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’.