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, decay will do more damage than in places that are dry and exposed to the sun.

  (2) Influence of soil:  Soil is made up of fine particles of sand and
    rock and of vegetable matter called humus.  A tree will require a
    certain soil, and unsuitable soils can be very often modified to
    suit the needs of the tree.  A deep, moderately loose, sandy loam,
    however, which is sufficiently aerated and well supplied with
    water, will support almost any tree.  Too much of any one constituent
    will make a soil unfit for the production of trees.  If too much clay
    is present the soil becomes “stiff.”  If too much vegetable matter is
    present, the soil becomes “sour.”  The physical character of the soil
    is also important.  By physical character is meant the porosity which
    results from breaking up the soil.  This is accomplished by ploughing
    or cultivation.  In nature, worms help to do this for the soil, but
    on streets an occasional digging up of the soil about the base of
    the tree is essential.

Humus or the organic matter in the soil is composed of litter, leaves and animal ingredients that have decayed under the influence of bacteria.  The more vegetable matter in the humus, the darker the soil; and therefore a good soil such as one finds on the upper surface of a well-tilled farm has quite a dark color.  When, however, a soil contains an unusual quantity of humus, it is known as “muck,” and when there is still more humus present we find peat.  Neither of these two soils is suitable for proper tree growth.

[Illustration:  FIG. 90.—­A Tree in the Open.  Note the full development of the wide crown with branches starting near the ground.  The tree is the European larch.]

  (3) Influence of light:  Light is required by the leaves in the process
    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