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Darwin Correspondence Project

From morphology to movement: observation and experiment


Pencils to simulate the action of an insect landing on a flower and brushing up against the sticky disc-shaped structure on the stamens
Pencils to simulate the action of an insect landing on a flower and brushing up against the sticky disc-shaped structure on the stamens
Orchids, p. 15

Darwin was a thoughtful observer of the natural world from an early age. Whether on a grand scale, as exemplified by his observations on geology, or a microscopic one, as shown by his early work on the eggs and larvae of tiny bryozoans, Darwin was fascinated with nature in all forms. He clearly enjoyed these pursuits, unlike his more formal academic training. But Darwin was also an experimenter, and many of his experiments were at the cutting edge for their time. We have seen that the idea of what constituted an ‘experiment’ evolved during Darwin’s lifetime (see What is an experiment?), and one of the principal areas illustrating this change was the German science of morphology.


Morphology as a science

The word ‘morphology’ had been coined by Johann Wolfgang von Goethe in 1797 to describe the study of the form, formation, and transformation of organic beings. Goethe’s conception of morphology as a science influenced both idealistic Naturphilosophie and later materialistic versions, notably that exemplified by Darwin’s German supporter Ernst Haeckel in his Generelle Morphologie der Organismen. Morphology was conceived to a large extent in opposition to static Linnaean taxonomy, which Goethe felt failed to take account of development and transformation of beings. While Linnaean higher categories were based on similar external structural features, Goethean morphological features were identified as derivations from a basic type modified by law-like processes. Morphology was the study of the transformations brought about by the conditions of existence.

Darwin had read some Goethe in translation (in his reading notebook, he referred to a translation of Goethe’s autobiography as ‘excellent’), but was most likely more familiar with morphology as a science from other sources, notably the works of German and French naturalists, as well as through his correspondence. Arguably, Darwin’s first extended foray into morphology was his study of barnacles. Although this work resulted in four taxonomic monographs, the research behind these volumes introduced a new experimental approach to classification (see Works in Letters: Living and fossil Cirripedia). Darwin studied larval as well as adult forms and tried to view several specimens of the same species from different locations in order to assess the amount of variability due to environmental factors. A couple of years into his research he told the American naturalist James Dana that it would be a long time before he could publish because the cirripedia were ‘very troublesome, from their great variability, & the necessity of examining whole animal & inside & outside of shell’. By including fossil species in his research and comparing these to similar extant forms, he was able to reclassify many species, while his study of the adaptation of organs to new circumstances over time allowed him to discern material relationships among seemingly disparate groups. His observations were the result of highly skilled experimental manipulation under the microscope.

Orchids and climbing plants: adaptation in action

As an experimenter, Darwin was guided by the concept of adaptation as a dynamic process. Whereas in pre-evolutionary thinking adaptation had been considered a state of being, in the context of Darwinian evolution theory, it became a process. In his research on orchids, therefore, Darwin began by carefully working out the structure of the sexual organs of the flower to see how fertilisation was achieved. He was struck by an apparent contradiction in the bee orchid (Ophrys apifera), which had different parts seemingly adapted for cross fertilisation (sticky glands containing pollen masses) or self fertilisation (pollen masses naturally fall on the stigma). In a letter to Gardeners’ Chronicle in June 1860, he asked readers living in other parts of the country to communicate their observations on what happened to the pollen masses. Darwin continued to investigate this anomalous structure after the publication of Orchids in 1862, and added information from observers in France and Germany to the second edition of 1877 (see Works in Letters: Orchids). 

Darwin was always hands-on in his approach, and after hearing about Australian orchids from Daniel Oliver, he wrote ‘I cannot quite understand the description, & without examining the live plants, with reference to visits of insects, I believe their means of fertilisation can never be understood.’ When he was not able to observe insect visits, he substituted experimental manipulation with simple devices such as pencils to simulate the action of an insect landing on a flower and brushing up against the sticky disc-shaped structure on the stamens that adheres to a visiting insect. 

Another puzzle for Darwin was the existence of nectaries with no nectar in many orchid species. He devised several experiments to test whether nectar might be present only at certain times of day or under certain conditions, but only ever observed dry nectaries. However, while examining the nectaries, he noticed the inner and outer membranes of the tube were separated by a fluid layer, and, moreover, that the inner membrane was extremely delicate. He postulated that insects penetrated the inner membrane to suck out the fluid (see Orchids pp. 49-53). By the time the second edition of Orchids was published, Darwin had performed more experimental work and received additional information from other researchers. Among other things, he compared the viscid matter of the discs in orchids with free nectar to those with inter-membrane fluid, finding a significant difference in the adherence properties of the two types of disc. ‘If this double relation is accidental,’ Darwin concluded, ‘it is a fortunate accident for the plants; but I cannot believe it to be so, and it appears to me one of the most wonderful cases of adaptation which has ever been recorded’ (Orchids 2d ed. pp. 43-4). 

