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THE POPULAR SCIENCE MONTHLY 



sary 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 

 registered 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 formation of some chemical substance which 

 remains in the protoplasm for a certain time, during which time the ef- 



Pig. 20. Dionwa muscvpula (Venus' Flytrap). Three leaves showing marginal 

 teeth and sensitive hairs (SH). The leaf at the left is fully expanded, the one at the 

 right is closed. 



fects 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 persistence 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 that it responds more rapidly and more accurately 

 than an untrained muscle; and the nervous mechanism through which 

 the stimulus is transmitted also becomes trained in the same way. In- 

 deed such training is probably chiefly a training of the nervous mechan- 

 ism. The skill of the pianist, of the tennis player, of the person who has 

 learned the difficult art of standing or walking, or the still more diffi- 

 cult art of talking, is probably due to the persistence in muscles and 

 nerves of the effects of many previous activities. All such phenomena 

 were called by Hering, " organic memory," to indicate that this persist- 

 ence of the effects of previous activities in muscles and other organs is 



