at first rises and then falls off, the process 

 being complicated at high temperatures by 

 a "time factor." The same effect has been 

 observed at high light intensities, and with 

 strong concentrations of carbon-dioxide 

 which have a narcotic effect. 



Disregarding these complications, we 

 will confine our attention to the first parts 

 of these curves. The ordinates of all three 

 curves are the same, namely, rates of carbon 

 assimilation, which can be measured in 

 terms either of CO2 absorbed or of sugar 

 produced. The former happens to be the 

 more convenient measure. At any given 

 temperature, the rate of assimilation which 

 is a function of that particular temperature 

 can be determined directly by the curve and 

 is equal to a certain distance measured off 

 from the origin on the Y-axis. Similar dis- 

 tances are given for any definite supply of 

 carbon dioxide and for any degree of il- 

 lumination. In any actual environmental 

 complex, where the temperature, light and 

 carbon-dioxide supply are known, the rate 

 of assimilation is equal to the shortest dis- 

 tance measured on the Y-axis. This is 

 stated as a general principle by Blackman 

 as follows: "When a process is conditioned 

 as to its rapidity by a number of separate 

 factors, the rate of the process is limited 

 by the pace of the 'slowest' factor." The 

 factor which gives the shortest distance on 

 the Y-axis — that is, the "slowest" factor, 

 he calls the limiting factor. 



As a matter of fact the carbon assimila- 

 tion of green plants is usually limited by 

 the seasonal variation in temperature and 

 the diurnal variation in light, the CO, con- 

 tent of the air being constant. Nothing has 

 been said of the other factors that effect 



