THE TREE. 17 
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 interfered with, their individual character- 
istics 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 derived 
irom the carbon dioxide of the air which enters into the leaves 
and under the action of light, air and water is there decom- 
posed; 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 bet- 
ter 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 fohage by hindering hght action. 
Transpiration. The flow of sap in trees is not well under- 
stood. Ina general way it may be said that the sapwood trans- 
mits the water irom 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 frees 1,0co pounds of carbon, 15 or 20 pounds of 
mineral substances and 5.cco pounds of water in a year, it may 
5) 
a 
