The Temperature Relations of Foliage Leaves . 77 
THE TEMPERATURE RELATIONS OF FOLIAGE LEAVES 
I N a comprehensive paper on the Physical Properties of Foliage 
Leaves, Ursprung has included, besides a large number of 
experiments on the mechanical properties of petiole and lamina, 
some experiments on the thermal properties of leaves, chiefly on 
the temperatures attained by foliage leaves in the sunlight. The 
paper contains a valuable survey of previous work, a clear analysis 
of the factors upon which leaf-temperature depends, and a number 
of original observations of leaf-temperature. For these experiments 
he rejects the thermo-electric method instituted by Stahl and uses 
a mercury thermometer. In the case of fleshy leaves, the bulb 
was stuck into the substance of the leaf. In the case of thin 
leaves, it was laid on the leaf surface and the leaf was folded up 
around it. He obtains very high temperatures for thick succulent 
leaves such as those of different species of Sempervivuw. These 
attained internal temperatures of from 48-52° C when the shade 
temperature of the surrounding air was 12-16° C. In other 
succulents, e.g., Opuntia Raftnesquii, Saxifraga crassifolia he obtains 
the high temperatures of 43° and 36° C respectively. For thin 
leaves Betula alba and Ulmus montana he obtains temperatures of 
only 28° and 29° respectively. From Angot’s figures of radiation at 
the equator he concludes that high internal temperatures in succulent 
leaves in the tropics must often occur, and therefore these 
temperatures cannot be so injurious as earlier observers, e.g., De 
Vries and Sachs, supposed. 
With regard to the effect of colour on leaf-temperature he 
makes the general statement that the decrease of diathermancy, due 
to the presence of anthocyan masking the green colour of a leaf, 
must tend to raise its internal temperature. 
The chief defect of his method is the use of a mercury thermo¬ 
meter by which the internal temperature of a thin leaf cannot be 
obtained. What is obtained is the surface temperature, in 
determining which, other factors, chiefly those causing heat loss by 
convection and radiation, play a greater part than in the case of 
true internal temperatures. 
Blackman and Matthaei, 1905, introduced an improved form of 
thermo-electric junction which was small enough to be inserted in a 
thin leaf and brought out the unexpected fact that even thin leaves 
attained very high internal temperatures, 45° C being reached by a 
cherry-laurel leaf exposed to direct radiation from the sun at noon 
in July. 
This thermo-electric method has been used more recently by 
Baumert in investigations as to the various methods by which 
leaves protect themselves from too intense light. As source of 
light he used a constant petroleum lamp, the light from which fell 
on the leaf in a moist chamber at an angle of 45°. The temperature 
was measured by a small thermo-junction of needle form to which 
was attached a D’Arsonval galvanometer. In this way he compared 
the temperature attained by leaves with, and leaves without, a 
protective covering of different kinds, e.g., a covering of hairs, a 
covering of wax, a glossy upper surface, a wet upper surface, strati¬ 
fication of outer epidermis wall as in species of Ferns, &c. In the 
