414 



NATURE 



[March 21 i 1878 



of both phenomena seems to be the repetition of a series of acts, 

 or the recalling of a series of impressions, in a certain order at 

 a certain time, because they have been repeated in that order 

 and at that time on many previous occasions. 



I will mention one more fact in connection with the move- 

 ments of Mimosa, in which the formation of habit is illustrated. 

 Every one knows that a noise regularly repeated ceases to dis- 

 turb us ; that one becomes habituated to it, and almost ceases to 

 hear it, A boy fast asleep inside an iron boiler while riveting 

 is going on, is an example of this power of habituation. The 

 same thing occurs with the Sensitive Plant. A single violent 

 shake causes the main stalk to drop, and the leaflets to shut up ; 

 in a minute or two the leaf recovers, and will again react on 

 being disturbed. In order to test the power of habituation, I 

 fastened one end of a thread to the leaf of a sensitive plant, and 

 the other to the pendulum of a metronome, and placed the plant 

 just at such a distance from the instrament that it received a pull at 

 every beat. The first shock caused the leaf to shut up, but after 

 a few repetitions it became accustomed to it, and I had the 

 curious sight of a highly- sensitive plant unaffected by a series of 

 blows. In nature this power no doubt enables the plant to 

 withstand the constant shaking of the wind. 



In spite of the amount of time which has been spent on the 

 study of sensitive and sleeping plants, no satisfactory explana- 

 tion of the use which the movements are to the plant has ever 

 been given. In the case of the carnivorous plants, we saw that 

 the movements of plants may be offensive, and like the move- 

 ments of animals in securing its prey. In the case of certain 

 flowers which we will now consider, the movements are flf(?fen- 

 sive, like the closing of a sea anemone. I shall describe these 

 movements with a view to showing the existence of periodicity 

 or habit, and some other general resemblances to animal 

 physiology. 



The crocus is perhaps the best example of a flower which 

 opens and shuts in accordance with changes of external circum- 

 stances. The crocus is especially sensitive to changes of tem- 

 perature. If a light index is fastened into one of the petals or 

 divisions of the flower, very small movements are made visible, 

 and in this way it has been shown that the crocus actually appre- 

 ciates a difference of temperature of one degree Fahrenheit.^ 

 I have seen a crocus distinctly open when a hot coal was brought 

 near it. The use of this power of movement is connected with 

 the fertilisation of the flower. In the warm sunshine the flower 

 opens wide, and the bees are soon hard at work, and carry pollen 

 from one flower to another. If, now, a cloud hides the sun, the 

 temperature falls, and the crocus begins to close, and by the 

 time the sky has become overcast and the first drops of rain 

 fall, the precious pollen is housed safe beneath the roof of 

 petals. The crocus is warned of the coming danger by 

 the shadow of the cloud just as the fly is warned by the shadow 

 of the approaching hand. The crocus is sensitive to changes of 

 light and darkness as well as to changes of temperature, and the 

 sum of these influences alternately acting by night and day produce 

 a periodic opening and shutting which resembles the periodic 

 movement or sleep of the Sensitive Plant. Corresponding to the 

 regular repetitionj of the stimulus of light and heat, an internal 

 periodicity has arisen in the flower which shows itself in a 

 curious manner. This phenomenon is best shown by certain 

 flowers which are not so sensitive to temporary changes, but 

 which open and close regularly by day and night. Raising the 

 temperature in the evening does not produce nearly the same 

 amount of divergence of the petals as a similar rise in the 

 morning. With the white waterlily, Oxalis rosea, and some 

 other flowers, the same thing is well seen. * If the flowers 

 have been allowed to close at the natural hour in the evening it 

 is hardly possible to perceive the least opening of the petals 

 even when the temperature is raised from 50° to 82°. On the 

 other hand a considerable lowering of temperature does not 

 produce so much effect in the morning as it does towards even- 

 ing. In all biological problems it is necessary to consider the 

 internal condition of the organism quite as much as the other 

 element, viz,, the external condition. It is a familiar fact that 

 similar external causes do not produce like results. A man may 

 fall ill after exposure to wet and cold at different times of his 

 life and the kind of illness may be very different. Once it may 

 be rheumatic fever, another time pleurisy, or some other malady, 

 so that in the case of the flowers which, under a given change 

 of temperature, behave differently at different times of day, 

 we see the variability in the internal condition or receptive 



' Pfeffer, "Physiologische Unters.," 1873, p, 183. 

