1390 



INDUCTION PHENOMENA 



CHAP. 33 



cates that an "optimum" light intensity exists for the enhancement of the 

 second maximum, D. 



(c) Temperature 



As mentioned before, Kautsky and Hirsch (1931) noticed that the 

 "upward slope," AB, is independent of temperature, whereas the "down- 

 ward slope," BC, is steeper the higher the temperature. Kautsky and 

 Spohn (1934) measured this effect visually and found the period AB to be 

 constant between and 40° C, and the period BC to decrease from 800 

 seconds at 5° to 20 seconds at 35° (in Pelargonium zonale). 



I 2 3456789 10 

 TIME, min. 



Fig. 33.32. Effect of temperature on rate of decay of 

 fluorescence outburst in Hydrangea (after Franck, French 

 and Puck 1941). / = 44 X 10* erg/cm.^ sec. 



According to Franck and Wood (1936), the fluorescence decay, BC, is 

 exponential at 25, 20 and 12° C, and can be attributed to a monomolecular 

 reaction with a temperature coefficient of approximately 2. The curves ob- 

 tained at low temperatures {e. g., 3°) show an accentuation of the second 

 wave. According to Franck, French and Puck (1941), the decay in Hy- 

 drangea lasts for 3 minutes at 23°, and for more than 10 minutes at 0° (fig. 

 33.32). 



The curves obtained by Kautsky and Marx (1937) with different leaves 

 confirmed the enhancement of the secondary wave by decreasing tempera- 

 ture. Leaves of different species showed analogous shapes at different 

 temperatures, e. g., the curve of Piper amplum at 25° was similar to that of 

 Ageratum mexicanum at 35°. 



Figure 33.33 shows the influence of temperature on fluorescence 

 curves of Ulva lactuca. They confirm that decrease in temperature in- 

 creases the heights of both the first and the second fluorescence peaks, and 

 shifts them further from the beginning of illumination. At 0°, the second 

 peak is shifted beyond the 3 minute registration limit. 



