INFLUENCE OF MECHANICAL AGENCIES 285 



which are heavier than the liquid they float on, is often accomplished by 

 small gas bubbles between the cells (Mycoderma). 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 exceptions, 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 liv- 

 ing organisms, and it is possible to sterilize cultures by continued shak- 

 ing. The effect is 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 organ- 

 isms show wide variations in their resistance to agitation. 



