March 21, 1878] 



NATURE 



413 



•well-known researches of Dr. Burdon Sanderson, in which he 

 compares the electrical disturbances which occur in the leaf of 

 Dionsea to those which take place in nerve and muscle. Again 

 Mr. Romanes has, in a recent lecture in this place, compared 

 the peculiar sensitiveness of Drosera to repeated touches wilh 

 the phenomenon known in animal physiology as the summation 

 of stimuli. But I have merely sought to show that we find in 

 Drosera a power of conduction of stimuli, an extreme sensitive- 

 ness to minute disturbances, and a power of discriminating 

 between different kinds of stimuli which we are accustomed to 

 associate with nervous action. To establish this analogy I 

 believe that the examples already mentioned may suffice. 



We will now inquire whether among plants anything similar 

 to memory or habit, as it exists among animals, may be found. 



The most fruitful ground for this inquiry will be the pheno- 

 menon known as the sleep of plants. The sleep of plants 

 consists in the leaves taking up one position by day and another 

 at night ; the two positions for night and day following each other 

 alternately. The common sentitive plant (Mimosa) is a good 

 example of a sleeping plant. The leaf consists of a main stalk 

 from which two or more secondary stalks branch off ; and on 

 these secondary stalks are borne a series of leaflets growing 

 in pairs. The most marked character of the night or sleeping 

 position is that these leaflets, instead of being spread out flat 

 as they are in the day, rise up and meet together, touching each 

 other by their upper surfaces. At the same time the secondary 

 stalks approach each other and ultimately bring the rows of 

 closed-up leaflets (two rows on each stalk) into contact. Besides 

 this well-marked change the main stalk alters its position. In 

 the afternoon it sinks rapidly, and in the evening it begins to rise, 

 and goes on rising all night, and does not begin to sink until 

 daylight. From that time it sinks again till evening, when it 

 again rises, and so on for every day and night. In reality the 

 movement is more complicated, but the essential features are as 

 I have described them. 



In comparing the sleep of plants with anything that occurs in 

 animal physiology, we must first give up the idea of there being 

 any resemblance between this phenomenon and the sleep of 

 animals. In animals, sleep is not necessarily connected with the 

 alternation of light and darkness, with day and night. We can 

 imagine an animal which by always keeping its nutrition at an 

 equal level with its waste would require no period of rest. The 

 heart which beats day and night shows us that continuous work 

 may go on side by side with continuous nutrition.^ Mr. Herbert 

 Spencer has suggested that since most animals are unable to lead 

 a life of even ordinary activity during the night because of the 

 darkness, therefore it answers best to lead an extremely active 

 life in day when they can see, and recover the waste of tissue by 

 complete rest at night. On the other hand, certain animals find 

 it more to their profit to sleep in the day and rest at night. But 

 there is nothing of this kind in plants ; their sleep movements 

 are not connected with resting. Although the leaflets close up, 

 yet the main stalk is at work all the night through.^ Moreover, 

 owing to the closing up of the secondary stalks of the leaf, the 

 length of the whole organ is increased, and therefore the work 

 done by the main stalk is also increased. So that, far from 

 resting at night, the main stalk is actually doing more work than 

 in the day. Besides this, instead of being more or less insensible, 

 as a sleeping animal is, the primary petiole of the Mimosa 

 remains fully sensitive at night, and displays the same property 

 which it shows by day, viz., that of faihng suddenly through a 

 large angle on its irritable joint being touched. Besides these 

 points of difference, there is the important distinction that the 

 movements of sleeping plants are strictly governed by light and 

 darkness without any reference to other circumstances. 



In Norway,^ in the region of continual day, the sensitive 

 plant remains continually in the daylight position — although no 

 animals probably remain continually awake. 



There is one — but only a fanciful resemblance — between the 

 sleeping plants and animals, namely, that both have the power of 

 dreaming. I have been sitting quietly in the hot-house at night 

 waiting to make an observation at a given hour, when suddenly 

 the leaf of a sensitive plant has been seen to drop rapidly to its 

 fullest extent and slowly rise to its old position. Now in this 

 action the plant is behaving exactly as if it had been touched on 

 its sensitive joint ; thus some internal process produces the same 

 impression on the plant as a real external stimulus. In the same 



* Leaving out of the question the repose during diastole, 

 z In Mimosa at least. 



