EFFECT OF HN0 3 ON PHOTOSYNTHETIC ACTIVITY 6j 



any change in the plant already submerged in water. 

 Again, this enhancement could not have been produced by 

 any stimulation caused by an electric disturbance during the 

 thunder-storm, for such a disturbance artificially produced 

 did not cause any responsive variation. This is what might 

 have been expected, since the electric disturbance in the air 

 could not affect the plant under water, which is a conductor. 

 The other possible explanation is the production, under 

 electric discharge during the storm, of oxides of nitrogen 

 in the atmosphere, which would be dissolved in the rain, 

 and thus cause an addition of traces of nitrates to the 

 pond-water. But the quantity so added would be incon- 

 ceivably small, and it is difficult to imagine how such 

 infinitesimal quantities could produce any effect on the 

 activity of the plant. 



Effect of HN0 3 on Photosynthetic Activity 



I thought, however, that it might be of interest to 

 investigate the effect of small traces of nitric acid on the 

 activity of the plant. I began by adding to the normal 

 C0 2 -solution one part of HN0 3 in 10,000 ; this, instead of 

 producing any enhancement of activity, caused a depression. 

 One part in 100,000 produced hardly any effect. This led 

 me to conclude that traces of HN0 3 could not exert any 

 stimulatory effect. Before giving up the inquiry I, however, 

 tried the effect of one part in a billion (billion in French 

 measure = 1000 millions). With this extraordinarily 

 minute quantity I obtained such a marked stimulation 

 of photosynthetic activity that it surpassed the limits of 

 belief. But repetition of the experiment, an account of 

 which will be presently given, left no possible doubt on the 

 subject. It was found that one part in ten billions produced 

 a very marked enhancement, which continued to increase 

 on further addition till at one part in two billions a maximum 

 increase of nearly 200 per cent, was produced. The activity 

 began to show a decline at a dilution of one part in a billion, 



