skip to content

Darwin Correspondence Project

Movement in Plants

MS-DAR-00209-00015-000-00089c.jpg

Leaves of Phyllanthus (leaf flower) and Cassia species showing variable movements, including twisting 180 degrees on axis, under different amounts of light.
https://cudl.lib.cam.ac.uk/view/MS-DAR-00209-00015/91
Leaves of Phyllanthus (leaf flower) and Cassia species showing variable movements, including twisting 180 degrees on axis, under different amounts of light.
DAR 209.15: 44br

The power of movement in plants, published on 7 November 1880, was the final large botanical work that Darwin wrote. It was the only work in which the assistance of one of his children, Francis Darwin, is mentioned on the title page. The research for this book is well documented in correspondence, partly because Francis visited the botanical institute at Würzburg for two summers and exchanged letters with his father about their research while he was away from home. Although Darwin lacked a state of the art research institute and assistants, he was able to co-opt the advantages of both while Francis was working abroad. Darwin was privy to the inner workings of the laboratory and kept informed about similar research being pursued by other naturalists who, like Francis, had come to this centre for the study of physiological botany to learn the latest experimental methods and use the most advanced laboratory equipment. Darwin also benefitted from the instrument-building prowess of his youngest son, Horace, who not only copied but also improved on some of the apparatuses that Francis had been introduced to at Würzburg. Darwin described his experimental practice throughout the book, but it is revealed in much greater detail in his correspondence. At first glance, this book seems to fit neatly into the realm of experimental plant physiology, but it was at its core informed by Darwin’s theory of evolution, particularly by his ideas about adaptive behaviour.

 

‘One general law or system’

In the early 1860s, at a time when his health was especially bad, Darwin had taken up the study of climbing plants, one of his ‘hobby-horses’, to keep himself busy when the task of writing his large work, The variation of animals and plants under domestication, eventually published in 1868, became too strenuous. His paper, ‘On the movement and habits of climbing plants’, appeared in the Journal of the Linnean Society (Botany) in 1865, and was an attempt to explain the evolution of climbing in all its forms. It was quickly reproduced as a small book, giving it a much wider audience. Darwin was not the first naturalist to study the mechanics of climbing, but he was the first to consider the topic within an evolutionary framework. He received a wealth of information from correspondents in response to the work, and by 1873 began preparing a second edition, which eventually appeared in 1875. In the same year, Darwin published a much longer work, Insectivorous plants, also the result of research begun in the early 1860s. Both books dealt with similar questions about the nature of movement, so much so, that at one point Darwin had considered combining the works in a single volume (letter to J. V. Carus, 7 February 1875). While Climbing plants focused mostly on the structure and structural changes of various plant organs and the mechanics of their movement, Insectivorous plants investigated the physiological aspects of both the movements involved in capturing prey and the subsequent digestive processes. With his final great botanical work, Darwin would attempt ‘to bring all the diversified movements of Plants under one general law or system’.

Darwin was no stranger to physiology in contexts other than botany. His 1872 work, Expression of the emotions in man and animals, relied on some of the most advanced work on human and animal physiology to explain how certain facial movements usually associated with uniquely human emotions had their origins in non-human animal expression. Darwin had not done experimental work in animal physiology himself, but he applied the methodology to his later investigations in the plant kingdom. Unlike some contemporaries who emphasised differences and viewed the division between animals and plants as absolute, Darwin was interested in similarities. What was the plant equivalent of digestion or reflex action at a physiological level? Was there a vegetable nervous system? How could existing organs become adapted to perform new functions, like climbing? For Darwin, physiology was a way of seeing how adaptation occurred.

Experimental plant physiology, while just beginning to gain followers in Britain, was already well established in German-speaking universities and agricultural institutes. Julius Sachs had set out the basic tenets of this research in his seminal handbook on experimental physiology of 1865. Sachs, who spent six years at the agricultural institute in Poppelsdorf, had devised several new instruments with which to quantitatively study aspects of growth, environmental influences, and the function of specific plant organs. This research had direct application for agriculture, so practical and theoretical studies were closely integrated when Sachs set up his laboratory in the botanical institute at Würzburg in 1868. His Lehrbuch der Botanik (Textbook of botany), published in the same year, became the standard work of plant physiology, and by the early 1870s, Sachs’s laboratory was attracting students from all over Europe and beyond. When Darwin’s son Francis worked in this laboratory in the summers of 1878 and 1879,  he encountered some of the most cutting edge research of the time.

 

‘Mad about drops of water’

Darwin’s interest in the diversified movements of plants was stimulated by a phenomenon seemingly unrelated to movement — the nature and function of bloom, the waxy or powdery coating often found on leaves or fruit. This connection is revealed only though correspondence because Darwin never published on bloom. In August 1873, while on holiday in Southampton at the home of his son William, Darwin wrote to his friend Thomas Farrer, ‘I am now become mad about drops of water injuring leaves’. He suspected that drops of water standing on the surface of a leaf might act like a lens focusing light rays, and burn sections of the leaf blade. Darwin asked whether Farrer’s gardener had observed the phenomenon. A few days later, Darwin wrote to Joseph Hooker, ‘Why are the leaves & fruit of so many plants protected by a thin layer of waxy matter (like the common cabbage) or with fine hair; so that when such leaves or fruit are immersed in water they appear as if encased in thin glass. It is really a pretty sight to put a pod of a common pea, or a raspberry into water. I find several leaves are thus protected on the under surface & not on the upper. … How can water injure the leaves? if indeed this is at all the case’. Hooker, who had also speculated on the topic, replied, ‘I can quite fancy water impeding both the actinic & calorific effects of sun-light on the leaf. We find watering most prejudicial in the hot sun. It is a splendid subject for experiments’. 

