September 3, iqo8] 



NATURE 



419 



current conception of stimulus ^ is practically identical 

 whether we look at the phenomena of movement or those 

 of structure. If this is allowable — and the weight of 

 evidence is strongly in its favour — a conclusion of some 

 interest follows. 



If we reconsider what I have called the indirectness of 

 stimulation, we shall see that it has a wider bearing than 

 is at first obvious. The " internal condition " or " physio- 

 logical state " is a factor in the regulation of the 

 organism's action, and it is a factor which owes its 

 character to external agencies which may no longer exist. 



The fact that stimuli are not momentary in effect but 

 leave a trace of themselves on the organism is in fact the 

 physical basis of the phenomena grouped under memory 

 in its widest sense as indicating that action is regulated 

 by past experience. Jennings" remarks; "In the higher 

 animals, and especially in man, the essential features in 

 behaviour depend very largely on the history of the in- 

 dividual ; in other words, upon the present physiological 

 condition of the individual, as determined by the stimuli 

 it has received and the reactions it has performed. But 

 in this respect the higher animals do not differ in prin- 

 ciple, but only in degree, from the lower organisms. ..." 

 I venture to believe that this is true of plants as well as 

 of animals, and that it is further broadly true not only 

 of physiological behaviour, but of the changes that are 

 classed as morphological. 



Semon in his interesting book, " Die Mneme,"^ has 

 used the word Engram for the trace or record of a stimulus 

 left on the organism. In this sense we may say that the 

 internal conditions of Pfeffer, the physiological states of 

 Jennings, and the internal conditions of Klebs are, broadly 

 speaking, Engrams. The authors of these theories may 

 perhaps object to this sweeping statement, but I venture 

 to think it is broadly true. 



The fact that in some cases we recognise the chemical 

 or physical character of the internal conditions does not by 

 any ineans prevent our ascribing a nniemic memorv-Iike 

 character to them, since they remain causal agencies built 

 up by external conditions which have, or may have, ceased 

 to exist. Memory will be none the less memorv when we 

 know something of the chemistry and physics of its neural 

 concomitant. 



Habit illustrated by Movement. 



In order to make mv meaning plain as to the existence 

 of a nmcmic 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 



1 With rejrard to the terminology of stimulation, I believe that it would 

 creatly simplify matters if our classification of causal conditions could be 

 based on the relation of th** nucleus to the rest of the cell. But our know- 

 ledge does not at present allow of more than a tentative statement of such a 

 scheme. It is now widely believed that the nu'"leus is the bearer of the 

 qua'ities transmitted from generation to generation, and the regulator of 

 ontogeny. May we not therefore consider it probable th.it the nucleus 

 plays in the cell the part of a_ central nervous system? In plants there is 

 evidence that the ectoplasm is the sensitive region, and, in fact, plays the 

 part of the cell's sense-organ. The chanee that occurs in the growth of a 

 cell, as a response to stimulus, would on this scheme be a reflex action 

 dependent for its character on the structure of the nucleus. The "indirect- 

 ness " of stimulation would then depend on the reception by the nucleus of 

 the excitation set up in the ectoplasm, and the secondary excitation 

 reflected from the nucleus, leading to certain changes in the growth of the 

 cell. 



If the nucleus be the bearer of the past history of the individual, the 

 scheme here sketched would accord with the adaptive character of normal 

 reactions and would fall into line with what we know of the reeulation of 

 actions in the higher organisms. Pfeffer (" Physiology of Plants," Eng. 

 trans., iii. lo) has briefly discussed the possibility of thus considering the 

 nucleus as a reflex centre, and has pointed out difiiculties in the way of 

 accepting such a view as universally holding good. Delage (" L'H^r^dit^," 

 2nd edit , 1903, p. 8S) gives a good summary of the evidence which indues 

 him to deny the mastery of the cell by the nucleus. Driesch, however 

 (" .Analytische Theorie der organischen Entwicklung," 1834. p. 8r). gives 

 reasons for believing that the cytoplasm Is the receptive region, while t'le 

 nucleus is responsible for the reaction, and it is on this that he bases his 

 earlier theory of ontogeny. 



- P. 124 (1004). 



'^ " Die Mneme, als erhaltendes Prinzip im Wechsel des organischen 

 Geschehens." von Richard Semon, rt*^ Auilage. 1904, 2*^ Auflage, 1908. It 

 is a pleasure to express my indebtedness to this work, as well as for the 

 suggestions and criticisms which I owe to Prof. Semon personally. 



NO. 2027, VOL. 78] 



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-f>osition, 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 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 internal conditions which habitually accompany the 

 movement. This is the characteristic par excellence of 

 habit — namely, a capacity, acquired by repetition, of react- 

 ing to a fraction of the original environment. We may 

 express it in simpler language. When 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 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 ' descrip- 

 tion of an imaginary case, that of a simple aquatic aniinal 

 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 in 

 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 suflice, 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 physiological state A 

 is produced, which, however, 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 stimulus, 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 treatment the organism " con- 

 tracts at once as soon as the carmine coines 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 contraction as a reaction. Thus we have in 

 an infusorian a case of short-circuiting precisely like the 

 case which has been quoted from Herbert Spencer as illus- 

 trating association. But Jennings' case has the advantage 

 of being based on actual observation. He generalises the 

 result as the " law of the resolution of physiological states " 

 in the following words : " The resolution of one physio- 

 logical state into another becomes easier and more rapid 

 after it has taken place a number of times." He goes on 

 to point out that the operation of this law is seen in the 

 higher organisms, " in the phenomena which we commonly 

 call memory, association, habit-formation, and learning." 



In spite of this evidence of mnemic power in the simplest 

 of organisms, objections will no doubt be made to the state- 

 ment that association of engrams can occur in plants. 



Pfeffer, whose authority none can question, accounts for 

 the behaviour of sleeping plants principally on the more 

 general ground that when any movement occurs in a plant 

 there is a tendency for it to be followed by a reversal — a 

 swing of the physiological pendulum in the other direc- 

 tion. Pfeffer' compares it to a released spring which 

 makes several alternate movements before it settles down 

 to equilibrium. But the fact that the return movements 



^ " Psvchology," 2nd edit., 1P70, vol. i. p. 435. 

 2 " Behaviour of the Lower Organisms," iqo6, p. 2^9. 

 ■' See Pfeffer. Af>ftafidl. A'. SrifJts. Gi's.. Bd. xxx. T007. It is impossible 

 to do justice to Pfeffer's point of view in the above brief statement. 



