THE GROWTH. 13 



actual shape of trees depends on the space they have to grow 

 irf, on the soil, situation, and on the age of the trees. Where 

 trees have plenty of room to grow, and their natural develop- 

 ment is not interfered with, their individual characteristics- 

 are most apparent. 



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 de- 

 rived from the carbon dioxide ( carbonic acid ) 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 under- 

 stood. In a general way it may be said that the sapwood 

 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 transpired varies greatly with the 

 species, age of the tree, amount of foliage at work, amount of 

 light at its disposal, climatic conditions and the condition of 

 tree growth. The amount of water transpired is so large in 

 comparison to the amount retained in the tree that while an 

 acre of forest may store in its trees 1000 pounds of carbon, 15 

 or 20 pounds of mineral substances and 5000 pounds of water 



