CHAPTER IX 



EFFECT OF INFINITESIMAL TRACES OF CHEMICAL SUBSTANCES 



ON PHOTOSYNTHESIS 



Sudden increase of photosynthetic activity after a thunder-storm — This 

 possibly due to nitrous fumes produced during electric discharge being 

 washed down by the rain — Determination of the effect of infinitesimal 

 traces of HN0 3 in enhancement of photosynthetic activity — Effect 

 of traces of extract of thyroid gland, of iodine, and of formaldehyde — 

 Opposite effects of large and small doses on physiological reactions. 



We saw in the last chapter how traces of copper sulphate 

 as minute as ten parts in a million gave rise to a pronounced 

 toxic effect, photosynthesis being thereby completely 

 abolished. The converse of this, namely the possible 

 influence of ultra-measurable traces of certain chemical 

 substances in enhancing the power of assimilation, is a 

 question of much importance in physiology. The C0 2 - 

 assimilation of water-plants affords an extremely sensitive 

 subject for this investigation. 



My attention was drawn to the matter by the extra- 

 ordinarily great increase in the photosynthetic activity of 

 Hydrilla after three days' thunder-storm and rain which 

 lasted from the loth to the 13th February of this year (1923). 

 The activity of a large number of these plants growing in a 

 particular pond of the Institute had been very carefully de- 

 termined from 3rd January to 9th February, and was found 

 to be practically the same in all the specimens. Immediately 

 after the thunder-storm, namely on the 14th February, the 

 photosynthetic activity was found to have been increased 

 by more than 100 per cent. This increased activity could 

 not have been due to any variation of temperature, for that 

 remained unchanged ; the rain-water could not produce 



