5. Chlorophyll, 



6. Enzymes. 



Where it is possible to vary one of these 

 factors independently of the rest, its effect 

 on the rate of assimilation can be measured, 

 under suitable conditions, and a curve 

 plotted. In this way a temperature-as- 

 similation curve, a light-assimilation curve 

 and a carbon-dioxide-assimilation curve 

 can be constructed. The other factors 

 are more difficult to control. The fol- 

 lowing curves were constructed by Black- 

 man and Smith^ from a study of the 

 rate of assimilation in Elodea. 



The light curve and the carbon-dioxide 

 curve are straight lines. The rate of as- 

 similation varies directly with the inten- 

 sity of light and the supply of carbon di- 

 oxide. The temperature curve shows that 



> 



^ 030 



FiQ. 1. Effect of external factors on assimilation 

 in Elodea. (After Blackman and Smith.) 



the rate of assimilation is an exponential 

 function of the temperature. In fact the 

 process of assimilation obeys van't Hoff's 

 law of reactions for temperatures under 

 30° C. Above this, the rate of assimilation 



6 Proc. B. Soc, B., 83 : 389-412, 1910. 



