172 CHEMICAL INFLUENCES. 



In a solid medium the hyphae of molds will grow toward the best source of 

 food supply. This growth on account of chemical stimulation is called 

 chemotropism, analogous to the phototropism or growth toward light. 

 If some injurious compound is offered, the hyphae will grow away from it. 

 Thus we have to distinguish between positive and negative chemotropism. 

 The motile organisms, bacteria as well as protozoa, demonstrate their 

 preference for certain food compounds by swimming toward them. This 

 is called chemotaxis (Fig. 55). Here also a positive and negative 

 chemotaxis must be distinguished, the latter taking place if injurious 

 substances are present. These reactions do not seem to be of much im- 

 portance in the life of microorganisms. 



