CHAPTER XVI 



CHEMOTROPISM 



1. When we create a center of diffusion in water or in 

 air we may theoretically expect orienting effects. Thus 

 when a fine capillary tube containing a solution of a salt, 

 e.g., sodium malate, is put into a drop of water containing 

 motile organisms, and the right side of an organism is 

 turned to the source of diffusion, the diffusing molecules 

 will collect in increasing concentration on that side. On 

 the left side of the organism, no such increase in the con- 

 centration of molecules will occur. If now the molecules 

 collecting on the right of the organism in increasing den- 

 sity are able to produce some chemical or some concen- 

 tration chain effect, the two sides of the organism will be 

 acted upon unequally and the tension of the symmetrical 

 motile organs will no longer be the same. As a conse- 

 quence the organism will turn until the mass of molecules 

 or ions striking the organism in the unit of time will be 

 the same for both sides. These effects only take place 

 when the organism is close to the opening of the capillary 

 tube, since the diffusion from the tube is slow. 



It is obvious, however, that it is difficult to provide 

 experimental conditions which give exact chemotropic 

 reactions. First of all, if the diffusion is rapid the differ- 

 ences in concentration of the effective chemotropic sub- 

 stance on two sides of an organism are too slight to result 

 in a turning movement. A second condition which is liable 

 to vitiate the result are the unavoidable convection cur- 

 rents due to changes or differences of temperature. In 



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