Darwin was clearly intrigued by bloom, but his main preoccupation in the summer of 1873 was his experimental work on insectivorous plants. Returning to bloom in October 1873, he asked his son George to calculate ‘what inclination a polished or waxy leaf ought to hold to the horizon, in order to let vertical rain rebound off as completely as possible’. He had also asked Horace to discuss the point with his friend Francis Balfour(258). Darwin promised to reflect on Balfour’s now missing reply, and mused, ‘As such a multitude of plants get their leaves wetted, & only a few are protected by a waxy secretion, I cannot but think that these latter must be injured in some special way— Moreover the yellow spots on the leaves look like some direct agency—’.

Movement in plants, p. 370.

Given that the function of bloom appeared to be protective, Darwin began to consider what other means plants might possess to protect themselves from the injurious effects of water. By November 1873, he was already devising experiments to show that movement was one such method. Working on Mimosa albidafrom Kew Gardens, he explained to Hooker, ‘I have never syringed (with tepid water) more than 1 leaf per day; but if it dies, I shall feel like a murderer. I am pretty well convinced that I shall make out my case of movements as a protection against rain lodging on the leaves’. Darwin then studied an even more interesting plant, a species of Cassia that was remarkable for its range of movement, which included being able to twist each separate leaflet around so that the lower surface of the leaf faced upward. He described to William Thiselton-Dyer how he ‘syringed the plant for 2 minutes, & it was really beautiful to see how each leaflet on the younger leaves twisted its short sub-petiole, so that the blade was immediately directed at an angle between 45o & 90o to the horizon’. By May 1874, Thiselton-Dyer had observed some cactus species in the genus Opuntia, and could report, ‘I confess I was astonished at the readines with which the lightest syringing we could give them elicited movement’. Darwin, however, had to finish his work on insectivorous plants, despite confessing to Thiselton-Dyer that deferring his work on bloom until the following summer ‘goes to my heart’. It would be another three years before Darwin would resume work on movement and bloom.

 

‘Very curious results’

In May 1877, Darwin asked one of his most trusted correspondents, Fritz Müller, to ‘observe whether any of your plants place their leaves during rain so as to shoot off the water; & if there are any such I should be very glad of a leaf or two to ascertain whether they are coated with a waxy secretion’. He told Hooker, ‘I have been looking over my old notes about the ‘bloom’ on plants, & I think that the subject is worth pursuing, though I am very doubtful of any success.'. Just two months later, Darwin put Francis in charge of this aspect of the investigation as he revealed to Thiselton-Dyer, ‘Frank & I are working very hard on bloom & sleep &c.; but I am horribly afraid all our hard work will yield uncommonly little if any fruit. …  I think Frank will do some good work on bloom & evaporation, & this is to be his share’. 

Movement in plants, p. 300.

Darwin now began to study so-called sleep in plants (the change in the position of leaves at different times of day) along with mechanical irritation and spontaneous movement. Richard Lynch, the foreman of the propagation department at Kew, provided information about Averrhoa bilimbi, a plant that exhibited all three types of movement (letter from R. I. Lynch, [before 28 July 1877]). ‘I do not believe I shd. have ever have noticed the movement had it not been for your information’, Darwin told Lynch, after carefully measuring the amount and time of movement in this species. Bloom was not neglected. Darwin told Thiselton-Dyer, ‘we have made out clearly that with some plants (chiefly succulent) the bloom checks evaporation.— with some certainly prevents attacks of insects— with some sea-shore plants prevents injury from salt-water—& I believe with a few prevents injury from pure water resting on leaves— This latter is as yet the most doubtful & the most interesting point in relation to the movement of plants’. Darwin vacillated between optimism and doubt, telling his daughter Henrietta, ‘Frank & I have been working very hard at bloom & the automatic movement of plants, from morning to night & we have made out a good deal’, but confiding to Hooker, ‘We have been working like slaves, & God only knows whether our results will prove worth the labour’. 

Darwin had first investigated movement in mature plants, but he increasingly focussed on the movements of the first leaves to appear when a plant emerged from the seed. Noticing that in a number of species these embryonic leaves, known as cotyledons, moved in ‘closely analogous manner’, Darwin told Thiselton-Dyer, ‘I expect to find such movements very general with cotyledons & I am inclined to look at them as the foundation for all the other adaptive movements of leaves’. He confirmed this view to Hooker, ‘From what Frank & I have seen, I think we shall be able to show that all the automatic movements of mature plants are developments of the wonderful automatic movements of the stem & cotyledons of all the plants which we have as yet observed.' With correspondents other than close friends, Darwin was more circumspect; he told Gaston de Saporta, ‘I am at present working with my son at some physiological subjects & we are arriving at very curious results, but they are not as yet sufficiently certain to be worth communicating to you’.

The next puzzle for Darwin was to distinguish autonomous movements of cotyledons and radicles (embryonic roots) from those caused by external agents such as the effects of light or of gravity, which had been called heliotropism or geotropism, respectively, by German physiologists. He told his American friend Asa Gray, ‘My son Frank & I have been observing the autonomous movements of seedlings & those due to Heliotropism, which latter from their complexity have almost driven us mad’. Gray replied, ‘Do work at heliotropism & the like. The Germans make me mad with their pottering pretence and their names, to stand in place of explanation’. 

