NO. 24 TIME COURSE OF PHOTOSYNTHESIS — McALISTER 1 3 



most of the time in the induction phase. The increased growth toward 

 higher frequencies found by Garner and Allard (1931) is a true inter- 

 mittency effect. The increased growth toward longer periods can be 

 explained as a decreasing percentage effect of the induction period. 

 It may be stated further that the induction relations found or re- 

 ported in this paper show again a striking similarity to the results of 

 Franck (1936) and of Kautsky and Flesch (1936), Kautsky and 

 Marx (1936), and Kautsky and Hormuth (1936) on the fluorescence 

 of photosynthesizing plants. Unquestionably, one is studying exactly 

 the same phenomenon by direct fluorescence observations and by mea- 

 surement of carbon dioxide. 



DISCUSSION 



Comparison of the induction period in wheat to that in algae may 

 be questioned because of possible stomatal effects in the higher plant. 

 In the present experiments it is believed that these effects — i( any — 

 have been eliminated by maintaining a high relative humidity around 

 the plant and by measuring the induction period (except as in fig. 4) 

 after a 20-minute dark period, the plants previously having been il- 

 luminated for more than an hour. Thus, presumably, the stomata 

 were kept open during illumination by high humidity and the induc- 

 tion period measured before they had time to close. Attempts were 

 made to close the stomata enough to limit carbon dioxide assimila- 

 tion by subjecting the plants to a relative humidity of 5 to 10 percent 

 for an hour. No difference could be detected between the assimilation 

 at this humidity and that at the usual high humidity of 70 percent. 

 This agrees with recent work by J. W. Mitchell (1936) and others. 

 It appears then that the induction period in the present experiments 

 (except as in fig. 4) is not affected by stomatal movement. 



A calculation of the length of the diffusion path (carbon dioxide 

 through water) in wheat leaves is of interest. Taking a case where 

 the carbon dioxide concentration in the air is the limiting factor (high 

 illumination and normal air concentration), when the thickness of 

 a water film which would have the same diffusion resistance is calcu- 

 lated, this thickness ought to be of the same order of magnitude as 

 the length of the diffusion path in the leaf. Making the same assump- 

 tions as Van den Honert (1930) did in his calculations, data from 

 the present experiments give a water film about one-fifth of the leaf 

 thickness. Since the area of both sides of the leaf was considered, this 

 is reasonable. Using this value for the water-film thickness, it is now 

 possible to calculate the time necessary for the carbon dioxide to 



