SUMMARY 



71 



in ten billions caused an increase of activity of about 40 per 

 cent. ; one part in a billion induced a further increase to 

 80 per cent, or so. The stimulating effect of traces of formal- 

 dehyde has a special significance in regard to the ' first 

 product ' of photosynthesis. There is reason to believe 

 that this first product is formaldehyde, which by polymerisa- 

 tion becomes converted into carbohydrate. The poisonous 

 nature of formaldehyde stood in the way of acceptance of 

 this theory ; but the experiments just described show that 

 traces of this substance, instead of being poisonous, have the 

 effect of increasing the photosynthetic activity. We shall 

 have occasion in a later chapter to refer to this subject. 



The opposite effects of small and large doses of a sub- 

 stance, as observed with formaldehyde, I find to be of 

 frequent occurrence in physiological reactions. Thus ether, 

 which in large doses kills the plant, in small quantities is a 

 stimulant for growth. The same is true of chloroform, but 

 the range of safety is here more circumscribed. With strong 

 poisons the range is still narrower. Copper sulphate acts 

 as a poison in a solution of one part in a million, but a 

 dilution of one part in 100 millions often acts as a stimulant 

 of photosynthetic activity ; the increase is, however, not 

 so great or so unfailing as in the case of formaldehyde. 



Summary 



The photosynthetic activity of Hydrilla was found to 

 have been increased by more than 100 per cent, immediately 

 after a thunder-storm. This appeared to be probably due 

 to traces of nitrous fumes produced by electric discharge 

 and washed down by the rain. 



Investigations on the effect of infinitesimal traces of 

 HNO3 showed that one part of this acid dissolved in two 

 billion parts of water induced an enhancement of nearly 

 200 per cent, in the rate of photosynthesis. 



Minute traces of the extract of thyroid gland, of iodine 

 and of formaldehyde showed similar enhancement of 