 ^ Pfeffer, "Physiologische Unters.," p. 195. 



state of the organism exemplified, the most interesting fact 

 being that the receptiveness varies not capricously but with 

 periodicity. 



The same phenomenon may also be seen when the cycle is 

 a yearly and not a daily one. A German physiologist has lately 

 made a long and patient research on the yearly periodicity in the 

 growth of buds. ^ The method consisted in ascertaining the 

 weight of 100 cherry buds gathered at frequently repeated 

 intervals throughout the year. In order to discover whether 

 the growth of buds would be equally increased in rapidity 

 at all times by a given increase of temperature, branches 

 were cut and kept in a greenhouse at a temperature of 

 60 to 70 at various times of the year. This experiment 

 showed that branches thus treated in the beginning of 

 December were hardly at all hurried on in growth, while the 

 rise of temperature at once produced energetic growth in buds in 

 the middle of January. If this fact is to be classed with the very 

 similar effects of temperature on the daily periodic changes in 

 flowers — and I can hardly doubt that it ought to be so classed 

 — a difficulty arises. The buds being new growths, have 

 never experienced a previous winter or spring, so that the 

 periodicity cannot originate in their tissues ; it must, therefore, 

 depend on some property common to all the branches, some 

 periodicity common to the nutrition of the tree, Askenasy 

 describes the case as the occurrence of some chemical change 

 which goes on in the buds, rendering them sensitive to rise of 

 temperature at a certain period. The case bears a resemblance 

 to the hybernation of animals. Thus, Berthold ^ says that when 

 the dormouse, Myoxus avellanarius first goes to sleep in the 

 autumn, it can be partly awakened, and then sent into deep 

 sleep by alternations of temperature, answering, like the crocus, 

 to alternations of heat and cold ; but when the winter sleep has 

 fairly set in, no effect could be produced by raising the tempe- 

 rature, — ^just as the oxalis and water lily when once shut for the 

 night could not be made to open. 



I have no doubt that many closer analogies will some day be 

 shown to exist between the behaviour of plants and animals, as 

 regards nerve-physiology. The after-effect of stimuli seems to 

 be represented in the movements of plants. If a stimulus is sud- 

 denly applied and then removed, the nerves acted on do not 

 cease to be disturbed the instant the stimulus ceases. The 

 molecular change, whatever it is, which goes on in the nerve, 

 cannot leave off directly the stimulus ceases. The molecular 

 action goes on like the vibration of a bell after it has been struck. 

 When a wheel is turned round rapidly before our eyes the image 

 of a new spoke strikes the retina before the image of the old 

 one has died away, so that we cannot distinguish one from 

 another. In the same way a burning stick whirled round looks 

 like a circle of fire. This after effect of stimuli is represented in 

 plants by heliotropism and geotropism. I have myself observed 

 it in the latter. I took a young growing shoot and put it through 

 a hole in a cork, so that it was firmly fixed into a bottle of water. 

 I then put the bottle on its side in a vessel filled with wet sand, 

 and fixed it firmly by piling wet sand over it. The shoot thus 

 projected horizontally from the vessel of sand. It now began to 

 straighten itself by geotropism, that is to say, the tip of the 

 shoot began to curve upwards. I applied a delicate means of 

 measuring this upward movement, and allowed it to continue for 

 some time. I then turned the bottle round on its axis, so as to 

 rest on what had been its upper surface, and the action of 

 gravity being now reversed as far as the shoot went, the tip 

 ought to have reversed its direction of growth, and curved up- 

 wards, but instead of this it went on curving towards the earth in 

 consequence of the after-effect of the old stimulus. And it was 

 more than an hour before it could reverse its movement, and 

 again grow upwards. 



With this case I conclude my comparison of plants and 

 animals. Some of the points of resemblance which I have 

 attempted to point out are purely analogical. Nevertheless, 

 I have tried to show that a true relationship exists between the 

 physiology of the two kingdoms. Until a man begins to work 

 at plants, he is apt to grant to them the word " alive " in rather 

 a meagre sense. But the more he works, the more vivid does 

 the sense of their vitality become. The plant physiologist has 

 much to learn from the worker who confines himself to animals. 

 Possibly, however, the process may be partly reversed — it may 

 be that from the study of plant-physiology we can learn some- 

 thing about the machinery of our own lives. 



I Askenasy, Bot. Zeitung, 1877, No. 50, 51, 52 ; abstract NatnrforscJtcr, 

 1878, p. 44. 

 « Bertholdj Mailers Archtv, 1837, p. 63. 



