These are two entirely different processes. The _Fire Blight_ (of the
middle and western States), is a disease of the circulatory system,
induced by a freezing of the sap while _the tree is in a growing and
excitable state_. It always _must_ occur before the leaves are shed in
the autumn. Winter-killing is of two kinds—resulting from severe cold,
and from untimely heat. The loss of tender shrubs, roses, etc., at
least, before they are fully established, and of half-hardy
fruit-trees, is occasioned by the winter sun shining warmly upon them
while frozen, and suddenly thawing them. The point of death is usually
near the surface of the ground, where the under-ground bark and upper
bark come together. Whole orchards are destroyed in this way; and, if
examined, the bark may be found _sprung off_ from the wood. This may
occur at any time during the winter.
We are in doubt whether the winter-stored sap exists in a state to be
affected by the expansion of the freezing fluids of the tree. If the
expansion of congelation _did_ produce the effect, it should have been
more _general_, for there are fluids in every part of the trunk—all
congeal or expand—and the bursting of the trunk in one place would not
relieve the contiguous portions. We should expect, if this were the
cause, that the tree would _explode_, rather than split. Capt. Bach,
when wintering near Great Slave Lake, about 63° north latitude,
experienced a cold of 70° below zero. Nor could any fire raise it _in
the house_ more than 12° above zero. Mathematical instrument cases,
and boxes of seasoned fir, split in pieces by the cold. Could it have
been the sap in _seasoned fir wood_ which split them by its expansion
in congealing?
We quote a paragraph from Loudon—“The history of frosts furnishes very
extraordinary facts. The trees are often scorched and burnt up, as
with the most excessive heat, in consequence of the separation of the
water from the air, which is therefore very drying. In the great frost
in 1683, the trunks of oak, ash, walnut, and other trees, were
miserably split and cleft, so that they might be seen through, and the
cracks often attended with dreadful noises like the explosion of
fire-arms.”
We don’t exactly know whether to take the first part as Loudon’s
explanation of the facts in the second.
There can be no doubt that the nature of the summer’s growth, very
much determines the power of a tree to resist the severity of winter.
When there is but an imperfect ripening in a cold and backward season,
the tissues formed will be feeble, and the juices stored in them thin.
Now the power to resist cold, among other things, is in proportion to
the viscidity of the fluids in a plant.
It is highly desirable that the chemical researches which have
revolutionized the art of cultivation, should be pushed into the
_morbid anatomy_ of vegetation. A close, exact analysis of all the
substances in an injured condition, will save a vast deal of bootless
ingenuity and fanciful speculation.