CHAPTER XIX 



THE DAILY VARIATION IN PHOTOSYNTHETIC ACTIVITY 



Effect of environmental changes on rate of photosynthesis — Variation 

 of light and temperature — Extreme sensitiveness of the plant to 

 external change — Method of simultaneous determination of photo- 

 synthetic activity, intensity of light and temperature — Complete 

 apparatus for the three determinations — The automatic records of 

 activity throughout the day — The effect of variation of intensity of 

 light — Effect of variation of temperature — The combined effect of light 

 and temperature — Resultant curve of variation. 



The change in the rate of photosynthesis under varia- 

 tion of one factor at a time has been studied in detail 

 in the previous chapters, including the effects of C0 2 - 

 concentration, of temperature, of light, and of the factor of 

 tonicity. The coefficient and the characteristic curve for 

 each factor have been determined, so that it is now possible 

 to obtain by simple calculation the value of photosynthetic 

 activity under variation of one or the other of these factors. 



There now remains the important question as to whether 

 the effect of any one of these factors is independent of those 

 of the others, or whether there is a mutual interaction 

 between the factors ; and further, to ascertain if a law can 

 be formulated which would enable us to calculate the result- 

 ing effect which would arise from the simultaneous indepen- 

 dent variation of different factors. In the present chapter 

 the inquiry is limited to the question of the effect of changes 

 in the natural environment on the photosynthetic activity 

 of the plant. The experiments described had hitherto been 

 carried out inside the laboratory, all the factors being main- 

 tained absolutely constant except the one the effect of whose 

 variation was to be determined. But out in the open two 

 important factors, light and temperature, are changing from 

 moment to moment. 



