264 PHOTOSYNTHESIS 



HO — C — OH HO — C — OH 



HO — C O > HO — C OH +0 > 



II I II 



O OH O 



HO — C = 



I + H2O 



HO — C = 



The oxalk acid may in turn condense in the same manner with car- 

 bonic acid to form mesoxalic acid, this by decarboxylation yielding glyoxylic 

 acid. Condensation of this with carbonic acid and decarboxylation yield- 

 ing glyceric acid and in this manner successively to hexonic acids and 

 hexoses. 



b. Formaldehyde. The Baeyer Theory. 



As a matter of fact most of the theories of the mechanism of photo- 

 synthesis, with the exception of those of Liebig, Ballo and of Tschelinzeff, 

 assume formaldehyde to be the final stage of the reduction. Difference 

 of opinion centers largely around the question as to whether the reaction, 

 carbon dioxide (or carbonic acid) -^ formaldehyde is accomplished in one 

 step or whether there are intermediate products. The formaldehyde theory 

 has been so very much in the center of the discussions of the mechanism 

 of photosynthesis that some analysis of the facts regarding the development 

 of this theory seems in place, though this can be done here only in the 

 briefest form. 



Already in 1863 Berthelot ^*^ expressed the idea that glucose was an 

 alcohol-aldehyde, that it was the first aldehyde derived from mannitol. 

 Berthelot also regarded as possible the dissociation of carbon dioxide into 

 carbon monoxide and oxygen and of water into hydrogen and oxygen, 

 that the carbon monoxide and hydrogen in the "nascent" condition were 

 capable of reacting and that the product, CHO (old atomic weights) now 

 written CHoO, was the beginning of the synthetic process. This idea was 

 adopted by Boussingault ^' who emphasized that with the formation of 

 carbon monoxide and hydrogen the proportion of carbon, hydrogen and 

 oxygen as existing in glucose was given. Thus Berthelot and Boussingault 

 may be considered the first proponents of the formaldehyde theory. 



At the same time it must be realized that the chemistry of the carbo- 

 hydrates was at this period only imperfectly worked out. Schorlemmer ^^ 

 in 1868 showed that the hydrocarbon obtained from the reduction of 

 mannitol was normal hexane and a few years later established that dipropyl 

 obtained from normal propyl iodide was identical with the hexane obtained 

 from mannitol. The relation of glucose to normal hexane was later defi- 



" Berthelot, "Sur les principes sucres," Supplement, 1862. "Legons sur les 



methodes general de svnthese en chimie organique," Paris, 1864, p. 180. 

 "Boussingault, Agronomic, 4, 301, 399 (1868). 



"Schorlemmer, Ann. Chcm., 147, 220 (1868) ; 161, 275 (1872). 



