27O CHAP. XXVIII. GENERAL REVIEW 



Photosynthetic Induction 



The total volume of oxygen given out in photo- 

 synthesis is proportional to the duration of the exposure 

 to continuous light ; but this quantitative relation does 

 not hold good when the light is intermittent. The 

 photosynthetic production of carbohydrates is brought 

 about by a series of chemical dissociations and com- 

 binations. Preliminary to dissociation, a state of strain 

 is induced in the molecule. When the light is tem- 

 porarily discontinued at this stage, the induced state of 

 molecular strain disappears ; thus the preliminary work is 

 undone. The undoing of the positive or uphill work brings 

 about a prolongation of the Induction-period (p. 89). 



Effect of rapid intermission. — When the intermission 

 is slow, the effect on photosynthesis is as described above. 

 But a very interesting phenomenon is discovered under 

 rapid intermission. It is found that a rapidly intermittent 

 light is photosynthetically more effective than continuous 

 light. A probable explanation is that, since under con- 

 tinuous light the resultant effect is A — D, on the cessation 

 of light A persists longer than D (p. 95). The results of a 

 gradual variation from rapid to slow intermission are : (1) 

 an effectiveness greater than under continuous light, (2) a 

 decreasing effectiveness reaching a limit, and (3) a partial 

 recovery (p. 96). 



Effect of Infinitesimal Traces of Chemical 

 Substances on Photosynthesis 



The photosynthetic activity of Hydrilla was found to 

 be very greatly enhanced after a thunder-storm. The only 

 plausible explanation for this was the production of oxides of 

 nitrogen, by electric discharge during the storm, which were 

 washed down by the rain into the pond and thus enhanced 

 the activity of the plant. Investigation on the effect of 



