principle holds true.

Requirements of trees: Trees are dependent upon certain soil and
atmospheric conditions which influence their growth and development.

(1) Influence of moisture: The form of the tree and its growth and
structure depend greatly upon the supply of moisture. Botanists
have taken the moisture factor as the basis of classification and
have subdivided trees into those that grow in moist places
(_hydrophytes_), those that grow in medium soils (_mesophytes_), and
those that grow in dry places (_xerophytes_). Water is taken up by
the roots of the tree from the soil. The liquid absorbed by the
roots carries in solution the mineral salts--the food of the
tree--and no food can be taken up unless it is in solution. Much of
the water is used by the tree and an enormous amount is given off in
the process of evaporation.

[Illustration: FIG. 89.--Dead Branches at the Top Caused by Insufficient
Water.]

These facts will explain some of the fundamental principles in the
care of trees. To a tree growing on a city street or on a lawn where
nature fails to supply the requisite amount of water, the latter
must be supplied artificially, especially during the hot summer
months, or else dead branches may result as seen in Fig. 89. Too
much thinning out of the crown causes excessive evaporation, and too
much cutting out in woodlands causes the soil to dry and the trees
to suffer for the want of moisture. This also explains why it is
essential, in wooded areas, to retain on the ground the fallen
leaves. In decomposing and mixing with the soil, the fallen leaves
not only supply the trees with food material, but also tend to
conserve moisture in the ground and to prevent the drying out of the
soil. Raking off the leaves from wooded areas, a practice common in
parks and on private estates--hurts the trees seriously. Some soils
may have plenty of moisture, but may also be so heavily saturated
with acids or salts that the tree cannot utilize the moisture, and
it suffers from drought just the same as if there had been no
moisture at all in the soil. Such soils are said to be
"physiologically dry" and need treatment.

In the development of disease, moisture is a contributing factor
and, therefore, in cavities or underneath bandages where there is
likely to be an accumulation of moisture,