144 GENERAL PHYSIOLOGY 



differentiating between a substance which is a good food and 

 a poison. As an example, Fischer has cited the fact that a 

 peptone solution to which a little corrosive sublimate has been 

 added exerts a positive chemotaxic influence, and the bacteria 

 rush into the solution only to be destroyed by the presence of 

 the sublimate. It has been thought that this attraction in- 

 dicated, on the part of the bacteria, a power of selection, and 

 that they were attracted and purposely moved towards these 

 substances. From the work of others, however, it seems 

 likely that this is not the case, but that the bacteria swim 

 about in an aimless fashion, and when they come in contact 

 with these chemotaxic substances are under their influence. 

 They find conditions there more favorable, and hence re- 

 main in the zone ; but certain bacteria in the solution may 

 fail to get into this zone, and therefore do not come under its 

 influence. 



The phenomenon of chemotropism is of general interest 

 throughout biology, and enables us to explain movements of 

 many kinds of life that would otherwise be very difficult to 

 understand. As examples of this we may cite the attractive 

 influence which the female cells have for the male cells among 

 plants and among animals. Overton noted that the bacteria 

 collect around conjugating spirogyra, where the tubes arise. 

 The pollen tube moves down the stamen of flowers, in all prob- 

 ability under the influence of this phenomenon. The sper- 

 matozoa of plant and animal cells are attractive to the egg cells 

 because of the excretion, by these egg cells, of certain sub- 



