RELATION TO ENVIRONMENT 14! 



Chemicals influence bacteria in much the same way that 

 other external agents do^ in that under certain conditions 

 chemical substances are tonic, i.e. they have a stimulating 

 effect. Other chemicals, or different concentrations of the 

 same chemical, may produce a repressive action leading, 

 ultimately, if the action is persisted in, to the death of the 

 bacteria. 



One of the most striking effects of chemicals on bacteria is 

 the power they have to induce and control locomotion. The 

 fact that bacteria seem to be attracted to certain substances is 

 a matter of long standing observation ; for instance, in stag- 

 nant water or in putrid solutions it is often seen that the bac- 

 teria, and other ciliated organisms, collect in masses around 

 certain particles of the decaying or putrefying substances. 

 With higher animals this would be called instinct. This 

 property is widespread throughout both the plant and 

 animal kingdom, and is very frequently noted among the 

 bacteria. This phenomenon is known as chemotaxis or 

 chemotropism. 



The phenomenon may be noted under different conditions ; 

 for instance, if the chemical substance has a nutritive value, 

 it is sometimes spoken of as trophotropism (attraction towards 

 food) ; if the chemical substance is a gas, it is sometimes called 

 aerotropism. The simplest manifestation of this phenomenon 

 is the movement of bacteria towards free oxygen. It has al- 

 ready been noted that bacteria in the hanging-drop prepara- 

 tion frequently collect at the outer edge of the drop. This is 



