ASSIMILATION OF CARBON BY PLANTS 



177 



color peculiar to it. With further increase in concentration, the 

 green rays also are completely absorbed. Finally, only the dark- 

 reel rays remain visible. A very thick layer of a chlorophyll 

 solution is no longer green but brick red. 



Besides the selective absorption of light energy, chlorophyll 

 possesses another important optical property, fluorescence. In 

 reflected light, it appears blood red, owing to the fact that a part 

 of the rays falling on it are transformed and reflected with an 



700 B 



Fig. 53. — Merging of bands in the absorption spectrum of chlorophyll with 

 increasing thickness of the solution layer (after WiUstdtter and Stoll). 



altered wave length. The faculty of fluorescence points to a 

 considerable photochemical activity in chlorophyll. 



The complete mechanism is not clear by which chlorophyll 

 renders the radiant energy that is absorbed by it effective in the 

 process of decomposition of carbon dioxide. It has been assumed 

 that chlorophyll plays the role of a sensitizer; i.e., it has the propn 

 erty of rendering the energy of the absorbed rays effective for 

 carbon dioxide decomposition, which cannot be accomplished 

 by visible wave lengths but only by the ultraviolet. According 

 to modern photochemistry, radiant energy shows some char- 

 acteristics that allow it to be absorbed in definite units called 

 ''quanta." Every substance subjected to photochemical decom- 

 position absorbs the radiant energy in a strictly defined quantity, 

 1 quantum or more per molecule. The quanta are constant 



