8 PRINCIPLES OF AMERICAN FORESTRY. 



erence to their stem. The actual shape of trees depends 

 on the space they have to grow in, on the soil, situation, 

 and on the age of the trees. Where trees have plenty of 

 room to grow, and their natural development is not inter- 

 fered with, their individual characteristics are most ap- 

 parent. 



TREE-GROWTH. 



Assimilation. Plants are made up of various tissues and 

 these are formed of numerous cells. The material of which 

 the cells are composed is largely carbon. This carbon is 

 derived from the carbon dioxide of the air which enters into 

 the leaves and under the action of light, air, and water is 

 there decomposed ; the oxygen is given off and the carbon 

 is retained, and, combined with water obtained from the 

 roots, forms starch, sugar, gum. and other plant-foods. 



This process of food-making is called assimilation and can 

 be carried on only in the green parts of the plant, and in 

 these only when exposed to light and air. Hence, foliage, 

 air, and light at the top are essential prerequisites for tree- 

 growth, and, other conditions being favorable, the greater 

 quantity and better development of foliage and the more 

 light this foliage has at its disposal for its work the more 

 vigorously will the tree grow. 



In general, therefore, the growth of wood may be reduced 

 either by the removal of foliage, which reduces the working 

 surface, or by shading, which somewhat checks the activity 

 of the foliage by hindering light action. 



Transpiration. The flow of sap in trees is not well 

 understood. In a general way it may be said that the sap- 

 wood transmits the water from the roots to the leaves, 

 where a part enters into the assimilated sap and goes to 

 build up the plant, and the remainder, which is by far the 

 greater part, passes off as vapor. The amount thus trans- 

 pired varies greatly with the species, age of the tree, amount 



