of assimilation. Cutting off some of the light from a tree affects
its form. This is why trees grown in the open have wide-spreading
crowns with branches starting near the ground as in Fig. 90, while
the same species growing in the forest produces tall, lanky trees,
free from branches to but a few feet from the top as in Fig. 91.
Some trees can endure more shade than others, but all will grow in
full light. This explains why trees like the beech, hemlock, sugar
maple, spruce, holly and dogwood can grow in the shade, while the
poplar, birch and willow require light. It also explains why, in
the forest, the lower branches die and fall off--a process known in
Forestry as "natural pruning," The influence of light on the form of
trees should be well understood by all those who plant trees and by
those designing landscape effects.

[Illustration: FIG. 91.--A Tree in The Forest. Note the tall stem free
from branches and the small, narrow crown.]

(4) Influence of heat: Trees require a certain amount of heat. They
receive it partly from the sun and partly from the soil. Evaporation
prevents the overheating of the crown. The main stem of the tree is
heated by water from the soil; therefore trees in the open begin
growth in the spring earlier than trees in the forest because the
soil in the open is warmer. Shrubs begin their growth earlier than
trees because of the nearness of their crowns to their root systems.
This also explains why a warm rain will start vegetation quickly.
Too much heat will naturally cause excessive drying of the roots or
excessive evaporation from the leaves and therefore more water is
needed by the tree in summer than in winter.

(5) Influence of season and frost: The life processes of a tree are