CHAPTER V 



PHOTOSYNTHESIS UNDER INCREASING INTENSITY OF 



LIGHT 



The unit intensity of light — Photosynthetic curve under increasing inten- 

 sity of light from Pointolite — Period for physiological adjustment to 

 changed intensity of light — Experiments with spring- and winter- 

 specimens — Formula for determination of photosynthetic activity 

 under changed intensity of light — Determination of the coefficient 

 by the Differential Method — Ratio of activities in spring and winter. 



As a test of the simplicity and high accuracy of the new 

 method, I will explain its working in the determination of 

 the effect of increasing intensity of light on photosynthesis. 

 We have to determine, among other things, the characteristics 

 of the photosynthetic curve, the coefficient for light, the 

 comparative effectiveness of artificial light and sunlight, 

 and the relation between the quantity of light and the 

 amount of photosynthesis. 



The essential conditions which must be secured for 

 this investigation are : (i) that the intensity of light should 

 remain constant during the period of the experiment, and 

 (2) that it should be capable of definite and known varia- 

 tion. The light from the sky does not remain constant, 

 nor can it be varied in a definite manner. The choice of 

 light is therefore restricted to artificial light or to sunlight. 

 I have already explained how the light given out by gas- 

 mantles is most unsuitable for quantitative measurements. 

 I therefore make use of Pointolite, the advantage of which 

 has already been referred to. It does not produce any 

 variation of temperature, and the intensity of light can be 

 easily varied in a quantitative manner. The effect of sun- 

 light will be described in a subsequent chapter. 



The unit intensity of light. — In photometry there is, 



