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36
Sanitary Value of Trees.
A good-sized tree— for instance, the plane, elm, or
lime, has been estimated to produce 7,000,000 leaves,
having a surface of 200,000 square feet, or five acres
of surface. The influence of such an absorbtion and
exhalation going on in the process of vegetable digestion
and nutrition should not be lost sight of as a
sanitary agency for large cities.
In summer, animal and vegetable putrefaction impregnates
the atmosphere with deleterious compounds.
Animal life is only kept active and healthy by getting
rid of its excretions and secretions. Carbonic acid, a
deadly poison given off by the lungs, shows its effects
if retained for a few minutes. This poison, together
with the ammoniacal organic products of man and
animal, would soon render the air pestiferous and incapable
of respiration did they not become quickly separated
and retained by the green leaves and shoots of
trees, and fixed in their substance. At the same time
vegetation is thus destroying and using up the malaria
it is giving out into the atmosphere the life-renewing
element "pure oxygen" for the respiration of man and
animals. These facts are too much ignored; but they
are not the only ones I would venture to notice.
Vegetation and trees with their copious foliage tend to
cool the atmosphere in hot weather. Trees, like
animals, have the power of maintaining a mean temperature
in winter and summer, and the air in contact
with the foliage in summer is much cooler than the
surrounding atmosphere in situations where there are
none. This influence, which depends upon the