RECENT ADVANCES IN SCIENCE 559 



corresponds to a particular concentration of a primary photo- 

 chemical product, which is active in the chemical reaction 

 according to the law of mass action. 



Experiments were also carried out at different temperatures 

 under different intensities of illumination. When the latter is 

 high, and the concentration of carbon dioxide high also, the 

 temperature coefficient is not constant, but decreases with in- 

 creasing temperature from about 4*3 at 5° C. to i*6 at 32° C. 

 When the light intensity is low the temperature coefficient is 

 about unity. This result forms very definite support to 

 Blackman's Theory of Limiting Factors, which has lately been 

 subjected to adverse criticism by W. H. Brown and G. W. Heise 

 (" The Theory of Limiting Factors," Philippine Journ. of 

 Science, C, 13, 34S-51, 191 8). Warburg further observes that, 

 if the light intensity is low, the same amount of light energy 

 can bring about the utilisation of the same amount of carbon 

 dioxide, whether the light is continuous or intermittent, whereas 

 if the light intensity is high, more carbon dioxide is utilised 

 by the same amount of light energy if the illumination is inter- 

 mittent than if it is continuous. This result is exactly what 

 would be expected on Blackman's view of the " time factor " 

 in assimilation applied to the action of light. 



The investigation of the internal factors controlling photo- 

 synthesis obviously presents considerably more difficulty than 

 the examination of the influence of external conditions. 

 Willstatter and Stoll (" Untersuchungen liber Kohlensaure- 

 assimilation," Berlin, J. Springer, 191 8) disputed the conclusion 

 of Miss Irving that, in the development of the seedling, chloro- 

 phyll content does not act as a limiting factor, but that some 

 other internal factor undergoing development during the de- 

 velopment of the seedling limits the assimilation. This question 

 has now been re-examined by G. E. Briggs (" The Development 

 of Photosynthetic Activity during Germination," Proc. Roy. 

 Soc, B, 91, 249-68, 1920), who, working with a new method 

 of measuring the rate of assimilation depending on combining 

 the oxygen evolved in assimilation with hydrogen at a surface 

 of palladium black, has followed the increase of the rate of 

 photosynthesis in seedling leaves of Phaseolus and other species 

 from zero to the stage when the photosynthetic activity is 

 nearly complete. In the experiments any increase in chloro- 

 phyll content was prevented, and so Briggs was able to confirm 

 the conclusion drawn earlier by Miss Irving, that some internal 

 factor other than chlorophyll is necessary for photosynthesis, 

 and that this other factor develops with increasing age of the 

 seedling. Briggs was also able to reconcile the apparently 

 contradictory results of Willstatter and Stoll on the one hand 

 and of Miss Irving on the other. 



