Trees derive their food and solid substance in part from the air and
in part from the soil. The solid part of their bodies is made up
of cellulose, which consists largely of carbon (44 per cent of its
weight), with hydrogen and oxygen added in almost the same proportions
as in water. The carbon is derived from the carbonic acid of the air,
which enters into the leaves and, under the influence of light, air,
and water, is there decomposed; the oxygen is exhaled; the carbon is
retained and combined with elements derived from the water, forming
compounds, such as starch, sugar, etc., which are used as food
materials, passing down the tree through its outer layers to the very
tips of the roots, making new wood all along the branches, trunk, and
roots.
This process of food preparation, called "assimilation," can be
carried on only in the green parts, 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 hence, other conditions being
favorable, the more foliage and the better developed it is, and the
more light this foliage has at its disposal for its work, the more
vigorously will the tree grow.
In general, therefore, pruning, since it reduces the amount of foliage,
reduces also, for the time, the amount of wood formed; and just so
shading, reducing the activity of foliage, reduces the growth of wood.