LIGHT CURVES OF FLUORESCENCE 1063 



0.1%) a strong stimulation of fluorescence. The effect of sodium azide 

 differs from that of either potassium cyanide or hydroxylamine in two 

 respects. Fluorescence is affected (c/. fig. 28.47) even in the absence of 

 reductants; and the typical effect is a decline, rather than a rise of the 

 yield of fluorescence. 



(/) Ion Concentration 



In Chromatium, the yield of fluorescence can be affected by changes in 

 pH. According to Wassink and co-workers (1942) the sign of this effect 

 depends on whether molecular hydrogen or thiosulfate is used as hydrogen 

 donor. (We saw on page 952 that the same is true of the influence of pH 

 on the yield of carbon dioxide reduction by these two reductants.) This is 

 illustrated by figure 28.48, page 1065. 



No experiments are available on the effect of other cations or anions on 

 the yield of fluorescence of green plants or purple bacteria. 



{g) Narcotics 



Narcotics were found by Kautsky and Hirsch (1935) to increase the 

 steady fluorescence of aquatic plants; this result was confirmed by Was- 

 sink, Vermeulen, Reman and Katz (1938), cf. fig. 28.49. According to 

 Franck, French and Puck (1941), very high concentrations of carbon di- 

 oxide (e. g., 20%) produce a similar effect. (It was stated in chapter 13 

 that the effect of excessive concentrations of carbon dioxide on photosynthe- 

 sis resembles narcotization.) 



The phenomenon was also studied in purple bacteria, by Wassink, Katz 

 and Dorrestein (1942). Figure 28.50 shows typical results obtained in the 

 presence and in the absence of reductants. The picture is similar to that 

 with sodium azide: In the absence of reductants, the addition of increas- 

 ing amounts of ethylurethan causes a progressive quenching (rather than 

 stimulation) of fluorescence (although the effect is reversed at > 1.5% ure- 

 than, at least below 15 kerg/cm.^ sec). In the presence of reductants, 

 moderate quantities of urethan have little if any effect on the fluorescence 

 curve, while quantities > 2% have a strong enhancing effect. 



(h) Oxygen 



Kautsky, whose theory of an exclusive transfer of excitation energy to 

 oxygen was described in Volume I, (page 514), was naturally interested in 

 the quenching of the fluorescence of leaves by oxygen. However, neither 

 Kautsky, Hirsch and Davidshofer (1932), Kautsky and Hirsch (1935) nor 

 Wassink, Vermeulen, Katz and Reman (1938) could find any distinct in- 

 fluence of changes in the external oxygen concentration (between 1 and 



