l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



approaching an asymptotic value at high intensities of illumination. 

 The induction period is prolonged to 12 or 15 minutes after a night 

 of darkness. The striking similarity betv^een the present data for 

 young wheat plants and the previous work on algae shows that we are 

 dealing with a mechanism fundamentally the same in both plants for 

 carbon dioxide assimilation. The importance of this induction period 

 as a source of information on the mechanism of photosynthesis should 

 be emphasized, for it is chemical in nature, is sensitive to temperature, 

 and is produced by light. 



Induction in intermittent illumination of equal light and dark periods 

 is shown to be very small at high frequencies — 1/60 second length 

 period — and is larger than normal for periods of from 5 to 15 seconds. 



2. Independence of respiration and illumination. The time rela- 

 tions of respiration, i. e., the immediate appearance of respiration at 

 the termination of illumination with a rate equal to that maintained 

 before illumination, together with its independence of light intensity 

 here reported for young wheat plants, lead to the conclusion that light 

 has no direct effect on respiration. 



3. Intermittent illumination. A minimum of carbon dioxide assimi- 

 lation in flashing light (equal light and dark periods) has been found 

 between 15-second periods and continuous illumination. This mini- 

 mum probably falls between periods of i and 5 minutes length. The 

 usual increase in efficiency of assimilation with increasing frequency 

 of intermittency (for periods shorter than 15 seconds) has been found 

 in young wheat plants and is seen to approach a limiting increase of 

 100 percent over continuous light. 



This minimum of carbon dioxide assimilation is strikingly sugges- 

 tive of Garner and Allard's results on the integrated growth effects of 

 intermittent illumination of equal light and dark periods. For several 

 higher plants they found a minimum of growth in the range of from 

 I to 5 minute periods. 



4. Correlation zvith the fluorescence of chlorophyll. The short time 

 relations herein reported for the respiration and carbon dioxide as- 

 similation of a higher plant strikingly confirm and correlate with 

 much of the work on the fluorescence of chlorophyll in a higher plant 

 reported by Franck and by Kautsky. 



Recapitulating, it may be said that besides these new results, most 

 of the previous work on the time course of photosynthesis with algae 

 has been verified with wheat. 



