Effect of Copper Compounds 35 



dilutions which were the most favourable to the growth of fungi. 

 Bokorny indicates that silver and copper salts work harm in unusually 

 dilute solutions. 



Attempts have been made to utilise the poisonous action of copper 

 on algae in clearing ponds of those plants. Lindsay (1913) describes 

 experiments carried on in a reservoir infested with Spirogyra. A 

 quantity of copper sulphate sufficient to make a solution of 1/50,000,000 

 was found necessary to kill off the Spirogyra, but it is suggested that 

 the solution was probably weaker before it reached the algae, owing to 

 the currents of fresh water. Anaboena needed 1/10,000,000 before it 

 was killed off, while Oscillatoria is less sensitive still, 1/5,000,000 usually 

 representing the mortal dose, though 1/4,000,000 was necessary in some 

 instances. Algae seem to be peculiarly sensitive to the copper sulphate, 

 far more so than the higher plants, as Nuphar lutea, Menyanthes 

 trifoliata, and Polygonum amphibium grew in the water unharmed by 

 the addition of the poisonous substance. For some unexplained reason 

 it seems that " the concentration of copper sulphate necessary to kill off 

 the algae in the laboratory is five to twenty times as great as that 

 needed to destroy the same species in its natural habitat." 



Conclusion. 



Altogether, after looking at the question from many points of view, 

 one is forced to the conclusion that under most typical circumstances 

 copper compounds act as poisons to the higher plants, and that it is 

 only under particular and peculiar conditions and in very great dilutions 

 that any stimulative action on their part can be clearly demonstrated. 



32 



