114 LIGHT, VEGETATION AND CHLOROPHYLL 



a process which is not properly speaking photochemical. 

 Because Ught does not play a direct part in it, it is called the 

 dark reaction (or the Blackman reaction). 



The occurence of the dark reaction is in accordance 

 with the results of the study of the law of the minimum and 

 of the observation of photosynthesis in intermittent light. 



By this law, the rate of photosynthesis is hnked with the 

 various factors which can influence it and is regulated by the 

 lowest; since all the factors have their part to play, the least 

 eff'ective retards the whole operation. Thus, when the tempera- 

 ture is adequate and the carbon dioxide abundant, but the 

 illumination is poor, variations of the first two factors are 

 unimportant because the rate of assimilation depends only 

 on the illumination. On the other hand, in strong illumina- 

 tion, the rate of photosynthesis depends on the CO2 content 

 of the air, or on the temperature, or on both, according to 

 which are deficient. 



Although the law of the minimum is not universally vaUd, 

 it best expresses the known facts. It is usually explained as 

 follows: the action of fight is independent of any other factor; 

 on the other hand, the dark reaction, fike every purely chemical 

 reaction, is accelerated by a rise of temperature and its speed 

 depends on the concentration of the substances taking part in it. 



The molecules activated by fight can also be classed 

 among these participants and their concentration obviously 

 depends on the illumination. Regarded in this way, the law 

 of the minimum is easily interpreted. Briefly, the "dark 

 reaction" includes the whole of the chemical processes 

 initiated by the presence of molecules activated by fight. 



Let us turn now to experiments on photosynthesis in 

 intermittent fight, or, more precisely, those in which the 

 plants are irradiated by very brief luminous flashes (j^„ 

 second) separated by dark intervals of variable duration. 



Each flash brings a certain quantity of luminous energy, 

 which is always the same. As the experimental conditions 

 remain invariable, we should expect the products of photo- 

 synthesis to be formed in a quantity proportional to the 



