president's address. 11 



Habit illustrated by Movement. 



In order to make my meaning plain as to the existence of a mnemic 

 factor in the life of plants, I shall for the moment leave the morphological 

 side of life and give an instance of habitual movement. 



Sleeping plants are those in which the leaves assume at night a 

 position markedly different from that shown by day. Thus the leaflets 

 of the scarlet-runner (Phaseolus) are more or less horizontal by day and 

 sink down at night. This change of position is known to be produced 

 by the alternation of day and night. But this statement by no means 

 exhausts the interest of the phenomenon. A sensitive photographic 

 plate behaves differently in light and darkness ; and so does a radiometer, 

 which spins by day and rests at night. 



If a sleeping-plant is placed in a dark room after it has gone to sleep 

 at night, it will be found next morning in the light-position, and will 

 again assume the nocturnal position as evening comes on. We have, in 

 fact, what seems to be a habit built by the alternation of day and night. 

 The plant normally drops its leaves at the stimulus of darkness and 

 raises them at the stimulus of light. But here we see the leaves risine: 

 and falling in the absence of the accustomed stimulation. Since this 

 change of position is not due to external conditions it must be the result 

 of the internal conditions which habitually accompany the movement. 

 This is the characteristic par excellence of habit— namely, a capacity, 

 acquired by repetition, of reacting to a fraction of the original environ- 

 ment. We may express it in simpler language. AVhen a series of actions 

 are compelled to follow each other by applying a series of stimuli 

 they become organically tied together, or associated, and follow each 

 other automatically, even when the whole series of stimuli are not 

 acting. Thus in the formation of habit post hoc comes to be equivalent 

 to projHer hoc. Action B automatically follows action A, because it has 

 repeatedly been compelled to follow it. 



This may be compared with Herbert Spencer's ' description of an 

 imaginary case, that of a simple aquatic animal which contracts its 

 tentacles on their being touched by a fish or a bit of seaweed washed 

 against it. If such a creature is also sensitive to light the circumstances 

 under which contraction takes place will be made up of two stimuli — 

 those of light and of contact — following each other in rapid succession. 

 And, according to the above statement of the essential character of 

 associative habit, it will result that the light-stimulus alone may suffice, 

 and the animal will contract without being touched. 



Jennings'^ has shown that the basis of memory by association exists in 

 so low an organism as the infusorian Stentor. When the animal is stimu- 

 lated by a jet of water containing carmine in suspension, a physiological 

 state A is produced, which, however, does not immediately lead to a visible 



' Psychology, 2nd edit., 1870, vol. i. p. 4S5. 



' Beha/vior offhe Lower Organisms, 1906, p. 289, 



