20 Livingston : Chemical stimulation of a green alga 



ceding paragraph. These three responses will now be considered 

 under the three heads: (i) The response of death, (2) The re- 

 sponse in phenomena of growth, and (3) The response in repro- 

 duction. 



III. Discussion of responses. 1. The response of death. — As 

 has been stated, death must be considered as truly a response to 

 stimuli as are any of the other alterations commonly passing under 

 that name. Whatever may be the ultimate nature of the vital 

 processes, they continue in such manner as to make up what is 

 called life only while variation of the conditions external to the 

 organism takes place within certain more or less narrow limits. 

 Within these limits, changes of environment produce changes in 

 the vital processes, and hence obvious changes in form, structure, 

 and activity of the organism as a whole. Beyond them the neces- 

 sary equilibrium of the many-sided system fails, and vitality ceases. 

 Thus, the least concentration producing death may fairly be re- 

 garded as a criterion for estimating the stimulating power of any 

 substance. The criterion forjudging whether or not this plant is 

 living is based on its loss of green color soon after death occurs. 



