440 Scientific Proceedings, Royal Dublin Society. 



re-established. Such a rise in the rate of photosynthesis during iUuraination, 

 as determined by the fixation of carbon dioxide, has been observed by 

 Osterhout and Haas (5), and explained by them, " by assuming that sunlight 

 decomposes a substance whose products catalyze photosynthesis or enter 

 directly into the reaction." 



It may be assumed that these reactions only take place when the leaf is 

 exposed to light, and that their velocity under normal conditions is dependent 

 on the intensity of illumination. The velocity of the first reaction would, 

 however, be increased by increased illumination only so long as carbon dioxide 

 is in excess, whereas the velocity of the second reaction would not be directly 

 influenced by this factor. It is probable, therefore, that in strong sunlight 

 the balance between the two reactions would be disturbed, tending to an 

 increased proportion of chlorophyll a. 



The following figures, deduced from the means of Willstatters' results (7), 

 seem to bear out this view : — 



Proportion of Chlorophyll a to Chlorophyll b. 



In the plant exposed to light in absence of carbon dioxide one would 

 expect that chlorophyll b would be very much diminished in amount, if not 

 altogether absent. This does not seem to have been tested by experiment. 



The authors wish to express their indebtedness to Mr.T. G. Mason, M. A., B.sc, 

 for some preliminary experimental work on the sensitization of gelatine 

 photographic plates, and also to the Eoyal Dublin Society for supplies of 

 liquid air, which made the experiments at low temperature possible. 



During the earlier stages of the work Mr. Ball was in receipt of a 

 Maintenance Grant from the Department of Scientific and Industrial 

 Eesearch. 



