CHAPTER XXVI 



THE PHYSIOLOGICAL SCALE AND THE LAW OF 

 PHOTOSYNTHESIS FOR THE DIFFERENT FACTORS 



Different variable factors in photosynthesis — The characteristic curve — 

 Scale of measurement — Thermometricand absolute zero — Physiological 

 scale and the determination of the zero point — Determination of the 

 coefficient and the law of variation of photosynthesis under variation 

 of temperature — C0 2 -coefficient and the photosynthetic law for varia- 

 tion of C0 2 -concentration — Coefficient for light and the law of photo- 

 synthesis for varying intensity of light — Photosynthesis under varia- 

 tion of tonic condition. 



Photosynthetic activity undergoes change under the varia- 

 tion not of one but of many factors. It is therefore quite 

 impossible to obtain any consistent results unless we succeed 

 in ascertaining the effect of each individual factor, as well as 

 the combined effects of the different factors. Without this 

 the results would be as anomalous as those obtained by an 

 investigator in physics who, ignorant of the effect of pressure, 

 attempted to determine the changing volume of a given 

 quantity of gas under variation of temperature. It was 

 the study of the separate effects of pressure and of tempera- 

 ture, and then of their combined effect, that led to the 

 formulation of a law which enables us to calculate the 

 volume of a gas under variations of temperature and 

 pressure. 



The principal factors which modify photosynthesis are 

 (a) the intensity of light, (b) the C0 2 -content of the medium, 

 (c) the temperature, and (d) the tonic condition of the plant ; 

 the problem of the effect of simultaneous variation of 

 different factors on photosynthesis therefore resolves 

 itself into the determination of the quantitative relation 

 between the variations of each of the above factors and 



