INTERNAL TEMPERATURE OF LEAVES. 253 



The figures in italics are the temperatures of the leaves with 

 stomata inwards. The observations are placed exactly in the 

 order in which they were made. They occupied about five 

 minutes in taking. Taking the average of the six observations 

 with each apparatus, we get stomata inwards 14*6°, stomata 

 outwards 12*9° above the temperature of the surrounding air — 

 a difference of 1 • 7° C. 



Effect of Air Movement. 



The internal temperatures reached by thin foliage leaves 

 placed normal to bright tropical sunlight and in still air are, 

 we have seen, from 38-43 C. Such temperatures as these have 

 hitherto often been considered injurious to the leaf. It is 

 worth while, therefore, to notice how far leaves are subjected 

 to such conditions in nature. The statement has been made 

 (Ewart 8) that in the tropics plants very rarely expose their 

 leaves so as to be normal to the sun's rays when the sun is 

 overhead. This statement is open to doubt , and would require 

 a large amount of detailed evidence to substantiate it. Full 

 evidence on this point, either for or against, is not at my 

 command. Apart from this, there is one condition, that of 

 air movement, which is most important in this connection, 

 upon which evidence will be brought forward. Even at 

 Peradeniya where strong winds are of very rare occurrence, 

 there are very few times when the air is quite still. During 

 the present experiments, which were all carried out on days 

 which were sunny and bright, a certain amount of fitful breeze 

 varying in strength was usually present. In all the tables the 

 highest readings were only obtained in certain still intervals, 

 which were in many cases very short. As has previously been 

 remarked, any air movement, however slight, is a most power- 

 ful factor in reducing the internal temperature, at any rate of 

 thin leaves. No attempt was made to measure the velocity 

 of the movement of air, but the condition of the air was 

 roughly classified as (1) still, (2) slight breeze, (3) breeze 

 with an occasional special note when the breeze was 

 strong. I 



