NO. 24 TIME COURSE OF PHOTOSYNTHESIS McALISTER 1 5 



It is sensitive to temperature and is caused by light. Van der Paauw 

 came to this conclusion in studying induction in Honnidiuin. 



All these results, taken together with the well-known experiments 

 of Emerson and Arnold (1932) with flashing light, the flashes being 

 very short and of very high intensity, make it evident that between 

 continuous light and intermittencies (equal light and dark periods) 

 of the order of 1/50 to i/ioo second, both light and dark periods are 

 too long for maximum effect, i. e., are being wasted. In intermit- 

 tencies more frequent than about i/ioo second, light is still being 

 wasted, but the dark, or Blackman reaction is proceeding very effi- 

 ciently. The quantitative aspects of the situation are markedly affected 

 and complicated by the existence of the induction period. 



It is now apparent that the induction period in a higher plant is an 

 important source of information on the mechanism of photosynthesis. 

 In this connection experiments are under way to correlate the amount 

 of carbon dioxide lost to photosynthesis during the induction period 

 with the amount of chlorophyll present in the plant. The asymptotic 

 value this amount approaches at high light intensity may be compared 

 to the total chlorophyll present since presumably at high illumination 

 we have all the chlorophyll working. Further experiments on the 

 effect of intermittent illumination of various ratios of light to dark 

 periods are planned. 



SUMMARY 



The application of this method of gas analysis to measurements of 

 the carbon dioxide exchange between a higher plant (wheat, variety 

 Marquis) and its surroundings yielded the following results: 



I. Induction period. The carbon dioxide assimilation measure- 

 ments herein reported are the first ever made on the time course of 

 photosynthesis during the first few seconds after illumination of a 

 higher plant. The power of the method is evident when it is realized 

 that these measurements are the first ever made continuously on any 

 organism, i. e., by turning on the light and watching what happens. 

 The work on the induction period previously reported (on algae only) 

 has been done by an indirect integrational method. The time rela- 

 tions of this induction period for young wheat and its variation with 

 temperature and intensity of illumination are found to be in excellent 

 agreement with those previously found for algae. Further, it is shown 

 that this induction represents a certain amount of carbon dioxide lost 

 to photosynthesis and that this amount lost approaches zero progres- 

 sively with decreasing intensities of illumination and is apparently 



