NO. 24 TIME COURSE OF PHOTOSYNTHESIS McALISTER II 



Figure 10 shows the results of a 3-^-hour " run " wherein the effect 

 of intermittent illumination was studied. At the top we see the effect 

 of continuous illumination for 15 minutes. The next graph shows the 

 effect of equal i -minute periods of light and darkness. Here the same 

 intensity was used, but only one-half the amount of light for the 15- 

 minute period. The system is fast enough to follow the plant proc- 

 esses. Next in order are shown the effects of equal periods of light 

 and darkness of 15 seconds, 5 seconds, 1/2 second, i/io second, and 

 1/60 second length respectively. It should be noted that the induc- 

 tion period is apparently vanishing at high frequencies and seems to 

 be amplified near the 5- and 15-second periods. 



The change in efficiency of carbon dioxide assimilation with fre- 

 quency of intermittency is clearly shown. The numbers to the right 

 of each curve give the assimilation rate. In all cases the plant received 

 the same quantity of Hght except the first, which receives twice as 

 much. At one-half the intensity (here light is the limiting factor) the 

 rate for continuous illumination would be 8.1. It is thus seen that 

 the 60-second and 15-second periods give lower rates, while all the 

 shorter intervals give higher rates, the shortest being a 95 percent 

 increase in efficiency of assimilation over the continuous light. Here 

 the rate per unit time is in fact essentially the same as that in con- 

 tinuous light, even though for only half the quantity of light. These 

 results agree almost exactly with Warburg's (1928, pp. 332-334), 

 even to the time relations. In both these experiments and Warburg's, 

 a limiting increase of 100 percent is approached as long as equal 

 periods of light and darkness are used; but if, as in the experiments 

 of Emerson and Arnold (1932) the light periods are shortened with 

 respect to the darkness, the efficiency per unit light may be increased 

 several hundred percent. Data from experiments not shown on this 

 figure give assimilation rates intermediate between that for continu- 

 ous light and i -minute intervals for 2-niinute and 5-minute periods. 

 Thus we see not only the previously observed increase of assimilation 

 rate with frequency of intermittency, but a minimum of assimilation 

 at about i -minute periods. 



This last-named finding — a minimum of photosynthesis — is strik- 

 ingly suggestive of Garner and Allard's (1931) results shown in 

 plate 2. This shows the integrated vegetative growth effects obtained 

 by them with higher plants in light of the intermittency periods indi- 

 cated. Their results have, in this part, been unexplained. It is now 

 indicated that this minimum of growth corresponds to a minimum 

 of photosynthesis because at these rates of intermittency the plant is 



