THE TEMPERATURE OF LEAVES OF PINUS IN WINTER 35 
For finding the temperature of thin leaves, the thermometer was 
placed on the upper surface of the leaf. The sides of the leaf were 
folded over with the midrib as an axis, and fastened with a light 
wooden clamp. In handling, the leaves were kept from contact with 
the fingers by using a pad of wadding. In the case of succulent 
leaves a small hole, the exact diameter of the thermometer bulb, 
was bored and the bulb inserted. The results obtained for succulent 
leaves under strong solar illumination confirmed the findings of 
previous investigators, i. e., for Opuntia 43.3° C, Mamillaria 43.5° C, 
and for Sempervivum 49.6° C. These were the maximal temperatures. 
The air temperatures were 27.5°, 28°, and 27.1° C. respectively. 
The temperatures found for thin leaves were much lower, Ulmus 
montana, Betula alba, and Saxifrdga crassifolia, showing maxima of 
0.7°, 2.3°, and 8.1° C. respectively, above the air temperature. Ur- 
sprung ascribes the lower temperature of thin leaves, as did Askenasy, 
to the very large surface exposed in proportion to the total mass of 
the leaf. 
In 1905 four important papers appeared, important because more 
refined methods were used in the determination of leaf temperatures 
than had been employed by previous investigators. Of these, one, 
prepared by Miss Matthaei (14), was communicated to the Royal 
Society of London by F. Darwin in 1903. Miss Matthaei, in the 
course of her researches on vegetable assimilation and respiration, 
found it necessary to determine the internal temperature of the leaves 
used. For this purpose detached leaves of cherry laurel {Prunus 
Laurocerasus) held in a special leaf chamber and placed in a constant 
temperature bath, were exposed to light of varying intensity from an 
incandescent gas lamp. For strong illumination, a powerful Keith 
burner was used. The leaf was protected from direct heating effect 
by a system of circulating water. The temperatures were determined 
by means of a very fine thermo-couple of copper and constantan. 
One junction was inserted in the midrib of the leaf, the other, insulated 
by a rubber tube, was placed in a water bath the temperature of which 
could be varied at will. The electromotive force produced by the 
difference in temperature at the two junctions was read by means 
of a Thomson mirror-galvanometer, about \ ohm in resistance. The 
deflections were read directly on the scale; no refinements, such as 
the use of a telescope, were considered necessary. This apparatus was 
found by calibration to be sensitive to 0.1° C. The temperatures were 
