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
    checked when the temperature sinks below a certain point.  The tree
    is thus, during the winter, in a period of rest and only a few
    chemical changes take place which lead up to the starting of
    vegetation.  In eastern United States, growth starts in April and
    ceases during the latter part of August or in early September.  The
    different parts of a tree may freeze solid during the winter without
    injury, provided the tree is a native one.  Exotic trees may suffer
    greatly from extreme cold.  This is one of the main reasons why it is
    always advisable to plant native trees rather than those that are
    imported and have not yet been acclimatized.  Frosts during
    mid-winter are not quite as injurious as early and late frosts and,
    therefore, if one is going to protect plants from the winter’s cold,
    it is well to apply the covering early enough and to keep it on
    late enough to overcome this difficulty.

The mechanical injuries from frost are also important.  Snow and sleet will weigh down branches but rarely break them, while frost will cause them to become brittle and to break easily.  Those who climb and prune trees should be especially cautious on frosty days.

  (6) Influence of air:  On the under side of leaves and on other
    surfaces of a tree little pores known as stomata may be found.  In
    the bark of birch and cherry