UNKNOWN FACTORS 



33 



leaves with but a small portion of their full chlorophyll 

 content developed can assimilate to a measurable degree. 



WILLSTATTER AND STOLL'S ASSIMILATION NUMBERS. 



This conclusion is contrary to that of Irving, a difference 

 probably due to the fact that Irving used young leaves whilst 

 Willstatter and Stoll employed older and sometimes much 

 older material. This explanation is due to Briggs,* in whose 

 memoir a critical examination of the work of the above- 

 mentioned authors will be found ; this author demonstrates that 

 the age of a leaf and the lapse of time from the greening to 

 the measurement of photosynthetic activity are all important. 

 If a leaf is cut from a seedling in the dark at an early stage 

 in its development and partly greened by exposure to light, 

 its photosynthetic activity will be zero or very small ; if, on 

 the other hand, the same procedure is repeated with a similar 

 leaf from the same plant after an interval of a few days, the 

 photosynthetic activity will be strongly marked. Briggs 

 confirms Irving's main conclusions : a young green leaf may 

 show no or very little carbon assimilation and the power 

 of photosynthesis lags behind the development of chloro- 

 phyll. This power increases with age whether the leaf be 

 in the dark or in the light even though there be no concurrent 

 increase in the chlorophyll content. 



THE UNKNOWN FACTOR. The fact that the temperature 

 relations of carbon assimilation are those of a chemical rather 

 than a photochemical reaction indicates the presence of an 

 internal factor, independent of the chlorophyll and associated 

 rather with the protoplasm, which controls the rate of carbon 



* Briggs : " Proc. Roy. Soc.," Lond., B. 1920, 91, 249. 

 VOL. II. 3 



