DIVISION III 

 Chemical Influences 



CHAPTER I 

 STIMULATION OF ©ROWTH 



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 they will stimulate the develop- 

 ment. Other substances too, which are not food, can also act as 

 stimulants. It is a recognized fact of long stand- 

 ing that many poisons in very small doses will 

 stimulate. This applies to the most highly 

 developed animals and plants as well as to micro- 

 organisms. Raulin noticed in 1869 that Asper- 

 gillus 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 

 ■}:'_■_ constituent of the mold cells. Alcoholic fermenta- 

 ■V tion can be stimulated by metallic salts. It is be- 

 ''•■■■•' Heved by some physiologists that, as a law of nature, 



Fig. io8.-Chem- every substance that is injurious in a certain con- 

 otaxis. {After centration is a stimulant in a lower concentration. 

 A similar action of certain chemical compounds 

 upon enzymes has been noticed, retarding in high concentrations, 

 stimulating in weaker solution. 



Chemotropism and Chkmotaxis. — 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 microorganisms may be accounted for largely by chemical or osmotic 



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