72 



Carbon Assimilation. 



bath to such a temperature that the E.M.P. of the combination of 

 two thermocouples was zero. The two couples are then at the same 

 temperature and the temperature of the water bath containing one 

 jiuiction is consequently the internal temperature of the leaf. 



in this way it was shown that the values obtained by Brown 

 and Escombe for the rise in temperature of the illuminated leaf 

 were much too small. The following table giving some of the 

 values obtained by Blackman and Matthaei shows how much 

 hijJlier the internal temperature of the leaf may be above that of 

 its surroundings when subjected to intense illumination. 



Table X. 

 E^ect of Light in Raising Internal Temperature of Leaves. 



In all experiments with high light intensities the internal 

 temperature of the leaf was therefore measured. 



In all experiments 800 c.c. of air containing from 0-8% to 2-8% 

 of carbon dioxide were passed over the leaf per hour. As this was 

 never used up it was supposed that carbon dioxide was not a 

 limiting factor. The experiment was allowed to run for 1^ to 2 

 hours before measurements were made, in order to render the 

 conditions constant. The amount of carbon dioxide absorbed 

 during consecutive hourly or two hourly periods was then measured. 

 This gives the value of the ' apparent assimilation ' to which is 

 added the value found for the respiration in order to obtain the 

 ' true assimilation.' 



Assimilation at Low and Medium Tetnperatures. 



As luiit intensity of light was used the light from a single 

 incandescent gas burner when the front of the mantle was 130 cms. 

 from the leaf. With this illumination, assimilation could be detected 

 at as low a temperature as — G^C. With increasing temperature 

 the assimilation rapidly increased up to S^C, above which, increase 