 

‘I cannot resist telling you’

Darwin’s experimental work required patience, acute observation, and new apparatus. ‘It is impossible for me to predict whether or not we should ever want this or that instrument, for we are guided in our work by what turns up’ he told Thiselton-Dyer. ‘Thus I am now observing something about geotropism, & I had no idea a few weeks ago that this would have been necessary. In a short time we might earnestly wish for a centrifugal apparatus or a heliostat’. Shortly after this, he reported some progress in understanding movement, telling Hooker, ‘I think we have proved that the sleep of plants is to lessen injury to leaves from radiation.—’. Darwin was still discovering new types of movement, as he explained to Müller, ‘Very many thanks for the seeds of the Viola; by an odd chance, I have just raised seedlings of Trifolium subterraneum & Arachis hypogæa, & now I shall have a third plant, so as to observe how the flowers penetrate the earth. For several months my son & self have been at work on the biology of seedling plants, & observations on the radicles make me wish to observe subterranean flowers,—that is if we can succeed in doing so’. 

The investigation of early movement was now totally absorbing Darwin. ‘At present I care for nothing in this wide world except the biology of seedling plants’, he confessed to Farrer. He was not above a little gloating when he told Thiselton-Dyer, ‘What trifles determine the success of experiments; Sachs missed a pretty little discovery solely by keeping his germinating beans too warm.— What magnificent work he has done on radicles. … I cannot resist telling you a little about the radicles. The apex is sensitive, & instead of turning to touching object like a tendril, it turns from it.’ The sensitivity of the tip of the radicle would turn out to be a pivotal but contested discovery by Darwin. Darwin would soon become more familiar with the research in Sachs’s laboratory as Francis’s departure for Würzburg was imminent, but he would miss working alongside his son.  He told Theiselton-Dyer, ‘I am working away like a slave at radicles & at movements of true leaves, for I have pretty well done with cotyledons. Alas Frank is off tomorrow to Wurzburg, & work by myself will be dull work’. Francis was in Würzburg until early August.

Francis lost no time in quizzing Sachs about plant movement and bloom, reporting back Sachs’s assessment of experiments made by Theophil Ciesielski, who had been a student of Ferdinand Cohn in Breslau: ‘One fact of his would be interesting if it were true but Sachs says it is “falsch’— If you cut the extreme tip of a root, it goes on growing pretty well though the punctum vegetationis is destroyed. Cieselski says that while this growth continues & before a new punctum vegis. is formed that the roots does not care for gravity & will grow in any direction it is placed in This Sachs says is not true. … Also in bloom being protection against dryness. He thinks blo⁠⟨⁠om⁠⟩⁠ must have many functions & said it was a protection against insects’. Sachs may have dismissed Ciesielski’s experiments out of hand, but his results would prove crucial as support for Darwin’s own experiments testing the sensitivity of the root tip to various stimuli.

Movement in plants, p. 179.

In May 1878, Darwin had pointed out the importance of temperature in affecting the sensitivity of bean radicles, and in early July he discovered that radicles of maize were ‘just reverse of bean’; that is, they required a much hotter temperature to become sensitive. Cotton radicles remained insensitive, a fact Darwin attributed to not being able to ‘get right temperature or keep air damp enough’. Questions related to experimental design and the need for good instruments were never far from Darwin’s thinking. Francis viewed the new instruments he was introduced to in Sachs’s lab with a critical eye, telling his father, ‘There is one machine we must have. A strong horizontal axis about 2 feet long which goes round by clock work slowly so that geotropism is quite excluded. We will get Jemmy to design one, the one here is far from well made’. Jemmy (a nickname for Darwin’s youngest son Horace) did, indeed, design an improved version of the instrument, a klinostat; Francis later described and illustrated Horace’s machine in a paper (F. Darwin 1880, pp. 449–55).

Diagram of a klinostat. Journal of the Linnean Society. Botany. 1881. Vol. XVIII, p. 450.

 

‘On evolution principles’

The question that continued to absorb Darwin was the relationship between bending movement and growth. Francis described the disagreements about the physiology of the pulvinus, a joint-like bundle of cells at the base of the leaf petiole in many plants, ‘I think the great distinct Pfeffer makes is between sleepers with joints & those without joints, & says that there is no growth but only variation in tension in those that have joints. But Batalin says there is very slight growth even in those with joints’. Darwin’s own investigation led him to conclude that ‘a pulvinus must be developed from ordinary cells, which secrete water into the inter-cellular spaces on the concave side of a bending organ; & that a pulvinus is developed only when the bending has to be continued for a period after growth has ceased or nearly ceased’. Sachs’s view that ‘the growth along the convex side is only in consequence of the increased turgescence which precedes it’ was reported by Francis, who added that Sachs ‘doesn’t think very much of Pfeffer, that is he says he is very learned & does very good work, but he doesn’t think clearly’. Darwin was not convinced. While he allowed that the contraction of a tendril into a spire could be a case of ‘growth prolonged on one side alone after it has ceased on all other sides’, he could not believe that a tendril curling when touched in less than a minute was due to growth.

Movement in plants, p. 113.

Unlike Sachs, who viewed the relationship between growth and curvature as a purely physiological question, Darwin thought of the problem in terms of adaptation. He argued, ‘from the necessity of the case on evolution principles’, that there was an emptying of the cells of water on one side and turgescence on the other; while growth was one possible outcome, the turgescent cells could also empty when repeated movement was required for a long period. Light, gravity, or contact with a solid body would produce some response on the affected side but turgescence would only result if the plant benefitted in some way, as determined by natural selection ‘according to requirements of the species’.

