8 



a limiting factor. Such is always the case 

 when a break occurs in the curve; usually 

 the curve rises at first and later runs paral- 

 lel with X-axis. Such curves were obtained 

 by Miss Matthaei^° in studying the 

 carbon assimilation of cherry laurel at 

 varying temperatures with unit light inten- 

 sity. The problem is much more compli- 

 cated, however, when variation of one factor 

 is accompanied by changes in one or more 

 other factors. This complication arises in 

 plotting the temperature curve for enzyme 

 activity. The curve rises at first according 

 to van't Hoff's law of reactions, but even- 

 tually a maximum value is reached and the 

 curve falls off. At some point near the end 

 of the ascending portion of the curve a 



M 



iu 



JO ao 



70 



go si? 



7imprra/tsr -f 



Fig. 2. Effect of temperature on the activity of 

 malt diastase. (After Kjeldahl.) 



break occurs: for all temperatures below 

 this point, temperature is the limiting fac- 

 tor and determines the activity of the 

 enzyme; for all temperatures above this 

 point, not temperature, but the amount of 

 enzyme is the limiting factor. The higher 

 temperatures cause a permanent inactiva- 

 tion or decomposition of the enzyme so that 

 its activity is conditioned only secondarily 

 by the temperature. There is also a time 

 factor involved here; the longer the tem- 



loPMi. Trans., B, 196: 47-105, 1904. 



