68 CHAP. IX. EFFECT OF TRACES OF CHEMICAL SUBSTANCES 



and it was reduced to the normal at a strength of 10,000 

 parts in a billion. Stronger solutions produced a depression 

 below the normal. 



The maximum point and the relative enhancement are 

 modified to a certain extent by the physiological condition 

 of the specimen. The stimulating effect of traces of HN0 3 

 is very marked in inactive specimens, where the change 

 induced is from no evolution of oxygen to a very vigorous 

 bubbling. Very active specimens do not show such a great 

 increase, the enhancement being then about 50 per cent. 

 In moderately active specimens the increase, as already 

 stated, is about 200 per cent. The following table shows 

 the effect of traces of HN0 3 on three different specimens 

 which were moderately active. 



Table XL — Showing Effect of HN0 3 on Photosynthetic Activity 



Method of the threshold. — I discovered another striking 

 method for the demonstration of the great enhancement of 

 activity induced by infinitesimal traces of HN0 3 . We take 

 a cut specimen of Hydrilla fixed in a test-tube in an inverted 

 position with the cut end uppermost. As a moderate inten- 

 sity of light is required to initiate photosynthesis, it follows 

 that the specimen ceases to give bubbles of oxygen when it is 

 moved away from the window which admits daylight into 

 the room. This usually happens when the plant is at a dis- 

 tance of about 2 metres from the window. If we now add 