Darwin focused on a particular type of adaptation in his research on climbing plants: the adaptation of an organ to perform a new function. Roots became hooks, stems became twiners, and leaves or even flowers became tendrils. Functions themselves changed over time. For example, tropical pitcher plants (Nepenthes) could climb by a twining stalk at the end of which was a nascent pitcher; upon reaching sufficient height, the pitcher, itself a modified leaf, would develop fully. As he observed both structural modifications and movement, Darwin began to notice the ubiquitous circular motion of tendrils. Readers of his article on ‘Climbing plants’ in which he announced his findings were amazed; Benjamin Walsh enthused, ‘this discovery of their sweeping circles & groping in the dark for support, like a blind Cyclops, is very astonishing’.

Although morphological adaptation was the main focus in Darwin’s work on both orchids and climbing plants, physiological features, such as the nature of the fluid in nectaries or the processes involved in the movement of twining, were noticed as an adjunct to structural features. When Darwin returned to research on plants, following his work on sexual selection and expression of emotions, plant physiology began to take centre stage.


Movement inside and out

Darwin had begun researching the insectivorous plant Drosera rotundifolia (common sundew) in 1860, around the same time he began work on orchid morphology. In a letter written in October 1860 to his botanical mentor, John Stevens Henslow, Darwin gave a detailed description of the sequence of events during the inflection of the tentacles of the sundew. After outlining the movement and changes of the fluid that was exuded from the tentacles, Darwin needed to ask whether the phenomenon was already known, as he was ‘so ignorant of vegetable physiology’. 

By the time he returned to this research over a decade later, not only was he more adept at physiological experiments, but also able to call on the expertise of chemists like Edward Franklin, animal physiologists like John Burdon Sanderson, physicians like Thomas Lauder Brunton, as well as his many botanical contacts. Darwin, although working from home, had extended his experimental practice to a more collegial style, benefitting from a network of fellow experimenters. Sanderson had been able to advise Darwin about many of his Drosera experiments, noting, for example, that some substances acted on animal muscle tissue, while others affected nerves. For example, he advised trying glycerine (a nerve agent) on Drosera, suggesting that if it produced inflection, it would ‘indicate that the excitability of Drosera is allied to that of nerve rather than to that of muscle’. Sanderson later performed experiments with a galvanometer (a device for detecting and measuring electric current) to record the nerve-like response of a leaf of Dionaea (Venus fly trap) when stimulated with an induction coil. Insectivorous plants, published in 1875, highlighted both differences and similarities in plant and animal physiology. Although the digestive fluids of sundews were remarkably similar to those of animals, substances that were poisonous to animals had no similar effect on insectivorous plants.

In his last major botanical work, Movement in plants, Darwin refined his experimental approach even further, collaborating with his son Francis and benefitting from his association with one of the most advanced plant physiological laboratories in Germany (see Works in letters: Movement in plants). Francis spent the summers of 1878 and 1879 in Würzburg in the laboratory of Julius Sachs and was able to work with new types of equipment designed by Sachs to facilitate a more quantitative analysis of various types of plant movement. He could also keep his father apprised of the work of other researchers in the laboratory and suggest instruments that could be improved on by Darwin’s youngest son, Horace, who was the founder of the Cambridge Scientific Instruments company. 

Although many of Darwin’s experiments were similar to those performed by Sachs, their results were often at odds. In a letter to William Turner Thiselton-Dyer of May 1878, Darwin mused over one such difference. ‘What trifles determine the success of experiments’, he commented, ‘Sachs missed a pretty little discovery solely by keeping his germinating beans too warm.’ Sachs had also dismissed the results of experiments made by Theophil Ciesielski, which had demonstrated that the response to gravity in the root was located in its apex. Darwin performed his own experiments on the sensitivity of the root tip and his work convinced him that Ciesielski had, in fact, got it right. 

Sachs’s criticism of Ciesielski revolved around the interpretation of his experimental results, but it was the Darwins’ entire methodology that irked Sachs. During his second summer at Sachs’s laboratory, a frustrated Francis complained to his father about Sachs’s critique of their methods. Sachs had argued against the use of caustic (silver nitrate) on root tips, arguing it would injure the root and prevent geotropism. In a letter to his father, Francis described his exchange with Sachs: ‘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.

Smoked glass was made by holding a flame to a glass plate, leaving a thin layer of carbon residue. 

As it happened, Darwin had performed similar experiments with clear glass and he mentioned this in his reply to Francis, adding that he was not surprised by Sachs’s negative view, although he would rather convert Sachs ‘than any other half-dozen-Botanists put together’. Darwin, in turn, was critical of Sachs’s experimental method when he suggested that Sachs, whose work he greatly admired, had failed to see a diminution of geotropism in his cut roots because he had cut them at an angle that allowed part of the tip to survive. While Darwin did not accept many of Sachs’s criticisms, he nevertheless tried other experimental protocols to reinforce his results. He told Francis, ‘Great man as Sachs is, I am not even staggered by him.

The experimental method was crucial to the development of both morphology and physiological botany. Morphology can be seen as a bridge between earlier systematic studies and later physiological research in that it moved from the description of structure to the study of formation and transformation of form, which in turn led to the investigation of the processes that underlay the development of form. Darwin’s personal development as an experimentalist mirrored these changes, but his guiding principle throughout was the view that adaptation was the engine of change.