UNKNOWN FACTORS 47 



was found that photosynthesis began immediately on exposure 

 to light ; at first the removal of carbon dioxide was rapid, 

 then more slowly until a maximum was reached when the rate 

 remained constant. These authors consider that under the 

 influence of light some catalyst is developed which facilitates 

 the photosynthetic process. Briggs concludes that the ac- 

 tivity of the photochemical phase of the carbon assimilation 

 mechanism in a seedling, as compared with that of more mature 

 leaves, is in some way limited : the photochemical phase 

 depends for its intensity not only on the chlorophyll but also 

 upon some other factor which increases with age during the 

 early stages of leaf development. 



The observations of Molisch * that leaves after slow drying 

 or freezing, treatments that do not destroy enzymes, retain 

 some power of carbon assimilation and can evolve oxygen, 

 whereas the same leaves killed by rapid drying or immersion 

 in hot water show no such power, again indicates the partici- 

 pation of an enzyme in the process. 



There is thus strong evidence of the existence of a control- 

 ling factor, probably enzymic, essential in the earlier phases 

 of carbon assimilation. Briggs f also draws attention to the 

 probability that the amount of " reactive chlorophyll surface," 

 which is not necessarily the same as the total chloroplast 

 surface, is an important internal factor and anything which 

 influences the amount of this surface, such as age, iron chlorosis 

 and lack of essential elements in the raw food material, is 

 indirectly a factor controlling the rate of carbon assimilation. 

 An increase or decrease in the reactive chlorophyll surface will 

 correspondingly increase or decrease the carbon assimilation 

 when a given temperature is limiting and also, in certain 

 conditions, when a definite light intensity and partial pressure 

 of carbon dioxide are limiting. This conception is an outcome 

 of measurements of the carbon assimilation of Phaseolus grown 

 in culture solutions lacking such essential elements as potas- 

 sium, magnesium, iron or phosphorus ; in these circumstances 

 the carbon assimilation is always less than in plants grown in 



* Molisch : " Zeit. fur Bot.," 1925, I7» 577- 

 f Briggs : " Proc. Roy. Soc," B, 1922, 94, 20. 



