1434 PHOTOSYNTHESIS IN INTERMITTENT LIGHT CHAP. 34 



After these preliminary remarks, we now proceed with the discussion of 

 intermittent hght experiments. 



In a quantitative discussion of the intermittency effect, a basis must 

 first be estabHshed for the comparison of yields in intermittent and con- 

 stant light. Three methods of comparison have been used (c/. fig. 34.5): 



(a) The yield obtained during a certain total period of intermittent 

 illumination (A'' light periods of t* seconds each, and A^ dark periods of ta 

 seconds each) has been compared with the yield produced in the same total 

 time, t = N{tci + i*), by uninterrupted light of equal intensity. We may 

 call the ratio of these two yields the intermittency factor for equal intensity, 

 and equal total time, in. This method of comparison has to be used, e. g., 

 to answer the question: How will periodic interruptions of illumination 

 by a rotating disc affect the yield of photosynthesis of a plant under a light 

 source of constant intensity? 



(6) The yield produced during A'' light periods of t* seconds each can be 

 compared \vith that of uninterrupted illumination of equal actual duration, 

 Nt* seconds (and equal intensity). This comparison answers the question: 

 Given a certain total amount of light energy of definite intensity, will it be 

 better utilized for photosynthesis by dividing it into several exposures 

 separated by dark intervals, or will it be used best in one continuous 

 stretch? The ratio of the yields obtained in these two ways— the inter- 

 mittency factor for equal intensity and equal total energy — will be designated 

 by iiE- A simple relation exists between in and ijE, namely: 



(34.1) ill = iiE 



td + V 



The quantity that Briggs (1941) called the "yield" of intermittency was: 



llE ~ 1- 



(c) The yield obtained in intermittent light can further be compared 

 with the yield produced by the same total amount of light energy dis- 

 tributed uniformly over the same total time; the intensity of the uninter- 

 rupted light is in this case smaller than that of intermittent light, in the 

 ratio t*/{td -\- t*). This method of comparison answers the question: 

 Given a certain amount of light energy to be used within a certain period 

 of time, will it be more advantageous to distribute this energy evenly over 

 the whole available time, or to concentrate it in separate exposures with 

 dark intervals between them? The ratio of the yields obtained in this way 

 can be called the intermittency factor for equal energy and equal time; 

 it will be designated by ist- 



The relation between Iei and the other two intermittency factors is as 

 follows : 



