PHYSIOLOGICAL STUDIES 55 



cyanide would not inhibit photosynthesis below the com- 

 pensation point and suggested that this was because respira- 

 tory intermediates, more reduced than carbon dioxide, could 

 be reduced in the light substituting for the products of pro- 

 cess I. This claim has been shown to be incorrect in at 

 least some strains of Chlorella. 



Urethanes depress photosynthesis and belong to the class 

 of narcotics. Here again in Chlorella photosynthesis is 



Intensity of illunnination 



FIG. 4.8. Rate of photosynthesis of Chlorella at different light in- 

 tensities in the presence (#) and absence (o) of hydroxylamine 

 hydrochloride. (After Weller and Franck.) 



inhibited to a greater extent than respiration but an impor- 

 tant difference between its action and that of cyanide is that 

 inhibition of photosynthesis is practically the same for all 

 intensities of illumination. Thus urethane probably inter- 

 feres in some way with the photochemical process in photo- 

 synthesis (although not exclusively, since if it did so the 

 inhibition would be greater at low light intensities) and in 

 agreement with this Warburg claims the inhibition to be 

 independent of the concentration of carbon dioxide. 



