and the growth is less strong, 

 the crown broadens and flattens out. This flat-topped 

 appearance shows that the tree has practically finished 

 its height growth and is growing only in diameter and 

 this only to a slight extent. Extremely old trees some- 

 times lack enough food for growth throughout the cam- 

 bium and lay on increment only in the upper portions. 

 This condition is usually followed very shortly by death, 

 a death, however, that requires many years for its 

 fulfillment. 



