ASSIMILATION OF CARBON BY PLANTS 185 



with the formation from CO 2 and H2O of those substances in 

 which hydrogen and oxygen are in the same proportion as in 

 water. 



The formaldehyde theory advanced by Baeyer in the seventies 

 of the last century fully meets these requirements. According 

 to this theory, the first product of assimilation of carbon dioxide 

 and water is formaldehyde. 



CO2 + H2O -> H-CHO + O2. 



The formaldehyde obtained may then be polymerized under 

 the influence of alkalies into a hexose as w^as first observed by 

 Butlerow. 



6HCH0 -^ C6H12O6. 



Against Baeyer's theory, arguments were advanced that 

 formaldehyde is a very toxic compound; that it is used with 

 great difficulty by plants for the synthesis of carbohydrates, and 

 that therefore it can hardly be considered as an intermediate 

 product of assimilation. These objections, however, could not 

 prevent this theory from being generally accepted; for formalde- 

 hyde as an intermediate product does not accumulate in concen- 

 trations that could be toxic, while its nonavailability for 

 synthesis when introduced into the cell artificially may depend 

 upon its presence in too great concentration. 



But Baeyer's theory has another weak point. It does not 

 explain the most difficult transformation occurring during photo- 

 s>mthesis, viz., the change of carbon dioxide, 0=C=0, into the 

 highly reduced compound, formaldehyde. 



^\ 



>C=0 



H/ . 



It is in this reduction that light energy is effective. Since 

 the photochemical reduction of carbon dioxide requires the 

 presence of a compound that can absorb light energy, such as 

 chlorophyll, and taking into consideration recent chemical data, 

 Willstatter revised the scheme given by Baeyer. According to 

 this idea, first a hydrate of carbon dioxide is combined with 

 magnesium of the nucleus of chlorophyll according to the fol- 

 lowing procedure: 