Francis was also thinking in terms of adaptation while further researching bloom. Sachs thought that the Darwins had ‘worked at bloom to some extent from a wrong point of view’ because he believed that ‘leaves want to keep dry in order that they may keep their stomata open to breathe’. This lead Francis to wonder why leaves were ‘so idiotic as to have stomata which shut when they are wet. Perhaps they do that in order to keep their intercellular spaces from being water logged & then found they were stifling themselves & found it a better plan to keep the water off altogether. Or perhaps those leaves produced bloom whose stomata did not shut well in wet’. Sachs suggested literature on stomata and Francis followed up, but his own observations often seemed to be at odds with what he read. Darwin advised: ‘It really seems that you must trust to your own observations alone on stomata. May not the stomata be variable even in the same species. Such variation may be expected in all characters which differ much in allied species of the same genus; & if I remember right the stomata do differ in the species of the same genus’.

 

‘This will be an awful job’

When Francis returned home, his detailed correspondence with his father ended. However, Darwin continued to discuss aspects of his work on movement with other correspondents.  He told Hermann Müller, ‘I am working away on some points in vegetable physiology; but though they interest me and my son, yet they have none of the fascination which the fertilisation of flowers possesses. Nothing in my life has ever interested me more than the fertilisation of such plants as Primula and Lythrum, or again Anacamptis or Listera’. Nonetheless, he continued to make requests for seeds in order to continue his observations on the emergence of young plants. William Williamson, who had raised plants of Drosera species from seed, was asked to send any spare seeds he might have. ‘I shd. like to see how the embryo breaks through the ground’, Darwin explained, before asking for information on how to sow them as he had ‘hitherto failed in raising seedling Droseras’. 

After the pleasure of the experimental work with Francis, Darwin was less enthusiastic about writing up their results. He told Thiselton-Dyer, ‘I have nothing to say about myself, except that we have almost finished the experimental part of our work, & must now begin modelling an enormous pile of notes into some sort of fashion, & this will be an awful job.’. It was also necessary for Darwin to plough through the literature on movement, much of it in German such as Wilhelm Pfeffer's book Die periodische Bewegungen der Blattorgane (The periodic movements of leaf organs), which detailed the leaf movements of several species that Darwin had not worked on. ‘I get to detest the German language more & more. What a job it is to read Pfeffer’, he grumbled to Thiselton-Dyer.  Darwin’s tone was even more pessimistic a month later, when he confessed that Thiselton-Dyer’s tireless generosity in fulfilling requests for plants and seeds ‘makes me feel ashamed of myself, & I cannot help thinking “oh Lord when he sees our book he will cry out is this all for which I have helped so much”.— In seriousness I hope that we have made out some points, but I fear that we have done very little for the labour which we have expended on our work.’. When he wrote this, Darwin was away from home in London, but Francis carried on with experiments at Down, reporting on work with ‘beastly horse chestnut roots’ and testing for dissolved nitrogenous matter from geranium leaves as part of his experiments on the function of bloom.

By December 1878, Darwin was thinking about the illustrations for his work on movement in plants. James Cooper, who had engraved diagrams for both Descent and Expression, and suggested a method of reducing the size of diagrams on the woodblock using photography for scientific accuracy (letter from J. D. Cooper, 13 December 1878). The method would be expensive, so Darwin hesitated, stating that the method was ‘all that I can desire, but as I shd  like to give a very large number of similar diagrams, I rather grudge so expensive a manner as Photographs for the more simple ones’. While he worked out practical details and continued organising notes, Darwin missed experimental work, telling Thiselton-Dyer, ‘trying to make out something new is so much more interesting than compiling old notes’. Thiselton-Dyer had continued to send plants from Kew for Darwin to observe, but Darwin was frustrated in his work because of the bad weather. He had been unable to observe apheliotropism (turning away from the sun) in a specimen of Darlingtonia californica, which required bright sun, and concluded, ‘I will give up all experiments until the Spring is well advanced for it is heart-breaking work now’.

Darwin was beginning to receive more letters from other researchers. The Russian plant physiologist, Alexander Batalin told Darwin about a series of experiments he designed to determine what benefits plants derived from leaf movements. He reported few results, noting that when leaflets of Mimosa were prevented from moving they lost their colour and died, but concluded, ‘About the cause of this phenomenon I know nothing’. Batalin followed up with his observations of Oxalis acetosella, noting that the leaflets bent down in strong sunlight but if prevented from doing so, even without damaging them, they lost colour, withered, and died within a couple of days (letter from A. F. Batalin, 28 February 1879). Darwin was especially keen for his results to be confirmed by trusted correspondents and worried when there were conflicts. After reading a paper by Hugo de Vries in which the author remarked that in many cases he noticed no influence of heliotropism on cotyledons, Darwin  wrote to determine how their observations could have been so much at odds (letter to Hugo de Vries 13 February 1879). He was reassured by De Vries, who cited special circumstances, and affirmed, ‘that Cotyledons and young leaves turn to a lateral light, independent of the movement of their stems’.

