34 
JOHN H. EHLERS 
Askenasy (i) in 1874 made observations on the temperature 
attained by succulent plants when exposed to solar radiation. Sem- 
pervivum and Opuntia were used for this purpose. A mercury thermo- 
meter was laid against the upper surface of the leaf, or inserted in a 
cut made for that purpose. With the thermometer in the shade 
registering 31° C, the leaves of Sempervivum attained a temperature 
of 43.7° to 51.2° C, an excess of 20.2° C. over the air temperature. 
With leaves of Opuntia he obtained a temperature of 15° C. above the 
shade temperature. Thin leaves {Auhrietia and Gentiana) on the 
other hand reached a temperature of only 7° C. above that of the air. 
The difference in temperature attained by succulent and by thin 
leaves he ascribes to two causes: (i) the lower rate of transpiration of 
the succulent leaves; (2) their massive structure, exposing less surface 
in proportion to mass than thin leaves expose. 
Worthy of mention because of the method employed is an investi- 
gation by Stahl (29) to determine the difference in temperature 
between the red and non-red parts of variegated leaves. Stahl seems 
to have been the first to apply a thermo-electric method to determine 
leaf temperatures. His apparatus consisted of a thermo-couple of 
German silver and copper, and a mirror galvanometer read by means 
of scale and telescope. The junctions were made spatulate in form 
and were pushed into the leaf lamina. Variegated succulent leaves 
were used. The source of light was a gas flame. Since no absolute 
temperatures are given his work needs no further mention here. 
Under tropical insolation, Ewart (5) obtained with leaves of Vanilla 
and Hoy a temperatures of 45° and 50° C. respectively. The leaves 
were suddenly bent around and pressed against a delicate mercury 
thermometer originally a little below the expected temperature. 
The results obtained compare fairly well with those obtained by 
Askenasy. 
Ursprung (31) in the fall of 1901 made observations on the temper- 
ature of both succulent and thin leaves under natural illumination. 
The observations were made in the Basel Botanical Garden at temper- 
atures ranging from 14° to 28° C. Ursprung rejected the thermo- 
electric method on the ground that the direct reading of temperatures 
with a mercury thermometer was more simple, and because of the 
difficulty of getting an instrument sufficiently sensitive over a wide 
range of temperatures. Thermometers with small cylindrical bulbs 
and graduated to .5 degrees over a range of 10° to 50° C. were used. 
