evere injury may have occurred and no
injury at the other place with plants normally having equal hardiness. A
careful analysis of the situation, however, would probably show that at
the place where the injury occurred a period of warm weather had existed
which was followed by a rapid drop in temperature to a killing low on a
single night, whereas the trees at the place where no injury occurred
were not subjected to such changes in temperature. On the other hand,
injury to the roots usually occurs only after prolonged periods of cold
weather. This is largely because the soil cools slowly and it requires a
long period of cold weather to reduce the soil temperature sufficiently
and to such depths as to cause injury to the roots.

Under northern conditions where low temperatures for a rather long
period are sometimes experienced, injury to the portion of the trees
above ground may occur as a result of drying out of the wood. It is well
known that a cake of ice will gradually evaporate and disappear when in
the open and exposed continuously to below-freezing temperatures. We all
know that the family wetwash when hung on a line and frozen will soon
dry, especially if the wind blows. The principles operating in these
cases may cause severe injury to trees. In the wintertime the root
systems of trees take up water from the soil that is not frozen and this
water moves in the tree to replace that lost by evaporation. Under
conditions where the soil is frozen to such an extent that the water
absorbed by the roots is continually less than that lost by the top of
the trees by evaporation, drying out of the top occurs. If this is
continued over a period of time a dryness of the wood and other tissues
occurs that causes death of the dried-out portions. This type of injury
does not show the typical symptoms of cold injury but rather those of
drying out. The conditions that are most likely to cause such injury are
a soil frozen to the effective rooting depths, a dry atmosphere, and a
moderately high wind velocity. Injury of a similar nature to that just
described very often affects trees transplanted in late fall or early
winter, especially those that did not have their tops cut back to
balance the loss of roots sustained in transplanting. During even very
mild winters the tops of such trees dry out to such an extent that the
small branches and even the leader may die. In extreme cases the entire
top may die back to the root. In plantin