checked when the temperature sinks below a certain point. The tree
is thus, during the winter, in a period of rest and only a few
chemical changes take place which lead up to the starting of
vegetation. In eastern United States, growth starts in April and
ceases during the latter part of August or in early September. The
different parts of a tree may freeze solid during the winter without
injury, provided the tree is a native one. Exotic trees may suffer
greatly from extreme cold. This is one of the main reasons why it is
always advisable to plant native trees rather than those that are
imported and have not yet been acclimatized. Frosts during
mid-winter are not quite as injurious as early and late frosts and,
therefore, if one is going to protect plants from the winter's cold,
it is well to apply the covering early enough and to keep it on
late enough to overcome this difficulty.

The mechanical injuries from frost are also important. Snow and
sleet will weigh down branches but rarely break them, while frost
will cause them to become brittle and to break easily. Those who
climb and prune trees should be especially cautious on frosty days.

(6) Influence of air: 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