depth and spread will vary with the species but will also depend
somewhat upon the condition of the soil around it. A deep or a dry
soil will tend to develop a deep root, while a shallow or moist soil
will produce a shallow root, Fig. 88.

The numerous fine hairs which cover the roots serve the purpose of
taking up food and water from the soil, while the heavy roots help
to support the tree. The root-hairs are extremely tender, are easily
dried out when exposed to the sun and wind, and are apt to become
overheated when permitted to remain tightly packed for any length of
time. These considerations are of practical importance in the
planting of trees and in the application of fertilizers. It is these
fine rootlets far away from the trunk of the tree that have to be
fed, and all fertilizers must, therefore, be applied at points some
distance from the trunk and not close to it, where merely the large,
supporting roots are located. In the cultivation of trees the same
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