FLUORESCENCE-TIME CURVES 



1395 



The association of two-maximum curves with increased carbon dioxide 

 concentration was confirmed by Franck, French and Puck (1941), as shown 

 by figure 33.38, obtained with Hydrangea leaves in 1% CO2. In this case, 

 the minimum occurs 1.5 minutes, and the secondary maximum 3 minutes 

 after the beginning of iUumination. It seems that different species require 

 different carbon dioxide concentrations to develop the second fluorescence 

 wave — some show it at 0.1% CO2, others only at 1% or more. It has been 

 mentioned above that, in ChloreUa, a very pronounced second maximum 

 was observed by McAhster and Myers even in ordinary air (c/. fig. 33.22B 

 and C). Kautsky and Franck (1943) found that Ulva lactuca shows no 

 second maximum in carbon dioxide-free medium, and a pronounced second 

 maximum in 0.1% CO2. 



01 2 3 4 5 6 7 

 TIME, min. 



Fig. 33.38. Fluorescence curve of Hydrangea in 1% COz 

 (after Franck, French and Puck 1941). A second maximum 

 occurs at about 3 min. / = 0.82 X 10* erg/cm." sec. 



The occurrence of the second fluorescence wave at the higher carbon 

 dioxide concentrations, and the fact that it is often associated with a mini- 

 mum of carbon dioxide absorption, suggests association with the carbon 

 dioxide gush. However, as mentioned before, there is no direct evidence 

 that the gush is accompanied by changes in fluorescence. 



(c) Poisons 



Kautsky and Hirsch (1935) noticed that the decay of fluorescence is 

 retarded by the presence of cyanide. Franck and Wood (1936) confirmed 

 this and found that the decay period, BC, is lengthened despite a decrease 

 in the height of peak B. According to Franck, French and Puck (1941), 

 this effect becomes apparent only at very high concentrations of the poison 

 (c. g., 2% HCN in air). 



A detailed study of the cyanide effect on two-maximum curves of 

 ChloreUa was made by Wassink and Katz (1939). Figure 33.39 shows the 