 

‘Almost too old’

When Francis spent a month in Algiers in early 1879, Darwin asked him to visit the botanist Gaetano Durando, to find plants and seeds (letter to Francis Darwin, [4 February – 8 March 1879]). He continued to write up the experiments on movement while Francis was away, telling him, ‘I have begun this day my chapters on Sleep of Plants & I can see that it will be an awesome job.’ On the same day, he told Thiselton-Dyer, ‘I am overwhelmed with my notes & almost too old to undertake the job which I have in Hand—ie movements of all kinds. Yet it is worse to be idle’. The lull in experimental work continued into March 1879, and Darwin seemed weary when he told Fritz Müller, ‘I have little or nothing to tell you about myself. I go on slowly crawling on with my present subject the various & complicated Movements of Plants’. More plants arrived from Kew in early April, but around this time Darwin’s work on plant movement was interrupted by his decision to work on Erasmus Darwin, a biographical sketch of his grandfather.

Darwin’s spirits picked up when Francis, spending another summer in Würzburg, reported, ‘Sachs seems to have completely changed his ideas about the cause of heliotropism & quite given up the idea that it is merely the shaded side growing quicker: he spoke as if these experiments of mine were hardly worth doing because it was so certain that the heliotropism does not depend on the mere difference of light on the two sides’. Francis also mentioned a new colleague Ernst Stahl, who was happy to help with Darwin’s research on movement in growths known as witches brooms that occurred on some fir trees. Stahl also spoke favourably about another researcher Albert Frank, who like Darwin, looked at plant movement as adaptive behaviour. Darwin was delighted to hear about these young researchers, whose views, he thought, echoed what he had ‘long been saying’. Frank had proposed that there were special forms of growth in plant organs, characterised by an inherent tendency to be horizontal or to be placed at a right angle to the direction of gravity or a light source; he referred to these as ‘Transversal-Geotropismus’ and ‘Transversal-Heliotropismus’. Darwin also believed this type of movement was a particular response to stimulus, not merely an averaging of conflicting geotropic and heliotropic forces, and later used the term diaheliotropism to refer to it in his book.

 

‘What-ever Sachs may say’

Francis was peppered with requests from his father, who asked, ‘when time allows remember I want much to know whether there is chlorophyll in the cots. of the Canary grass (Phalaris) & Oat. Also remember to learn about cutting thin sections of soft leaves &c.— Lastly the instrument for making marks at equal distances on stems &c.’ The instrument was the self-registering auxanometer, invented by Sachs around 1870, but a version of it was made by Darwin’s youngest son Horace, who also made an improved version of another of Sachs’s inventions, the klinostat. This was now the direction of Darwin’s research, and when Thiselton-Dyer sent information about bloom, Darwin informed him that ‘this subject ‘for the present only, has gone to the dogs with me’. This appears to be the only indication that Darwin would decide to omit the subject of bloom from his book. 

In mid-June 1879, Darwin was pleased to get back to experimental work, telling Francis, ‘My work has been almost exclusively writing. & I am now finishing Summary on Sleeping Plants, which has been excessively difficult, but the result is, I think, satisfactory & makes a good essay. …I have begun cauterizing  tips of cotyledons of Phalaris & I think(?) this acts in same manner as black caps, ie. stops  basal part bending to light.’ Darwin realised how important it would be to his theory if he could demonstrate that the root tip acted like a primitive neural transmitter, receiving input (light, gravity, etc.) and relaying an appropriate response, based on the needs of the plant. In Würzburg, Francis was also experimenting with caustic (silver nitrate) to cauterise root tips, but told Darwin that he was using beans in sawdust since ‘Sachs wants the bean caustic experiment done in loose earth as he seems to suspect abnormalities in air experiments & I suppose thinks earth better than water’. Darwin was also working on roots by the end of June and reported, ‘the tip of radicle of Gossypium herbaceum is very sensitive to touch of caustic & bends from the touched side. This is good for bits of card did not act at all well. Also the tip when blackened for 1/2mm stops the geotropism of horizontally extended roots completely.

The work on roots was some of the most important and Darwin hoped to convince Sachs that root movement was controlled by the sensitivity of the tip. Darwin advised Francis to demonstrate some of their bean experiments to Sachs, suggesting, ‘Would it not be well to show Sachs effect of touching apex once lightly one side’, and added, ‘What-ever Sachs may say, it seems to me important to prove that an aphelic organ grows quicker in dark— it is good concurrent evidence that light is only the regulator & not cause of movement’. In the same letter, Darwin discussed terminology, a subject he had been mulling over for a while and discussing with his sons. Francis had consulted Karl Goebel, an assistant of Sachs and an expert in classical languages. Darwin wanted terminology to be as short and straightforward as possible and would have accepted Goebel’s suggestion of ‘proshelic’ and ‘aphelic’ as adjectives if substantives could be made, concluding, ‘If not I think I will stick to Heliotropism & apheliotropism—to heliotropic & apheliotropic’. This terminology worked well with a later coinage, ‘diaheliotropism’, suggested by Darwin’s son William in February 1880, probably to replace Frank’s ‘Transversal-Heliotropismus’ (letter from W. E. Darwin, 10 February [1880]).

Francis became increasingly frustrated trying to convince Sachs of the significance of their root experiments. He complained to his father, ‘I did the caustic experiment with Faba & Phaseolus in damp earth & by evening they had all grown well & the caustic ones had not bent, but next morning many of the caustic ones were bent—so Sachs doesn’t believe in it a bit; he says the growth is disturbed and that anything that disturbs growth prevents geotropism— as for instance merely growing in damp air in some cases. He also says caustic is not a proper thing for the work because nitric acid will be set free & this will be diffused back into the root & injure it!!!! … I said you had done experiments with smoked glass and that the roots grew down sloping surfaces not by pressing hard against them, but only touching in a number of places or at least touching very lightly Then he said that the smoke may cause injury to the root! one feels inclined to say— If you say that its no use talking with you’. Darwin was philosophic, replying, ‘I am very sorry that Sachs is so sceptical, for I wd. rather convert him than any other half-dozen-Botanists put together; but I expected it’. Darwin also pointed out that his early bean experiments had been done with clear glass. Glass was smoked by holding it over a flame, which resulted in a fine layer of carbon residue that could easily be marked by being touched by the root.

