he case without any hypothetical vital power of resisting
temperatures below the freezing point, such as is hinted at even by
Becquerel.

Reaumur, indeed, is said to have observed temperatures in slender trees
nearly thirty degrees higher than the temperature of the air in the sun;
but we are not informed as to the conditions under which this
observation was made, and it is therefore impossible to assign to it its
proper value. The sap of the ice-plant is said to be materially colder
than the surrounding atmosphere; and there are several other somewhat
incongruous facts, which tend, at first sight, to favour the view of
some inherent power of resistance in some plants to high temperatures,
and in others to low temperatures.[50] But such a supposition seems in
the meantime to be gratuitous. Keeping in view the thermal
redispositions, which must be greatly favoured by the ascent of the sap,
and the difference between the condition as to temperature of such parts
as the root, the heart of the trunk, and the extreme foliage, and never
forgetting the unknown factor of specific heat, we may still regard it
as possible to account for all anomalies without the aid of any such
hypothesis. We may, therefore, I think, disregard small exceptions, and
state the result as follows:--

If, after every rise or fall, the temperature of the air remained
stationary for a length of time proportional to the amount of the
change, it seems probable--setting aside all question of vital
heat--that the temperature of the tree would always finally equalise
itself with the new temperature of the air, and that the range in tree
and atmosphere would thus become the same. This pause, however, does not
occur: the variations follow each other without interval; and the
slow-conducting wood is never allowed enough time to overtake the rapid
changes of the more sensitive air. Hence, so far as we can see at
present, trees appear to be simply bad conductors, and to have no more
influence upon the temperature of their surroundings than is fully
accounted for by the consequent tardiness of their thermal variations.

Observations bearing on the second of the three points have been made by
Becquerel in France, by La Cour in Jutland and Iceland, and by Rivoli at
Posen. The results are perfectly congruous. Becquerel's observations[51]
were made under wood, and about a hundred yards outside in open ground,
at three stations in the district of Montargis, Loiret. T