128 TEXT-BOOK OF FUNGI 



A concentration of from two to four hundred times the 

 strength fatal to the higher plants is necessary to inhibit 

 the germination of fungus spores when mineral acids are 

 used. The protoplasm is much more susceptible to the 

 action of such agents during the conidial stage than at any 

 other period of the life-cycle. 



On the other hand, formaldehyde, which is non-toxic to 

 the higher animals, is very deadly in its action on fungi, 

 being surpassed only by mercury, silver, and two chromates, 

 out of the compounds experimented with by Clark. The 

 fact of formaldehyde being both a reducing and an oxydis- 

 ing agent, coupled with its great instability, may account 

 for its extreme toxic effect on fungi. As represented by 

 equimolecular formulae, the toxicity of formaldehyde is as 



follows : proved fatal to Aspergillus and Penidllium : 

 5 12 



toSterigmatocystis&n&Botrytis', whereas - - proved 



2048 8192 



fatal to Oedocephalum. 



As contrasted with the above, sulphate of copper, the 



basis of most fungicides, requires a concentration to kill 



the spores of Penidllium. Sulphate of zinc was found to 

 have a very low effect on fungi, and so far as Aspergillus is 

 concerned, zinc is practically non-toxic, its spores surviving 



an immersion of forty-eight hours in a - concentration 



( = 7 per cent). Clark expresses surprise at the low toxicity 

 of this salt, considering its extensive use for impregnating 

 railway-sleepers as a preventive against fungi. 



Acetic acid is, as a rule, more toxic to fungi than are the 

 mineral acids. 



