LIGHT AND VEGETATION 117 



It is possible that each granum represents one of Emerson's 

 functional units, but it is also possible that there is no relation 

 between these two kinds of groups, since one granum may, 

 for example, attack several m.olecules of COg and constitute 

 several functional units and another granum may be inactive. 



An hypothesis has been put forward that the molecules of 

 chlorophyll may be arranged in monomolecular layers oriented 

 at the surface of contact between a lipid phase, towards which 

 the hydrophobe phytol groups would be turned, and a 

 proteinic phase towards which would be directed the tetra- 

 pyrrol groups, which are hydrophile. 



It is certain, in any case, that chlorophyll manifests its 

 photosynthetic activity only in the particular structure of the 

 chloroplast, without which it is powerless. 



The Reducing Reactions of Photosynthesis 



It is doubltess premature to try to describe in detail the 

 chemical transformations of which the final result is inter- 

 preted simply by the formula: 



CO2+H2O — ^H CH O+O2 



a formula which expresses a reduction of CO 2 and of H2O, 

 i.e., an extraction of oxygen. 



Nevertheless, several theories have been propounded, 

 although they have not been proved experimentally. 



As early as 1918 Willstatter and Stoll advanced the 

 following hypothesis: as carbon dioxide and water are fixed 

 to the atom of magnesium with a molecule of chlorophyll, 

 the luminous energy causes a displacement of the atoms of 

 oxygen and the formation of a structure of peroxide : 



/° 



/ 



— O— C— OH would become — O- CH 



II \ 



O ' \o 



The peroxide group easily loses an atom of oxygen. 



