The root:  The root develops in much the same manner as the crown.  Its
    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
    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