48 THE SYNTHESIS OF CARBOHYDRATES 



The assimilation number is the ratio of the number of milli- 

 grams of carbon dioxide assimilated to the number of milligrams 



CO 



of chlorophyll in the assimilating leaf surface, i.e. 



chlorophyll 



In leaves in which the chlorophyll content is small, the ratio 

 will be higher than in those in which it is large. In such 

 leaves the proportion of enzyme will be comparatively large, 

 with the result that conditions which favour enzyme activity, 

 such as temperature, will have but little effect upon the 

 assimilation number ; increased illumination, however, should 

 assist the chlorophyll which is comparatively deficient ; on the 

 other hand, in cases where the chlorophyll is in excess, only 

 those conditions which can assist enzyme activity will have any 

 effect upon the assimilation number. Experimental results 

 entirely bear out these theoretical considerations and so support 

 their correctness. 



In this connection further reference to the views of Baly and 

 Heilbron upon the formation of carbohydrates in the plant 

 may be made. The authors point out that to effect the change 

 represented by the equation 



CO 2 + H 2 O = HCHO + O 2 



energy must be supplied since the energy contents of the 

 products are greater than those of the reactants. Since an 

 aqueous solution of carbon dioxide is unable to absorb visible 

 light but will absorb ultra-violet light of short wave length, it is 

 the latter kind of radiation which will normally be required to 

 effect the change. Since, however, ordinary sunlight contains 

 but little of such short wave-length light, it is assumed that the 

 function of the chlorophyll is to absorb the visible light and to 

 radiate infra-red energy of a frequency identical with that of 

 the carbon dioxide, which can then absorb it ; thus the photo- 

 synthetic process can proceed in the absence of the ultra-violet 

 light normally required for the process. In this capacity the 

 chlorophyll is said to act as a photocatalyst. The authors 

 have shown experimentally that while formaldehyde is synthe- 

 sized from carbon dioxide and water by short wave-length 

 ultra-voilet light, it is polymerized to carbohydrate by ultra- 

 violet light of long wave length. By adding to a solution of 

 carbon dioxide some substance such as sodium phenpxide. or 



