230 SMITH : 



PART I. 



Introduction. 



VERY little attention has previously been paid to the 

 ascertaining of the internal temperature of leaves 

 exposed to natural illumination. For succulents Askenasy (1) 

 has shown that high internal temperatures up to 49° C. occur. 

 Keeble (14) took some temperatures of leaves of Amherstia 

 nobilis at Peradeniya by the method of placing a mercury 

 thermometer in contact with the leaves. His object, however, 

 was not to find the internal temperature of the leaves, but to 

 estimate how much heat was transmitted and how much 

 reflected. Stahl (28) used a thermo-electric method of deter- 

 mining the internal temperature of succulent leaves exposed 

 to artificial illumination, with the special object of finding 

 whether there was a difference in temperature between red 

 and non-red parts of the leaves. His spatula-like thermo- 

 junction, however, was too large to be placed in thin leaves. 

 Ewart (8) took one or two isolated observations on the 

 temperature of Hoya leaves by using a mercury thermometer 

 in contact with the leaf. 



Ursprung (29) in the course of his work on the heat and 

 light relations of leaves took some temperatures of both 

 succulents and thin leaves exposed to natural illumination. 

 He deliberately abandoned the thermo-electric method on 

 the ground that it was difiicult to get the instrument suffi- 

 ciently sensitive over a large range. Since he -did not use 

 artificial light, he required an easily portable and yet sensitive 

 galvanometer, and this he could not obtain. In fleshy leaves 

 he made a small hole just big enough to contain the thermo- 

 meter bulb. In thin leaves he placed the thermometer on 

 the upper surface, and then folded the leaf upwards over its 

 midrib and fastened it, thus folded and insulated by a pad of 

 wadding in a clamp supported on a stand. His results 

 agreed with those previously obtained for siicculents, and he 

 found some very high temperatures occurring in them, e.g. : — 



Opuntia Rafinesquii 43 '3° C. 



Mammillaria fulvispina 43 '5° C 



Semper vivum tectorum 49*6° C. 



Saxifraga crassi folia 35*9° C. 



