360 



SCIENCE 



[N. S. VoL.XXVni. No. 716 



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 {Phase- 

 olus) 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 in- 

 terest of the phenomenon. A sensitive 

 photographic plate behaves differently in 

 light and darkness; and so does a radiom- 

 eter, 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 noc- 

 turnal 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 rising 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 in- 

 ternal conditions which habitually accom- 

 pany the movement. This is the charac- 

 teristic 'par excellence of habit — namely, a 

 capacity, acquired by repetition, of react- 

 ing to a fraction of the original environ- 

 ment. "We may express it in simpler lan- 

 guage. "When a series of actions is com- 

 pelled 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 propter 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 stimulated by a jet of water 

 containing carmine in suspension, a physi- 

 ological state A is produced, which, how- 

 ever, does not immediately lead to a visible 

 reaction. As the carmine stimulus is con- 

 tinued or repeated, state B is produced, to 

 which the Stentor reacts by bending to one 

 side. After several repetitions of the stim- 

 ulus, state C is produced, to which the 

 animal responds by reversing its ciliary 

 movement, and C finally passes into D, 

 which results in the Stentor contracting 

 into its tube. The important thing is that 

 after many repetitions of the above treat- 

 ment the organism "contracts at once as 

 soon as the carmine comes in contact with 

 it." In other words, states B and C are 

 apparently omitted, and A passes directly 

 into D, i. e., into the state which gives con- 

 traction as a reaction. Thus we have in 



~ " Psychology," 2d edition, 1870, Vol. I., p. 435. 

 ^ " Behavior of the Lower Organisms," 1906, 



