LIGHT IN RELATION TO TREE GROWTH. 
INTRODUCTION. 
Light is indispensable for the life and growth of trees. In com- 
mon with other green plants a tree, in order to live, must produce 
organic substance for the building of new tissues. Certain low forms 
of vegetable life, such as bacteria and fungi, do not require light. 
They exist by absorbing organic substance from other living bodies; 
but the higher forms of plants manufacture their own organic mate- 
rial by extracting carbon from the air. The leaves, through the 
agency of their chlorophyll, or green coloring matter, absorb from 
the air carbon dioxide, and give off a nearly equal volume of oxygen. 
The carbon dioxide is then broken up into its elements and converted 
into organic substances which are used in building up new tissues. 
Light is not only indispensable for photosynthesis, but it is essential 
for the formation of chlorophyll itself. Only in exceptional cases, 
as in the embryo of fir, pine, and cedar seeds, does chlorophyll form 
in the dark, and, with the exception of some microbes, the green cell 
is the only place where organic material is built up from inorganic 
substances. 
Light also influences transpiration, and consequently the metab- 
olism of green plants. It influences largely the structure, the form, 
and the color of the leaf, and the form of the stem and the crown 
of the tree. In the forest it largely determines the height growth of 
trees, the rate at which stands thin out with age, the progress of 
natural pruning, the character of the living ground cover, the vigor 
- of young tree growth, the existence of several-storied forests, and 
many other phenomena upon which the management of forests de- 
pends. A thorough understanding, therefore, of the effect of hght 
upon the hfe of individual trees, and especially on trees in the forest, 
and a knowledge of the methods by which the extent of this effect 
can be determined are essential for successful cultural operations in 
the forest. 
The aim of this bulletin is to bring together the principal facts 
with regard to the part which light plays in the life of the forest, 
and the different methods of measuring it. It should prove of ma- 
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