Movement in plants, p. 531.

Francis was undeterred by Sachs’s negative response to his bean experiments and devised a new test, which he described in a letter to his mother, ‘I did some beans extended horizontally in damp earth some causticed above others below & the difference was very striking 2 of those causticed above being more geotropic than the control beans, while the under caustic were only faintly geotropic (tho’ they were somewhat bent)’. Darwin praised Francis’s ‘splendid idea’ that showed that the application of caustic on one side did not interfere with the bending, and leaving Darwin in no doubt that the apex was ‘a kind of brain for certain movements, like the gland of Drosera for inflection—or the hairs on Dionæa—ie a specialised centre for receiving certain irritation’. Darwin wrote again the same day pointing out, ‘it seems to me highly important that cauterising tips of radicles should not prevent apheliotropism … for this shows that it is no absolute consequence of the application of cautic, that the radicles cannot bend … They do not when cauterised bend geotropically & why shd we say this is owing to injury, when they do bend when cauterised to darkness’. 

While reviewing Sachs’s experiments with roots, Darwin told Francis, ‘It will be very important for us to learn whether it is the tips of radicles that perceive & cause them to bend to damp surfaces, so learn if you can how Sachs tried Beans’. Francis also wanted to replicate Sachs’s own experimental design, telling his father, ‘I have been talking about the sieve experiment. It all depends on getting the right degree of dampness of the surrounding air— Phaseolus are said to act well, I will try it here with his sieve in the same place where he does it’. Sachs’s experiment involved sieves containing seeds germinating in damp sawdust being suspended so that the bottom was inclined at 40° to the horizon, and the tips of the radicles being coated to exclude moisture. A few days later, Francis confirmed, ‘Caustic mustards grew as much as the not caustics & were aphelic again today’. Francis’s results regarding growth in roots of Sinapis alba (white mustard) whose tips had been cauterised confirmed that the use of lunar caustic (silver nitrate) did not cause systemic injury to the root as had been suggested by Sachs.

 

‘Safe at home again’

Darwin had already begun to think about not only the publication, but also the translation of his work. He told his German translator, Julius Carus, ‘Together with my son Francis, I am preparing a rather large volume on the general movements of Plants, & I think that we have made out a good many new points & views. I fear that our views will meet a good deal of opposition in Germany; but we have been working very hard for some years at the subject. …I shall be much pleased if you think the book worth translating & proof-sheets shall be sent you, whenever they are ready’. As July drew to a close, Darwin seemed less hesitant than usual at the prospect of his annual family holiday telling his close friend Hooker, ‘I have been working pretty hard of late & want rest & change, so we all go on August 1st to Coniston for a month’. As the date approached, Darwin returned some of the plants he had received from Kew, but kept specimens of two vines, one of which had only just begun to produce aerial roots. Darwin had hoped to study the tropic movements of such roots, but since their movement was so slow, he had failed to confirm that they were apheliotropic. He now seemed more reluctant to spend a month away from work, telling Thiselton-Dyer, ‘I wish that my holiday were over & that I was safe at home again’.

As Francis’s time abroad drew to a close, he was excited to tell his father that Sachs had asked him to publish his research on roots in the Institute’s own journal, Arbeiten des botanischen Instituts in Würzburg. The only problem was that his results conflicted with Sachs’s published views. Francis explained, ‘It is rather difficult to write as I must not quote the last edition even of the Lehrbuch as there Sachs evidently expected negatively heliotropic roots to grow quicker in light’. Nevertheless, Sachs even agreed to translate the paper into German, and it appeared in 1880 (F. Darwin 1880b). In the same letter, Francis revealed the frustration of some of his colleagues, who were pressured by Sachs to conform to some of Sach’s theories. One friend, Fredrik Elfving(1485), thought Sachs’s view on the cause of horizontal growth in certain roots ‘pure bosh’, and said he would not alter his manuscript. Francis also mentioned he wanted to visit the laboratory of Anton de Bary in Strasbourg on his way home. Francis was clearly impressed with De Bary and his laboratory, and would later spend three months there from May 1881.

While on holiday in the Lake District, Darwin received a long letter from De Vries detailing his latest research on the relationship between turgescence and growth (letter from Hugo de Vries, 7 August 1879). Darwin replied, ‘I thank you much for your letter, which has interested me more than anything which I have read for a long time.— I have gradually been coming to the opinion that in all the cases to which you refer, growth was preceded by a change in the turgescence of the cells, or by some such change; but then I had very little evidence, & my opinion was chiefly founded on general considerations, which are often deceptive.— I hope that you will publish in the course of the Winter, so that I may be able to read & refer to your evidence before the Spring’. Luckily, De Vries published two papers in 1879 and 1880 that Darwin was later able to refer to in his book. De Vries continued his turgor experiments, and in September 1879 reported that movement increased the water-absorbing ability of the parenchyma, the soft inner tissue composed of thin walled cells, and concluded, ‘it is the water-absorbing power, that plays the principal part in the growth and the movements caused by stimulus’. Darwin was somewhat doubtful, asking, ‘Do you feel sure that the cell walls have not a power of contraction; for I could not avoid suspecting that they had this power, whilst observing the movements of Drosera and Dionæa’.
 

