222 SCIENCE PROGRESS 



is made to furnish a physiological proof for this well-known 

 hypothesis. Unfortunately the basis on which this proof rests 

 is not very satisfactory. It consists of an examination of the 

 C0 2 /0 2 ratio in gaseous exchange. If this is equal to unity 

 under all conditions the authors appear to consider it proved 

 that formaldehyde can be the only intermediate product in 

 carbon assimilation. They do indeed find that under all condi- 

 tions of experiment, even with prolonged intense assimilation, 

 the ratio between the carbon dioxide taken in and the oxygen 

 evolved is constant and equal to unity. They conclude, there- 

 fore, that no intermediate products of reduction other than 

 formaldehyde are formed and accumulated, as otherwise the 

 C0 2 /0 2 ratio would undergo alteration. Apart from theoretical 

 considerations, this investigation presents many novelties in 

 regard to technique. For the first time since Boussingault 

 a dynamic method is employed in which currents of gas pass 

 over the leaves. That part of the investigation dealing with 

 the assimilation of succulents is of particular interest, as it is 

 shown that even here the C0 2 /0 3 ratio approaches unity if 

 assimilation continues over a long enough period. 



The rapid growth of interest in the problems of carbon 

 assimilation has resulted in a certain amount of discussion and 

 criticism of previous work. The most substantial contribu- 

 tions in this respect come from W. H. Brown and G. W. Heise, 

 " The Application of Photochemical Temperature Coefficients 

 to the Velocity of Carbon Dioxide Assimilation " {Philippine 

 Journal of Science, C. 12, 1-25, 191 7), "The Relation between 

 Light Intensity and Carbon Dioxide Assimilation " (ibid. 85-97). 

 These papers constitute a critical review of earlier work, but deal 

 especially with the writings of F. F. Blackman and his pupils. 

 It was clear, after the criticism to which Willstatter and Stoll 

 subjected the work of Miss Irving on the assimilation of 

 etiolated leaves, that Blackman 's hypothesis regarding the 

 interaction of factors conditioning assimilation (the principle 

 of limiting factors) could no longer be regarded as valid. 

 Brown and Heise 's criticism renders Blackman 's interpretation 

 of his own results and those of his pupils still more improbable. 

 In their first paper Brown and Heise consider the work of 

 Miss Matthaei and of Blackman and Smith on the influence 

 of temperature on carbon assimilation. It is shown that a 

 critical examination of the work of these writers leads to 



