1400 



INDUCTION PHENOMENA 



CHAP. 33 



latter to shift the search for the effect of oxygen on fluorescence to very low- 

 concentrations (in a hope of substituting a weakly dissociable complex, 

 XO2, for free ox^^gen as energy carrier). Marked changes in the fluores- 

 cence curves were, in fact, found in leaves almost completely deprived of 

 oxygen; but whether these curves were caused, as alleged, by the absence 

 of oxygen during the induction period (and not by anaerobic incubation) 

 can be argued, since there is no way of depriving the cells of oxygen except 

 by sweeping them with an oxygen-free gas for some length of time. 



51.8 



51.8 



50 

 40 

 30 

 20 

 10 







20 30 



40 



51.8 



40 



51.8 



10 20 30 40 



TIME, sec 



51.8 



10 20 30 

 TIME, sec. 



Fig. 33.44. Effect of O2 on fluorescence in Ageratum mexicana leaves (after 



Kautsky and Hormuth 1937). 



Kautsky (1939) said the "low Oa-curves" (fig. 33.44) cannot be at- 

 tributed to anaerobic incubation for three reasons : first, because the sweep- 

 ing out with nitrogen lasted only 60-90 minutes; second, because certain 

 types of fluorescence ciu'ves were obtained at definite oxygen concentra- 

 tions independently of the length of the incubation period; and third, be- 

 cause the normal shape of the induction curves was restored within 1 

 minute upon the admission of oxygen. None of these arguments is con- 

 vincing: The duration of anaerobic incubation needed to produce "after 

 effects" varies widely from species to species (as mentioned in section AG) ; 

 the degree — perhaps even the character — of the anaerobic metabolism 

 may depend on oxygen concentration; and 1 minute may be sufficient time 

 to burn up the fermentation products obstructing chlorophyll. 



Because of these considerations, the significance of the extensive collec- 



