FACTS AND FACTORS OF DEVELOPMENT 57 



elapses after the first stimulus and before the 

 second no visible response follows, i. e., two 

 successive stimuli are necessary to cause the 

 leaves to close, and the two must not be more 

 than three minutes apart; the effects of the 

 first stimulus are in some way stored or regis- 

 tered in the leaf for this brief time. This kind 

 of phenomenon is widespread among living 

 things and is known as "summation of stim- 

 uli." In all such cases the effects of a former 

 stimulus are in some way stored up for a 

 longer or shorter time in the protoplasm. It 

 is possible that this is the result of the forma- 

 tion of some chemical substance which remains 

 in the protoplasm for a certain time, during 

 which time the effects of the stimulus are said 

 to persist, or it may be due to some physical 

 change in the protoplasm analogous to the 

 "set" in metals which have been subjected to 

 mechanical strain. 



Probably of a similar character is the per- 

 sistence of the effects of repeated stimuli and 

 responses on any organ of a higher animal. A 

 muscle which has contracted many times in a 

 definite way ultimately becomes "trained" so 