‘A horrid bore’

In late October 1879, Darwin told Gray, ‘I have written a rather big book,—more is the pity—on the movements of plants, & I am now just beginning to go over the M.S. for the second time, which is a horrid bore’. He now turned to Thiselton-Dyer for help with identification of a cryptogam and asked about correct terminology for referring to parts he described as ‘little discs’ and ‘greenish bodies’ (letter to W. T. Thiselton-Dyer, 29 October 1879). Thiselton-Dyer, who had assisted in the English translation of Sachs’s Text-book of botany, referred to the terms used in their translation, but added that Darwin’s suggestion of ‘fronds’ for the discs would ‘meet with very general approval’, and noted that the greenish bodies were ‘usually called gemmæ’. Although Darwin was supposed to be going over his manuscript, he requested seeds of a species of cotton that he had not been able to observe earlier (letter to W. T. Thiselton-Dyer, 20 November 1879). Hooker offered to write to Egypt for the seeds (From J. D. Hooker   29 November 1879; DCP-LETT-12336). Darwin replied that he would be ‘very glad of a few cotton seeds’, explaining, ‘it is only one little point which I somehow overlooked: the cotyledons, when old & large, sink downwards at night, & I neglected to prove that it was not merely their weight, with reduced tension of the tissues at night, which caused this periodical movement’.

Darwin’s next request was for more apheliotropic plants; he assured Thiselton-Dyer, ‘we see our way to good results if we had more plants to work on’. Having received seeds of two unusual American gourds from Gray, Darwin repeated his request for cotton seeds. He also worried, ‘I have just put 5 of the seeds of Megarrhiza to soak, but only one sinks & this alarms me.— I very much want to see whether the curious heel-like projection at the base of the hypocotyledenous stem, …which splits the seed-coats so beautifully in other Cucurbitaceæ, is here absent, as I hope & as ought to be the case as the Cotyledons are not with-drawn from the seed-coats’. A few days later, Darwin sent his thanks for cotton seeds from Kew and promised Hooker, ‘My work must & shall soon end, otherwise you & Dyer will wish me dead & buried’. A month later, he told Thiselton-Dyer his cotton seeds had germinated and added, ‘I hope my work will some day end, but new points are continually turning up’. His early results with Megarrhiza were also mentioned, bringing on a request for more seeds of ‘any Trichosanthes’; Darwin concluded, ‘Am I not a superb bore?!’.

Darwin was intrigued by the unusual germination of the seed in Megarrhiza, but his observations were at odds with those of Gray, who had written an article on the subject in 1877 (A. Gray 1877e). Gray had reported that the body of the seed was raised well outside the soil on what seemed to be a well-developed radicle. Darwin told Gray, ‘You have been misinformed about their germination, for I think you cannot have watched the whole process … the radicle bends down & penetrates the ground, but grows only to a length of about half an inch or less … its growth is arrested, and the lower ends of the tubular petioles grow quickly & penetrate the ground just like a root to a depth of nearly 2 1/2 inches; then their growth ceases, and now the radicle takes up the game & grows very quickly’. Gray suggested Darwin’s plants might have been too weak to lift the weight of the seed (letter from Asa Gray, 3 February 1880). The matter was finally settled by an informant of Gray’s in California, Volney Rattan, who had observed the plant in its native habitat and described the sprout (tubular petiole) as having penetrated the ground, the plumule remaining unopened underground. Rattan had noted that in one not so well covered germinating seed, a part that remained above ground had blackened from frost; he speculated that if the plumule remained above ground when germination occurred, the plant would be killed by frost (letter from Asa Gray, 4 April 1880). Darwin agreed, ‘It seems almost certain that the protection of the plumule from frost has determined the curious mode of germination’ and concluded, ‘Mr Rattan seems to be a real good observer, & that is a rare species of animal’.

 

‘Right well pleased’

Darwin was almost ready to send his manuscript to the printers, but had still not decided on a title. No one in the family liked his proposal of ‘Circumnutating Movements of Plants’, he told Robert Cooke of John Murray publishers, before suggesting ‘The Movements of Plants’ or ‘The Nature of the Movements of Plants’ (letter to R. F. Cooke, 23 April [1880]). Cooke replied, ‘We are as much puzzled as yourself, as to a Title for your new work’, and he enclosed a proof-sheet of the title page for Darwin to alter and return. After Darwin decided on ‘The power of movement in plants’, he immediately wrote to Carus, telling him, ‘I shall be right well pleased if you will translate my new book’. The manuscript was sent off towards the end of May 1880 and Darwin then spent a fortnight at his son William’s in Southampton. Just before he left, he received a copy of Alphonse de Candolle’s Phytographie (A. de Candolle 1880). In his letter of thanks for the book, Darwin promised to send a copy of his own book, explaining, ‘I think that I have succeeded in showing that all the more important great classes of movements are due to the modification of a kind of movement common to all parts of all plants from their earliest youth’. 

