Sept. 1, 1925 
TTiermostasy of Growth of the Date Palm 
449 
tance of the ascending sap current in regulating the temperature of the 
trunk interior and stated that “ Buff on was the first author to observe 
that when we cut trees in the winter, the interior part of the trunk 
appears to be warm, especially close to the center, * * 
He cites Hunter (p. 879) as the first to try to explain the phenomena 
by placing a thermometer in a hole bored in the trunk of a large 
walnut tree, where he obtained in autumn, temperatures 2 or 3 
degree above the surroundings. He was followed by Schoepff of New 
York (p. 879) and Bierkander of Sweden, confirming the earlier work. 
DeCandolle cited John Schopf (21), who published “Ueber die Tem- 
peratur der Pflanzen,” in Naturforscher, Halle, 1788. This is a 
record of observations made at New York in 1783 on the temperatures 
within the trunks of several species of trees, black, red and white oak, 
wild cherry, chestnut, and beech, compared with those of the sur¬ 
rounding air. He observed distinct gradients between tree and air 
temperatures, notably one morning with the air at 22° at 7.45, when 
the interior of a 3-ioot chestnut showed 36°. Schopf is quoted 
as having “repeated and enlarged 77 Hunter’s experiments. These 
records, probably the beginning of the study of plant physiology in 
the United States, he interpreted as proving that plants generated in 
themselves a vital heat analogous to that of warm-blooded animals. 
He failed to associate the higher interior tree temperatures with the 
ascending sap current and soil temperatures. 
DeCandolle further records that Pictit and Mam-ice at Geneva, 
by the use of several thermometers, some in the trunk of a large 
chestnut tree, others in the soil at various depths, arrived at a true 
solution of the problem: 
They noticed that the variations in the internal temperature (in the trunk) 
corresponded plainly with the temperature given by the thermometer placed in 
the soil at the depth of 4 feet, thus in the vicinity of the roots of the tree. 
Hermstaedt (p. 880) is quoted as observing a temperature of plus 
1° R. (34.25° F.) within a tree when the air temperature was minus 
10° R. (9.25° F.). DeCandolle sums up this part of the problem as 
follows: 
In bringing these facts together, and especially the last ones mentioned, Rum- 
ford's theory about the ascension of the sap helps us to appreciate the problem 
dealing with the internal temperature of plants as I have attempted to explain 
in 1805 in the “Principes de Botanique” in the introduction to the Flore Fran- 
$aise. * * * 
The water absorbed by the roots ascends vertically in the trunk. This water 
shows the temperature of the soil at the middle depth of the tree roots; it is 
consequently warmer than the atmosphere in the winter and cooler in the summer. 
The insulating effect of the bark and the outer layers of the trunk 
in conserving the heat of the ascending sap is recognized in the 
following paragraph: 
Therefore, while the ascension of the sap tends continually to bring the tempera¬ 
ture of the center of the trunk to an equilibrium with that of the soil, the whole 
structure of the woody body of the trunk and especially that of the bark, prevents 
the temperature from reaching an equilibrium with the surrounding air. 
A summary of these observations on the interior temperatures of 
trees, dating back from DeCandolle in 1832 to Schopf in 1783, and 
Hunter in 1775 shows the following points generally accepted: 
(1) The paramount influence of the air temperature and, in sun¬ 
light, the sun’s rays incident upon the surface of the trunk and 
branches, in controlling the interior temperature of the tree. 
