Factors Influencing the Intake of Carbon Dioxide. 67 



temperatures is measured over a considerable time it is certain 

 that a temperature will be found at which assimilation is at a 

 maximum. This apparent optimum temperature will however vary 

 according to the time elapsing between the commencement of 

 assimilation at that temperature and the actual measurement of 

 assimilation owing to the rapid falling off in assimilation due to the 

 time factor. 



We shall later discuss the time factor in more detail, but in 

 this place we would comment on the use of such expressions as 

 ' time factor.' It is of course desirable that the same terminology 

 should be used wherever possible in physiology as is employed in 

 pure chemistry and physics. But while analysis of physiological 

 processes has not proceeded far this is not always possible, and it 

 is to the credit of F. F. Blackman that he has introduced the terms 

 'limiting factor,' 'time factor,' which permit discussion of 

 physiological processes without involving premature assumptions 

 as to their nature. 



The work of P. F. Blackman and his pupils has been largely 

 concerned with the influence of the various factors temperature, 

 light and carbon dioxide supply on the rate of carbon assimilation. 

 Before the publication of his * Experimental Researches on Vegetable 

 Assimilation and Respiration ' there had indeed been much 

 work on the influence of these factors on assimilation, but as none 

 of these previous workers had recognised the principle of limiting 

 factors it seems unnecessary for us to discuss their results here. 

 Again the search for an optimum value of the various factors has 

 not helped to elucidate the problems. 



I We shall first deal with the work of Blackman and his pupils 

 on the relation between assimilation and the chief environmental 

 factors, carbon dioxide supply, light intensity and temperature. 

 Blackman expresses this relation as being such that " the magnitude 

 of this function in every combination of these factors is determined 

 by one or other of them acting as a limiting factor. The identification 

 of the particular limiting factor in any definite case is carried out 

 by applying experimentally the following general principle. When 

 the magnitude of a function is limited by one of a set of possible 

 factors, increase of that factor, and of that one alone, will be found 

 to bring about an increase of the magnitude of the function." 



We give here (Fig. 6) the curves obtained by Blackman and 

 Smith (1911 b) showing the inter-relationship between carbon 

 assimilation and the three external factors in the case of Elodea, 



