INTERNAL TEMPERATURE OF LEAVES. 239 



been registered. It seems reasonable for the future, therefore, 

 to take the tempsrature of the midrib as being the tem- 

 perature of the whole lamina, and this was accordingly done. 



In considering the experiments as a whole it must be 

 remembered that though a good deal of sunshine was vouch- 

 safed, yet there were times when clouds appeared, obscured 

 the sun , and caused the experiment to be abandoned. Though 

 this is not always stated, it is often the cause of the abrupt 

 conclusion of the experiments. 



Later, temperatures were registered by dry and wet bulb 

 thermometers to obtain humidity of air and by the black 

 bulb thermometer to give the intensity of solar radiation , and 

 these Avill be inserted where they seem to bear on the argument. 



It is perhaps worth while recording the results of an experi- 

 ment intended to give some idea of the conductivity of heat 

 of a green foliage leaf. For this purpose a leaf of Magnolia 

 sphenscarpa was exposed to direct sunlight with a thermo- 

 junction in the midrib. After the temperature in sunlight 

 had been observed, the junction alone was shaded by a small 

 piece of pith. The temperature attained by the junction 

 under these conditions was not due to absorption of direct 

 radiation by the part of the leaf in which the junction was 

 placed, but was due chiefly to conduction of heat from the 

 illuminated parts all round. That leaf substance is not a 

 perfect insulator, and at the same time not a good conductor 

 is shown by the temperatures attained. 



Thus, while in full sun, the leaf attains a temperature 

 of from 13-9 (10.9 a.m.) to 16-6 (11.19 a.m.) above the 

 surrounding air, shading only the junction reduces the tem- 

 perature to 6-7 (10-13 a.m.) and 64 (11.13 a.m.) above 

 the surrounding air. It is thus seen how large a part in the 

 high temperatures produced is played by the absorption of 

 radiation by the restricted area immediately surrounding 

 the junction. 



High Temperatures reached by Illuminated Leaves. 



Tables will first be given showing the temperatures to which 

 green foliage leaves of different kinds attain under still air 

 conditions in direct bright sunhght. 



