Collected works > Edinburgh edition, 1894-98 - Works of Robert Louis Stevenson > Volume 28, 1898 - Appendix

ON THE THERMAL
of the tree ; and that those in the tree are always less in
amount and considerably slower of occurrence than those in
the air. This thermal sluggishness, so to speak, seems capable
of explaining all the phenomena of the 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. 1 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
1 Professor Balfour's Class Book of Botany, Physiology, chap, xii.,
p. 670.
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