24 CHEMISTRY OF PLANT LIFE 



and action have not yet been discovered, come into play. It is a 

 noteworthy fact, however, that the mechanics of this apparently 

 simple chemical change, upon which the whole nutrition of the 

 plant depends, and which furnishes the whole animal kingdom, 

 including the human race, with so large a proportion of its food 

 supplies, is as yet wholly unknown. 



It is the common practice to represent the whole results 

 of the photosynthetic action by the empirical equation 



6H 2 0+6C0 2 = C 6 Hi 2 6 +60 2 ; 



but here again the only value to be attached to such an algebraic 

 expression is that it accurately represents the gaseous exchange of 

 carbon dioxide and oxygen involved in the process. Certainly, 

 it throws no light upon the nature of the process itself. 



THEORIES CONCERNING PHOTOSYNTHESIS 



The many theories which have been advanced concerning the 

 nature of the chemical changes which are involved in photosyn- 

 thesis have served as the basis for much experimental study of the 

 problem. The following brief summary will serve to point out 

 the general trend of these investigations and the present state of 

 knowledge concerning the chemistry of photosynthesis. 



Von Baeyer, in 1870, advanced the hypothesis that the first 

 step in the process is the breaking down of carbon dioxide into 

 carbon monoxide and oxygen and of water into hydrogen and 

 oxygen; that the carbon monoxide and hydrogen then unite to 

 produce formaldehyde, which is immediately polymerized to form 

 a hexose. These theoretical changes may be represented by the 

 following equations: 



|CO 2 =CO+0 

 \H 2 O =H 2 +O 



2. H 2 +CO = CH 2 O 



3. 6(CH 2 0) = C 6 H 12 6 



In the investigations and discussions of this hypothesis, it 

 has been ascertained: first, that carbon monoxide has never been 

 found in the free form in plant tissues; second, that when Tropaeo- 

 lum plants were surrounded with an atmosphere in which there 



