370 PHOTOSYNTHESIS 



has received very little thorough study and constitutes one of the least 

 explored fields in the whole subject of photosynthesis. 



The theories which have been advanced to explain the function of 

 chlorophyll in photosynthesis- are of two types. In one type the physical 

 properties of chlorophyll, such as the absorption of light and fluorescence 

 are chiefly responsible for its action ; in another type, certain chemical 

 properties or reactions of chlorophyll are also made use of. 



The discovery of the fact that chlorophyll "sensitized'' a photographic 

 plate to the longer wave lengths lead Timiriazefif ^* to propose the theory 

 that chlorophyll plays a similar role in photosynthesis. Carbon dioxide 

 as well as carbonic acid and its salts do not absorb light of the visible 

 spectrum ; photochemical reaction in this portion of the spectrum is there- 

 fore excluded, unless the system can be "sensitized" to these rays. Al- 

 though this theory does not throw much light on the kinetics of the 

 photosynthetic reaction, it has been accepted by many. One of the chief 

 difficulties with the theory is that it merely supplies a term and, in the 

 absence of preciser knowledge regarding the mechanism of optical sen- 

 sitizers, has not aided greatly in elucidating the chemical reactions of 

 photosynthesis.^^ 



Tswett ^^ considers that the fluorescent light of chlorophyll is the main 

 source of energy in the photosynthetic process. He assumes that some 

 compound of carbon dioxide is capable of absorbing these rays. He also 

 assumes that fluorescence is due to a reversible reaction of tautomeric 

 molecules. Light is absorbed by some one of the chlorophyll components 

 which is thereby changed into another form. When the original molecule 

 is reformed, the energy taken up in the first change is radiated as photo- 

 luminescence or phosphorescence and it is this light which is used in 

 photosynthesis. The function of the chlorophyll would thus be the con- 

 version of polychromatic light into monochromatic red light. Tswett 

 assumes that it is really phosi^horescence which is the immediate source 

 of energy for photosynthesis, that there is an interval between the excita- 

 tion (fir?t tautomeric change) and the emission of light (reversal of first 

 tautomeric change ) . In this connection it is interesting to note that War- 

 burg " noticed a higher rate of photosynthesis with intermittent light than 

 with the same light intensity of continuous radiation. This latter observa- 

 tion is in accord with Tswett's predictions. While the theory of Tswett 

 has some very interesting features, it has got us little farther, for it offers 

 nothing as to the mode of action of the phosphorescent rays on the carbonic 

 acid. Stern '^ has pointed out that if Tswett's theory is correct and 



"Timiriazefif, Compt. rend., 96, 875 (1883); 100, 851 (1885); 102, 686 (1886); 

 109, 379 (1889) ; Ann. Set. Nat. Bot. (7), 2, 99 (1885). Hoppe-Seyler, Zeit. physiol. 

 Chem., 3, 339 (1879). Reinke, Bcr. bot. Ges., 1, 419 (1883). Englemann, Bot. 

 Zeitg., 46, 717 (1888). 



■^^Kayser, Handb. dcr Spektroskopic. 4, 1078 (1907). 



°* Tswett, Zeit. pliysik. Chem., 76, 413 (1911). 



"Warburg, Biochcm. Zeit., 100. 262 (1919). 

 • "^ Stern, Ber. bot. Ges., 38, 2,2, (1920). 



