174 ' MICRO-ORGANISMS AND FERMENTATION. 



well known, form the principal and most important food- 

 stuffs. We shall return to these in the subsequent chapter 

 on yeasts. 



Speaking generally, one may assert that constructive 

 activity predominates during the nutrition of green plants, 

 whilst destructive activity predominates in the case of moulds ; 

 the enzymes which are of such general occurrence are the 

 special destructive factors. 



It may be remarked that certain substances, which are 

 not necessarily incorporated by the fungi, are nevertheless of 

 importance to them. The influence which these exert on the 

 metabolism and growth of fungi suggests the term " chemical 

 stimuli." The presence of water and oxygen, for instance, 

 are essential in bringing about the germination of spores, 

 but it frequently happens that germination can only take place 

 if certain substances are also present in solution. Klebs has 

 shown that the spores of Aspergillus repens will not germinate 

 either in pure water or in inorganic nutrient solutions, or 

 even on peptone, unless some inorganic salt, such as saltpetre, 

 is added, but that, in a 0-5 per cent, solution of grape-sugar, 

 germination does take place. Light also acts as a stimulant 

 in certain cases, whilst in others it may have a retarding 

 effect. It has been ascertained that minute doses of certain 

 poisons have the effect of stimulating the growth of fungi, 

 and of accelerating fermentative phenomena, an action which 

 may, perhaps, be ascribed to physiological reaction on the 

 part of the organisms concerned. Thus, a small quantity of 

 zinc sulphate (0-002 per cent.) added to a solution of sugar 

 and inorganic salts has the effect of making the growth of 

 Aspergillus niger twice as strong as under ordinary conditions. 

 Copper sulphate has a corresponding action under certain 

 conditions. In the case of yeasts, the addition of the merest 

 trace of substances such as mercuric chloride (1 : 500,000), 

 iodine, potassium iodide, and chromic or salicylic acids has a 

 very beneficial action on the fermentation. Lactic acid 

 bacteria, grown in milk free from casein, are also stimulated 

 by the addition of minute quantities of mercuric chloride. 



The interesting observation has recently been made that 

 products formed by the metabolism of the fungi have a stimu- 



