STIMULATION OF GROWTH 287 



forces. 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. 120). Here also a positive and negative chemotaxis must be 

 distinguished, the latter taking place if injurious substances are present. 