3 Schubler, quoted by Pfeflfer ("Die periodische Beweguugen der Blatt- 

 organe," 1875, p. 36). 



way a dog dreaming by the fire will yelp and move his legs as if 

 he were hunting a real instead of an imaginary rabbit^ 



I said that in the regions of perpetual light the sensitive plant 

 remains constantly in the day position. We might fairly expect, 

 therefore, that we should be able to produce the same effect by 

 artificial light constantly maintained. This experiment has, in 

 fact, been made by A. de Candolle,^ Pfeffer, and others with 

 perfect success. But before the leaves come to rest a remarkable 

 thing takes place. In spite of the continuous illumination, the 

 sleeping movements are executed for a few days exactly as if the 

 plant were still exposed to the alternation of day and night. 

 The plant wakes in the morning at the right time and goes to 

 sleep in the evening ; the only difference between these move- 

 ments and those of a plant under ordinary circumstances is that 

 under constant illumination the movements become gradually 

 smaller and smaller, until at last they cease altogether. When 

 the plant has been brought to rest it can be made to sleep and 

 wake by artificial alternations of darkness and light. This fact 

 seems to me extremely remarkable, and one which, in the domain 

 of animal physiology, can only be paralleled by facts connected 

 with habit. The following case is given me by a friend and is 

 probably a common experience with many people : — Having to be 

 at work at a certain time every day, he has to get up at an early 

 hour, and wakes with great regularity at the proper time. When 

 he goes away for his holiday he continues for a time waking at the 

 proper hour to go to work, but at last the body breaks through 

 the habit, and learns to accommodate itself to holiday hours. 



It seems to me that this case may fairly be likened to that of the 

 sensitive plant in constant illumination. There is the same con- 

 tinuance of the periodic movement on the first removal of a 

 stimulus, and the same gradual loss of periodicity consequent on 

 the continued absence of the stimulus. 



From this kind of habitual action there is but a small step to 

 those actions in which we say that memory comes into play. Dr. 

 Carpenter ^ relates the case of a boy who, in consequence of an 

 injury to his brain^ never acquired the, power of speech or of 

 recognising in any way the minds of other people. In spite of this 

 mental incapacity he had an extraordinary sense of order or 

 regularity. Thus although he disliked personal interference, his 

 hair having been one day cut at ten minutes past eleven, the next 

 day and every following day he presented himself at ten minutes 

 past eleven, as if by fate, and brought comb, towel, and scissors, 

 and it was necessary to cut a snip of hair before he would be 

 satisfied. Yet he had no knowledge whatever of clocks or 

 watches, and was no less minutely punctual when placed beyond 

 the reach of these aids. 



It is hard to say whether this boy actually remembered at ten 

 minutes past eleven that now was the time to have his hair cut, 

 or whether it was an unconscious impulse that made him do so. 

 But whether we call it habit or memory, there is the same know- 

 ledge of the lapse of time, the internal chronometry, as Dr. 

 Carpenter calls it, which exists in the sensitive plant, and the 

 same tendency to perform an action because it has been done 

 previously. There is, in fact, hardly any distinction between 

 habit and memory ; if a man neglects to wind up his watch at 

 night, he says that he forgot it, and this implies that memory nor- 

 mally impels him to wind it ; but how little memory has to do with 

 the process is proved by the fact that we have often to examine our 

 watches again to see that they are wound up. It is the old problem 

 of conscious and unconscious action. If a friend, in order to 

 test our powers of self-control,* moves his hand rapidly near 

 the face, we cannot help winking, though we know he will not 

 hurt us ; and when we are breaking through a hedge or thicket, 

 we close our eyes voluntarily to keep twigs out. Here are 

 two actions performed with the same object by the same muscles 

 under command of the same nerves, yet one is said to be directed 

 by the will and the other by instinct, and a great distinction is 

 drawn between them. It seems to me that the presence ot what 

 Mr. Lewes calls "thought consciousness" is not the crucial 

 point, and that if it is allowed that; the sensitive plant is subject 

 to habit (and this cannot be denied), it must, in fact, possess 

 the germ of what, as it occurs in man, forms the groundwork of 

 all mental physiology. 



I am far from wishing to make a paradoxical or exaggerated 

 statement of this resemblance between the periodic movements 

 of plants and memory of the human mind. But the groundwork 



» This curious phenomenon was first observed by Millardet, who describes 

 it as of rare occurrence. (Millardet, loc. cit., p. 29.) 



* Quoted by Pffiflfer (" Periodische Bewegungen," p. 31). 

 3 " Mental Physiology," p. 349. 



* See " Physiology of^ Common Life," vol. ii. p. 200. 



