280 CHAP. XXV11I. GENERAL REVIEW 



The volume is thus a linear function of the absolute 

 temperature. 



It may be said in general that when A and B are so 

 related that any change in B produces a corresponding 

 variation in A, then A is a function of B. The relation 

 between B and A is discovered by plotting a curve 

 in which the ordinate represents the induced change 

 and the abscissa the variation that induces it. Photo- 

 synthesis is a function of light, of C0 2 -concentration, of 

 temperature and of the tonic condition of the plant. The 

 relation between photosynthesis and each of these factors 

 is found from the characteristics of the different photo- 

 synthetic curves. 



It is shown that more satisfactory results can be obtained 

 by basing the measurement upon a physiological rather than 

 upon a physical scale. The physiological zero is found by 

 producing backwards the straight median portion of the 

 photosynthetic curve until it intersects the abscissa. 



It is also shown that when measurements are taken on 

 the physiological scale the photosynthetic activity is found 

 to be a linear function of each of the different factors. 



It has been explained how the average coefficient is 

 obtained by the Differential Method, dividing the increment 

 of activity by the increment of the particular factor. In 

 the absolute determination we obtain the coefficient for 

 every point in the median range by dividing the activity by 

 the value of the factor on the physiological scale. 



The general law of photosynthesis for variation of the 

 individual factors of temperature, of C0 2 -concentration, of 

 light and of tonic condition is expressed by 



- is a constant ; -^ is a constant ; — is a constant ; 



— is a constant. 

 P 



The change in photosynthetic activity under the varia- 

 tion of one factor is found from the laws of photosynthesis 



