INTERNAL TEMPERATURE OF LEAVES. 231 



For thin leaves, however, he got much lower temperatures, 

 e.g. :— • 



Betula alba 28-3° C. 

 Ulmus montana 29^3° C. 



Blackmail improved the thermo-electric method by reducing 

 the size of the junction until it was small enough to be inserted 

 in the midrib and even in the lamina of thin leaves. He also 

 arranged that the junction, threaded through the leaf in the 

 open air on the roof of the laboratory at Cambridge, should 

 be electrically connected with a sensitive reflecting galvano- 

 meter in one of the rooms of the laboratory. By the use of 

 this apparatus, described by Matthaei (18) and again by Black- 

 man and Matthsei (3), he was for the first time able to 

 overcome the difficulty of taking the internal temperatures of 

 thin leaves in natural illumination. These were obtained by a 

 sensitive instrument to a high degree of accuracy. Though 

 the main object of their use of the apparatus was to obtain 

 the internal temperature of the leaves in their assimilation 

 apparatus, yet they obtained figures which showed that, in 

 natural illumination in the open air, thin leaves attained 

 higher temperatures than had been previously recorded, and 

 more closely approximating to the figures obtained previously 

 only for succulents, e.g., a leaf of Cherry Laurel reached 

 44-45° C. when the temperature of the surrounding air was 

 27° C. 



The subject of internal leaf temperature was incidentally 

 touched on by Brown and Escombe (4) in a research dealing 

 with the interchange of energy between a leaf and its environ- 

 ment. Their conclusions are quantitatively different from 

 those found by direct experiment by Blackman and Matthsei 

 (3). Their research had, however, a very different primary \ 

 object from that of ascertaining the internal temperature of 

 the leaf, and little significance perhaps is to be attached to 

 the actual figures on this point, which was quite a side issue in 

 the investigation. It must be pointed out too that their 

 figures are obtained in, an indirect manner by an elaborate 

 calculation based on several different observations, in each of 

 which a small error may have occurred. 



