558 SCIENCE PROGRESS 



and the rate of the process have continued, and in the main 

 support the conclusions already drawn. With regard to tem- 

 perature, the generally held opinion, resulting from the work 

 of Miss Matthaei, that the rate of carbon assimilation is a 

 little more than doubled by an increase of temperature of io° C, 

 has been called in question by W. H. Brown and G. W. Heise, 

 who subjected the published results of previous investigators 

 of this subject to a searching criticism. A, M. Smith (" The 

 Temperature Coefficient of Photosynthesis : A Reply to Criti- 

 cism," Ann. of Bot., 35, 517-36, 1919) has now repUed in 

 vindication of the generally held opinion, and defends the 

 conclusions previously drawn by Miss Matthaei, F. F. Blackman, 

 and himself. In an experiment on the influence of temperature 

 on the rate of photosynthesis of Ulva rigida, W. J. V. Osterhout 

 and A. R. C. Haas (" The Temperature Coefficient of Photo- 

 synthesis," Journ. Gen. Physiol., 1, 295-8, 191 9) found that 

 the rate of carbon assimilation in this alga at 27° C. is i-8i 

 times the rate of the process at 17° C. This value of the 

 temperature coefficient, though somewhat smaller than the 

 values found for other species by Miss Matthaei and Blackman 

 and Smith, is yet of the same order, and is confirmatory of 

 the earlier findings. Osterhout and Haas suppose that the 

 assimilatory process involves two reactions, a light reaction 

 with a low temperature coefficient and an ordinary chemical 

 reaction with a high temperature coefficient. There is little 

 that is new in this supposition, and among recent writers a 

 somewhat similar suggestion has been made by Warburg, whose 

 work is cited below, and also earlier by Willstatter and Stoll. 

 In the experiments of O. Warburg {Biochem. Zeitsch., 100, 

 230-70, 191 9) on the influence of external conditions on the 

 rate of photosynthesis, the experimental plant was the uni- 

 cellular alga Chlorella, the determination of the carbon dioxide 

 assimilated being made by the method of gas analysis of 

 Haldane and Barcroft. With regard to the influence of carbon 

 dioxide concentration, the rate of assimilation increases nearly 

 proportionally to the concentration from zero up to about a 

 concentration of 2 X lo"'. Above this the rate of assimilation 

 does not increase at so rapid a rate with increasing concentra- 

 tion of carbon dioxide until a concentration of about 10 X 10 ~* 

 further increase in concentration of carbon dioxide produces 

 little further change. The relation of light intensity (" con- 

 centration of light energy ") to the rate of assimilation is similar 

 to that of carbon dioxide concentration. From these results 

 the author assumes that at low concentrations of carbon 

 dioxide the rate of assimilation is determined by a chemical 

 action and not by diffusion. From the relation of light intensity 

 to rate of assimilation it is supposed that each light intensity 



