119 
of Edinburgh , Session 1872-73. 
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 slen- 
der 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.* 
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 un- 
known 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 tempera- 
ture of their surroundings than is fully accounted for by the conse- 
quent 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 con- 
* Prof. Balfour’s Class-Book of Botany, Physiology, chap. xii. page 670. 
VOL. VIII. 
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