170 PHYSICAL INFLUENCES. 



small gas bubbles between the cells. In other instances, it may be 

 just the floating of cells having oily surfaces. 



The growth is influenced by gravity very little. The sporangia of 

 molds are the only exception, growing decidedly away from the center of 

 gravity (negative geotropism). 



AGITATION. For the majority of microorganisms, the quiet, undis- 

 turbed growth of the laboratory culture is the normal or the ideal one. 

 Such cultures, if shaken for a considerable time, show a decrease of living 

 organisms, and it is possible to sterilize cultures by continued shaking. 

 The effect it not a simple mechanical breaking or tearing of the cells. 

 The bacteria break up into the finest particles. This is also the case if 

 cultures are exposed for several days to the trembling motion caused by 

 the working of very heavy machines. There is no grinding or tearing 

 effect but the cells break to pieces just the same. 



A slight and slow agitation seems to be advantageous for many cul- 

 tures, only continuous heavy motion proves harmful. Different organisms 

 show wide variations in their resistance to agitation. 



