SUMMARY 65 



kept in a vessel inside the laboratory, its activity is some- 

 times found to be abolished. This is often due to traces 

 of metallic contamination ; for the plants do not suffer 

 on being kept in glass or earthenware vessels, but if kept in 

 brass vessels they lose their activity in a short time. In 

 order to find out what was the minimum dose of metallic 

 poison which produced a toxic effect, I made dilute solu- 

 tions of copper sulphate. A solution of one part of CuS0 4 

 in 100 millions of water caused a depression cf activity 

 from the normal 100 to 88 ; one part in a million caused a 

 further depression to 70, and ten parts in a million induced 

 abolition of response. 



Summary 



The effect of feeble stimulus is an enhancement of photo- 

 synthesis, as also of the rate of growth in growing organs, 

 and of the rate of transpiration of leaves. In all, strong 

 stimulus induces the opposite effect of depression. 



Anaesthetics such as ether induce a depression in the 

 rate of photosynthetic evolution of oxygen ; removal of 

 the anaesthetic is followed by restoration of the original 

 activity. 



Formaldehyde acts as a poison and abolishes the activity 

 in a dilution of one part in a thousand. 



Copper sulphate solution one part in a million causes 

 a great depression ; a solution containing ten parts in a 

 million acts as a poison and abolishes the activity. 



