1216 THE TEMPERATURE FACTOR CHAP. 31 



comparatively small temperature differences between leaf and air. Ac- 

 cording to Miller and Saunders, the average surface temperature of leaves 

 in the field is practically equal to that of the air. (In direct sunlight, the 

 surface temperature of the leaf often was, in their experiments, 1 or 2° 

 above that of the air; but, when the sun was covered by a passing cloud, 

 transpiration caused the leaf temperature to drop immediately to 1 or 2° 

 below that of the surrounding atmosphere. Averaging over all atmospheric 

 conditions gave a mean value of temperature difference close to zero.) 

 Eaton and Belden (1929) found that turgid leaves of cotton plants had a 

 temperature lower than that of the air during the greater part of the hot 

 and dry summer day; only on mornings and evenings was this relation 

 reversed. Wilted cotton leaves, on the other hand, showed positive tem- 

 perature differences during the whole day. (The authors considered these 

 results evidence of the strong influence of transpiration.) 



In contrast to these examples of negligible or even negative tempera- 

 ture differences between illuminated leaves and air, others, from Asken- 

 asy (1875) to Clum (1926), have observed in direct sunlight, internal tem- 

 peratures 10°, 15° or even 20° C. above the temperature of the ambient air. 

 The fact that Miller and Saunders conducted their experiments with 

 much thinner leaves than Blackman and Matthaei, and Clum may explain 

 some of the discrepancies. Askenasy (1875) first suggested that thick suc- 

 culent leaves are particularly hable to overheating in the sun. Table 31.1 

 shows that this conclusion was confirmed by Ursprung (1903), but not by 

 Smith (1909). The figures of Seybold and Brambring in this table (c/. 

 also fig. 31.1) also show no significant differences among the temperatures of 

 xeromorphic, hygromorphic and succulent leaves. This is not as strange 

 as it may appear; because of the lower ratio of surface to volume, succulents 

 undoubtedly are handicapped in the dissipation of heat; but, since Hght 

 absorption is proportional to the area rather than to the volume (while 

 heat capacity is proportional to the volume) of the leaf, the heating by 

 light absorption also is much slower in the case of succulents than in that 

 of thin leaves (c/. Bnjophylhmi curve in fig. 31.1). (Conditions are dif- 

 ferent in the dark, since heat production by respiration is approximately 

 proportional to the volume of the tissue.) 



The difference between leaf temperature and air temperature may be 

 of paramount importance for the photosynthetic activity of evergreens in 

 winter. According to Ehlers' data in Table 31.1, the needles of conifers 

 may have, during a winter day, an internal temperature 7 or 8° C. above 

 that of the air. The temperature of alpine plants may rise even higher, 

 because of the intense radiation to which they are exposed, even while the 

 temperature of the air may be quite low. 



The danger of overheating is particularly great in experiments with 



