INTRODUCTION. 5 



form bast or bark. This ring or layer method of growth does 

 not apply with trees such as palms.* 



Moisture is the most important element that the tree draws 

 from the soil. Trees require comparatively little mineral 

 matter and this is in large part returned when they shed their 

 leaves in the autumn. Carbon is the principal food of trees 

 and carbon is derived from the atmosphere. 



Leaves perform their functions through the instrumentality 

 of chlorophyll, a substance upon which they depend for their 

 color. The health of a tree depends upon that of its foliage, 

 and the health of a tree can be told by the condition of its 

 foliage. 



Trees need sunlight, and are influenced by the way in which 

 they receive it. Sunlight reaches the sides of trees that stand 

 by themselves and these trees produce many branches. The 

 sides of trees that stand together in the forest are shaded and 

 the lower branches of such trees do not thrive. The foliage of 

 forest trees reaches upward toward the sunlight; their trunks 

 are thus lengthened and wood is deposited upon the trunks 

 rather than upon the branches. 



In this way it is possible to influence the shape of any tree. 

 A full-branched tree such as is prized in landscape effects or a 

 long straight trunk that is valued by lumbermen may be obtained 

 by the required direction of sunlight. Lower branches die and 

 fall away in forests, or they may be removed by pruning, and 

 in either case wood diverted from the branches will concentrate 

 upon the trunk. 



The energy of a young tree is largely expended upon the 

 development of its roots and foliage. The trunk grows very 

 slowly during this interval. It is particularly wasteful to cut 

 down young trees and saplings just as wood is about to be 

 diverted to the trunk, f 



Top-soil of forests is porous and loose. Roots break the 

 originally compacted ground and leaves and twigs falling from 



* See Exogens, p. 189. 



f In Europe branches removed by pruning are used as firewood. These 

 trimmings take the place of some better material that would otherwise be used 

 for fuel. Practices are much more wasteful in America. 



