: On the under side of leaves and on other
surfaces of a tree little pores known as _stomata_ may be found. In
the bark of birch and cherry trees these openings are very
conspicuous and are there known as _lenticels_. These pores are
necessary for the breathing of the tree (respiration), whereby
carbonic acid gas is taken in from the air and oxygen given out. The
process of assimilation depends upon this breathing process and it
is therefore evident that when the stomata are clogged as may occur
where a tree is subjected to smoke or dust, the life processes of
the tree will be interfered with. The same injurious effect results
when the stomata of the roots are interfered with. Such interference
may occur in cases where a heavy layer of soil is piled around the
base of a tree, where the soil about the base of a tree is allowed
to become compact, where a tree is planted too deep, or where the
roots are submerged under water for any length of time. In any case
the air cannot get to the roots and the tree suffers. Nature takes
special cognizance of this important requirement in the case of
cypress trees, which habitually grow under water. Here the trees are
provided with special woody protuberances known as "cypress knees,"
which emerge above water and take the necessary air. See Fig. 18.

Conclusions: From the foregoing it will be seen that trees have certain
needs that nature or man must supply. These requirements differ
with the different species, and in all work of planting and care as
well as in the natural distribution of trees it is both interesting
and necessary to observe these individual wants, to select species
in accordance with local conditions and to care for trees in
conformity with their natural needs.

CHAPTER V

WHAT TREES TO PLANT AND HOW

The following classification will show the value of the more important
trees for different kinds of planting. The species are arranged in the
order of their merit for the particular object under consideration and
the comments accompanying each tree are intended to bring out its
special qualifications for that purpose.

Conditions for tree growth in one part of the country differ from those
of another and these lists, especially applicable to the Eastern States,
may not at all fit some other locality.

TREES BEST FOR THE LAWN

DECIDUOUS

1. America