METABOLISM 67 



cupies a peculiar position among life 

 processes. Growth, reproduction, mo- 

 tion and irritability, and constructive 

 metabolism may cease, as in the resting 

 seed, yet life may go on for many years. 

 But when destructive metabolism ceases, 

 life ceases. We can apply this test to the 

 resting seed: as long as it is alive it pro- 

 duces carbon dioxide, when this ceases 

 it is dead. 



The cessation of this process is there- 

 fore a test of life but we can not say that 

 its continuance is a test for life for we 

 know that when an organism is killed in 

 certain ways it can go on producing car- 

 bon dioxide after death. 



These facts would seem to indicate 

 that life as manifested in the simplest 

 organisms is a physicochemical process 

 in which destructive metabolism plays 

 a fundamental role. This leads us to 

 attempt to control the process. Some 

 very remarkable experiments have been 

 made in this field. It is possible to stop 

 the development of certain organisms 



