346 VARIOUS CHEMICAL AND PHYSICAL AGENTS CHAP. 13 



"reduction centers" is similar to that of the chlorophyll molecules. 

 However, Arnold found that, even after prolonged irradiation and 

 considerable deactivation, chlorophyll is still unbleached and apparently 

 unchanged chemically. This reminds us of observations on the "CO2 

 acceptor" described in chapter 8; both the radioactive indicator method 

 (page 204) and the study of the "pickup" phenomenon (page 207) have 

 indicated that this acceptor, although not identical with chlorophyll, is 

 present in a concentration approximately equal to that of the green 

 pigment. We may thus tentatively ascribe the sensitivity of photo- 

 synthesis to ultraviolet light to the destruction of the carbon dioxide 

 acceptor. The observation of Ruben, Kamen, and Hassid (1940) that 

 ultraviolet light (X = 253.6 mn) destroys the capacity of Chlorella cells 

 for taking up radioactive carbon dioxide in the dark fits well into this 

 picture. 



In a second paper, Arnold compared the effects of ultraviolet radia- 

 tions on the rates of photosynthesis in continuous and flashing light, and 

 found that both are reduced in the same proportion. The bearing of 

 this result on the theory of the kinetic mechanism of photosynthesis will 

 be discussed in volume II, chapter 34. 



2. Electric Fields and Currents 



Some rather unreliable information has been gathered on the effect 

 of electric currents and potentials on photosynthesis. Thouvenin (1896) 

 claimed that the passage of direct current through Elodea stimulates pho- 

 tosynthesis. Pollacci (1905, 1907) and Koltonski (1908) observed that 

 the effect depends on the direction of the current, stimulation occurring 

 when the apex of the shoot was positive and inhibition when it was nega- 

 tive. Chouchak (1929) asserted that corn leaves assimilated more car- 

 bon dioxide than ordinarily when they were positively charged, and less 

 when the charge was negative. Gorski (1931) found that, if Elodea 

 sprigs are made to assimilate in water through which a direct current 

 (0.2-0.8 ma./cm.2) is passed, no change in the rate of oxygen evolution 

 could be observed in 0.5% potassium acid carbonate, calcium nitrate, 

 potassium chloride, magnesium sulfate, or sodium dihydrogen phosphate 

 solutions, but a slight increase in rate occurred in an ammonium sulfate 

 solution. 



3. Radioactive Rays 



Henrici (1921) noticed the effect of radioactive radiations on the rate 

 of photosynthesis. In her experiments, the plants were protected from 

 the direct action of the rays so that the effect had to be ascribed to the 

 ionization of the air. In weak light, ionization sometimes increased the 

 rate of photosynthesis by as much as a factor of 4; the effect disappeared 



