44 THE SYNTHESIS OF CARBOHYDRATES 



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-men- 

 tioned 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 chlorophyll. 

 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. But the same phenomenon does not 

 necessarily occur in seedlings. In a further communication, 

 Briggs f has shown that in Helianthus, Acer and Cucurbita, for 

 example, where the cotyledons serve as organs of storage and, 

 on becoming green, are the first organs of carbon assimilation, 

 there is no lag between greening and photosynthesis when 

 light or temperature is limiting. On the other hand, in 

 Phaseohis, Ricinus and Zea, plants in which there is formed a 

 carbon-assimilating organ distinct from the storage organ, the 

 photosynthetic power is not developed until some time after 

 germination, and in natural growth conditions the lag ob- 

 tains. It will be remembered that the cotyledons of Phaseolus 

 are entirely organs of storage, the first foliage leaves being the 

 first organs of assimilation ; similarly in Zea, the first foliage leaf 

 is concerned with photosynthesis, the cotyledon being occupied 

 with the absorption of food from the adjacent endosperm ; 

 in Ricinus, on the other hand, the cotyledons are embedded in 

 the endosperm and afterwards expand to form the first carbon 

 assimilating organs. In these so specialized plants, the com- 



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

 f Ibid., 1922, 94, 12. 



