DIVISION III. 



CHEMICAL INFLUENCES. 



CHAPTER I. 

 STIMULATION OF GROWTH. 



The influence of chemical substances upon microorganisms may be 

 helpful or harmful, or not noticeable. As helpful must be considered 

 above all the food compounds. Unless given in such large doses as to 

 cause a physical or osmotic effect (see chapter I, page 147) they will 

 stimulate the development. Other substances too, which are not food, 

 can also act as stimulants. It is a recognized fact of long standing that 

 many poisons in very small doses will stimulate. This applies to the 

 msot highly developed animals and plants as well as 

 to microorganisms. Raulin noticed in 1869 that 

 Aspergillus niger grew very much better in a nutrient 

 solution if a small amount of zinc salt was added. He 

 considered the zinc, therefore, as a necessary constit- 

 uent of the mold cells. Alcoholic fermentation can 

 be stimulated by metallic salts. It is believed by 

 some physiologists that, as a law of nature, every 

 substance that is injurious in a certain concentration 

 is a stimulant in a lower concentration. A similar 

 action of certain chemical compounds upon enzymes 

 has been noticed, retarding in high concentrations, FlG 55 _chemo- 

 stimulating in weaker solution. An explanation for taxis. (After Fischer.) 

 these facts cannot be given at the present time. 



CHEMOTROPISM AND CHEMOTAXIS. Microorganisms manifest their 

 preference for certain foods not by a stimulated growth alone. They also 

 make efforts to obtain better food by growing or moving toward it, which 

 is not a manifestation of a rudimentary intellect. Such reactions of micro- 

 organisms, may be accounted for largely by chemical or osmotic forces. 



171 



