32 THE SYNTHESIS OF CARBOHYDRATES 



transformation of energy in the photosynthetic phase of carbon 

 assimilation. 



INTERNAL FACTORS. 



CHLOROPHYLL.* Plant physiologists for long have recog- 

 nized that the intensity of carbon assimilation must be de- 

 pendent on the chlorophyll and its amount ; it is, however, 

 but recently that the problems involved have been critically 

 examined. Irving,f who used the leaf's carbon dioxide of 

 respiration in her experiments, found by gasometric methods 

 that etiolated leaves, either when they are orange-yellow or 

 when they have attained a considerable degree of greenness, 

 do not possess any appreciable power of synthesizing carbon 

 dioxide. If there be any photosynthetic activity, it cannot 

 be greater than one-tenth part of respiration nor come within 

 I per cent of the activity subsequently developed. Carbon 

 assimilation begins when the leaves are fully green and 

 develops very quickly ; wherefore it follows that the first 

 origin of this function is not correlative to the amount of 

 chlorophyll produced, or, in other words, that the amount of 

 chlorophyll is not a conditioning factor in the early stages 

 of carbon assimilation. 



Willstatter and Stoll J were the first to make quantitative 

 estimations of the amount of chlorophyll in leaves, by the 

 methods already outlined. Also they measured the amount 

 of carbon assimilation of the leaves of different plants and 

 of the same plant in different conditions normal, etiolated, 

 autumnal, and so on and thus arrived at the assimilation 

 number which is the ratio between the amount of carbon 

 dioxide assimilated per hour and the chlorophyll content 

 both expressed in milligrams. A selection of the values ob- 

 tained are tabulated below. 



Willstatter and Stoll, whose experimental methods were 

 similar to Irving' s, with the chief exception that they used 

 a 5 per cent concentration of carbon dioxide, found that 



* A general account of chlorophyll, its chemistry and constitution, will 

 be found in Vol. I. 



t Irving : " Ann. Bot.," 1910, 24, 805. 



I Willstatter and Stoll : " Ber. deut. chem. Gesells.," 1915, 48, 1540. 



See Vol. I., Section on Pigments. 



