130 GENERAL PHYSIOLOGY 



fact be considered as aquatic plants. The reason for this is 

 perfectly evident. They secure their food through absorp- 

 tion, and it is absolutely necessary that they be surrounded by 

 moisture. It is true that bacteria may grow upon solid me- 

 dia, but they can only do so when they are surrounded by, at 

 least, a film of moisture. If they become completely dried, 

 they are unable to feed. They are, however, able to maintain 

 their existence for some time in a desiccated condition, but are 

 not able to grow when in this state. So far as the ability of a 

 microorganism to withstand desiccation is concerned, different 

 kinds of bacteria vary greatly. The spores are able to with- 

 stand an almost unlimited amount of desiccation. It is for 

 this reason, largely, that they are so valuable to the bacteria. 

 The vegetative cell is not without this power, however, but 

 the different kinds vary greatly in their ability to withstand 

 complete drying. For example, such an organism as the 

 Msp. comma of Asiatic cholera can withstand drying for 

 only a few hours, or a few days at most. The same is true 

 also of the M. pneumonise and Bact. influenzas ; but other 

 vegetative cells, such as the Bact. diphtheriae, may withstand 

 the effect of desiccation for many days, in some cases as 

 long as 194 days. Reference was made in a preceding para- 

 graph to the effect of the density of the medium on the bacteria, 

 and it will be recalled that when the culture medium is less 

 dense than the protoplasm of the cell, the cell is in a state of 

 turgescence ; that when the culture medium is more dense than 

 the protoplasm, the cell is plasmolyzed. It is true also that 



