PHYSIOLOGICAL STUDIES 69 



between fluorescence and photochemical reaction. Another 

 example of a change in rate of photosynthesis being accom- 

 panied by an opposite change in fluorescence intensity can 

 be observed during a steady state of photosynthesis. If the 

 temperature of a leaf is lowered, after an initial transitory 

 'burst' of fluorescence, the fluorescence intensity remain^, 

 higher corresponding to the lowered rate of photosynthesis. 

 The existence of such a 'burst' phenomenon together with 

 a number of other observations led Franck (1949) to doubt 

 the correctness of this simple view of 'anti-parallelism'. 

 Franck, French, and Puck (1941) investigated the variation 

 in fluorescence yield as a function of light intensity. At low 

 light intensities the yield was at first constant but as the 

 intensity was increased the fluorescence yield increased 

 until it finally attained a new steady value some 70% 

 greater. The intensity at which the higher yield is attained 

 may be higher than that at which photosynthesis approaches 

 its 'saturation' rate, as in Chlorella, or it may be about the 

 same intensity, as in Hydrangea. The transition from the 

 lower to the higher yield occurs at lower light intensities 

 the lower the concentration of carbon dioxide or in the 

 presence of cyanide. Similarly, as shown by Wassink et al.y 

 in the purple photosynthetic bacteria the transition occurs 

 at lower intensities the lower the concentration of hydrogen 

 donor. An exception was found however in the diatom 

 Nitzschia in which Wassink and Kersten (1946) observed a 

 decrease in fluorescence yield with increasing light intensity. 

 It thus became apparent, as Franck has frequently pointed 

 out, that factors other than a simple competition between 

 fluorescence and photosynthesis must be considered. 

 Fluorescence can only compete with the primary photo- 

 chemical process in photosynthesis. Some factors, however, 

 e.g. cyanide, affect the over-all process by changing the rate 

 of a dark process and thus must affect the concentration of 

 intermediates associated with the chlorophyll. This might 

 easily result in a change in the rates of reactions resulting in 

 dissipation of energy, e.g. as heat. Since these involve a large 

 proportion of the energy available, a small percentage change 

 in their activity will result in a large percentage change in 