By July 1880, Darwin was correcting the first sets of proof sheets, but now wrote to his publisher for advice about the number of copies they should print (letter to John Murray, 10 July 1880). Moreover, since he worried about sales of what would be a long and highly technical work, he thought Murray might not want to publish on their usual terms (Murray bearing the initial cost of publication with Darwin receiving 2/3s of the profits), but on commission (Darwin paying the initial cost). Although the length of the book was projected at 600 pages, which, Cooke pointed out was ‘a good deal more’ than any of Darwin’s previous works, Murray was willing to publish on the usual terms (letter from R. F. Cooke, 15 July 1880). This was also preferable to Darwin, despite his already having paid Cooper for around 200 woodcuts, several using the more expensive photographic method. He also noted that although the book contained ‘much new & curious matter’, the profits would be small because there were ‘very few persons who care for physiological Botany in this country’. Cooke’s reply was not encouraging; taking into account the cost of illustrations, he admitted, ‘the prospect is rather more gloomy’. He could only suggest printing more copies or raising the price (letter from R. F. Cooke, 20 July 1880). Darwin demurred, however, stating, ‘I must take the risk & loss on my own shoulders. As I have made some money by science, I must now lose some for science. I will have 1000 copies printed off & from what you say charge 15s’.

Even at this very late stage, Darwin could not resist adding a reference to a recently published paper by Stahl (Stahl 1880a) on the influence of the direction and intensity of illumination on plant movement, and asked Francis, who was assisting with the proof sheets while on a holiday in Wales and had Stahl’s paper with him, for the relevant page numbers (letter to Francis Darwin, 5 August [1880]). Darwin was also very taken by Balfour’s address to the British Association for the Advancement of Science on the role of Darwinian theory in the growth of experimental embryology, which he read when it was published in Nature, (https://www.nature.com/articles/022410a0), and told Balfour, ‘In my recent work on plants I have been astonished to find to how many very different stimuli the same small part,, viz the tip of the radicle, is sensitive & has the power of transmitting some influence to the adjoining part of the radicle, exciting it to bend to or from the source of irritation according to the needs of the plant; & all this takes place without any nervous system! I think that such facts shd. be kept in mind, when speculating on the genesis of the nervous system’.

By mid-September 1880, Darwin was actively engaged with the foreign publication and translation of the work. He warned Carus that ‘the work appears to me to possess some value & novelty; but it is very dull’, and mentioned that there were 195 wood-cuts, which he would try to get stereotyped as cheaply as possible if Carus decided to translate the work into German. Darwin needn’t have worried. Carus was ‘most happy to translate the book’ and informed Darwin that the publisher, Eduard Koch had already agreed to publish it (letter from J. V. Carus, 18 September 1880). The American publisher, D. Appleton & Co, also agreed to publish as soon as stereotypes of the text were available from Murray (letter from D. Appleton & Co., 17 September 1880). Darwin was fortunate in having as his French translator Édouard Heckel, who had worked on the physiology of plant movement; having read the book, he told Darwin he had been surprised that there was no discussion of the movement of flower parts, like sleep in corollas or the reactive movement of pistils and stamens (letter from Édouard Heckel, 23 September 1880). 

Darwin wanted electrotypes of the illustrations made for the translations and asked about the cost of these (letter to R. F. Cooke, 16 October 1880). Cooke replied that although the actual cost would be £10, he advised, ‘you should make these foreign publishers pay more for at the usual rate of charging per inch &c they wd. be over £40’; he suggested charging £25 or £30, noting, ‘This sum would help to repay your own expenses on the work’. Darwin was keen not to charge the French more than the cost, since the publisher was even hesitant about paying for a translator, but added, ‘As my Books sell well in Germany it will, from what you say, be fair to charge Herr Koch 25£;; but he must not know that I have let the Frenchman have them for 10£’.

With the November publication imminent, Darwin began alerting friends that they would soon receive copies. He must have been gratified when Gray had written in September to tell him, ‘I long to see your Circumnutating book, and must write one, if not two notices of it in U.S.’ However, he continued to doubt the appeal of the book, especially to non-botanists. He told Alfred Wallace, ‘In 2 or 3 weeks you will receive a book from me if you care to know what it is about, read paragraph in Introduction about new terms & then the last chapter & you will know whole contents of book’. No letter to Balfour survives, but Darwin may have offered him the same advice because Balfour told him, ‘I have as yet only read the introduction & the last chapter. … The remarkable nervous system without nerves, for I do not know what else to call it, the existence of wh you have proved, must have a most important bearing on speculations as to the origin of the nervous system in animals’. As soon as the book was published, Darwin worried about the loss it would incur if no more than a thousand copies sold. He was heartened by an anonymous but favourable review in The Timesand told Cooke, ‘I wonder who in the world has been glorifying me in the Times—: it ought to sell a few more copies & then I shall not lose’. Darwin was happy to report to William, who also looked after Darwin’s financial affairs, ‘The publication will not cost me quite so much as I expected. Murray has sold 800 copies. The Times ought to help’ and repeated his advice, ‘Read only last Chapt. of my book’. Within a few days Cooke wrote, ‘We must print off immediately 500 more copies of the Movement of Plants, as we are in want of copies’, leading Darwin to exclaim to his son George, ‘Hurrah for the old bloody Times, Murray says 500 copies urgently required’. Darwin told William the news as well, noting, ‘instead of losing 1 or 2 hundred pound, Frank & I shall make a few pounds’. 

Darwin received thanks and positive comments from colleagues at home and abroad. Cohn concluded his letter of praise, remarking, ‘I don’t know if I should dare to express how much I admire in your last book as much as in your former, all the qualities of a great biologist and philosopher’. Darwin soon shifted his attention to his worm book, but briefly revisited Movement in plants a year later when Julius Wiesner published a book-length critique of Darwin’s work (Wiesner 1881). Francis would later respond to Wiesner’s critique in an article published in Nature the day after his father’s death (F. Darwin 1882). Darwin’s study of plant movement went beyond physiology in its scope, focusing not only how plants moved, but also why they did.