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Darwin Correspondence Project

Power of movement in plants

Sources|Discussion Questions|Experiment

Family experiments

Darwin was an active and engaged father during his children's youth, involving them in his experiments and even occasionally using them as observational subjects. When his children became adults, he continued to collaborate with them; for instance his daughter Henrietta helped edit manuscripts of The Descent of Man (1872). This teaching module focuses on work done by Darwin with his son Francis on experiments for The Power of Movement in Plants (1880). The correspondence between Darwin and Francis reveals two wonderful sides of Darwin’s character: his abilities as a precise, meticulous, and inventive experimentalist, and his role as an attentive and affectionate father. Darwin's letters to Francis mix advice on scientific matters with more practical concerns (such as the necessity of keeping proper financial records). As you read through these letters, notice that although they come from the last decade of Darwin’s life, he is still interested in his scientific questions and ideas.

Plants that move

In The Power of movement in plants Darwin continued his experiments with and observations of climbing plants. In this book Darwin provided detailed observations of the response of plants to external stimuli. Along with his other specialised books (including his books on orchids and earthworms), this monograph was intended to answer a specific set of objections to his theory of evolution by natural selection. This 1880 book sought to illustrate that evolution could account for changes in behavioral responses. In the conclusion of the book Darwin argues that gradual modifications in the development of plants in response to natural forces such as water and light could account for all botanical adaptations.

You can download the entire packet of reading materials here: Power of movement in plants source pack

SOURCES

Books

Darwin, C.R. The power of movement in plants. 1880. London: John Murray.

Chapter nine: Sensitiveness of plants to light: its transmitted effects

Letters

Letter Pack: Power of movement in plants

Letter 7346- Darwin to Francis Darwin, 18 October 1870

Darwin writes to Francis to remonstrate Francis for getting himself to debt, but also sends along the money to help Francis clear his debt. Darwin cautions Francis that he knows of no one who is truly too busy to keep proper account of his finances, and further that those who land themselves in debt are in danger of compromising their character.

Letter 10517- Darwin to Francis Darwin, 29 May 1876

Darwin writes to Francis to inquire about a variety of Francis’ experiments. He makes several detailed suggestions about Francis’ experimental technique.

Letter 11586- Darwin to Francis Darwin, 2 July 1878

Darwin writes to Francis to suggest that observing the cotyledons of Oats bending toward the light would do as an appropriate observation for heliotropism. He remarks that he cannot remember if such observations have been made, but that they would be worth making.

Letter 11628- Francis Darwin to Darwin, 24 July 1878

Francis writes to his father with some comments about heliotropism in molds. He remarks on the lab technician’s views on mold in walnuts. He also writes about his inability to get any labwork done, remarking that he was “too floppy to work”.

Letter 12152- Francis Darwin to Darwin, 12 July 1879

Francis writes to his father with some results from his latest experiment on movement in plants. By the time this letter was written, he and Darwin were well into the publishing stage for The Power of Movement in Plants. Francis notes that he sent a chapter on sleeping plants to their publisher.

DISCUSSION QUESTIONS

1. Why do you think it is important to study these later, specialised books by Darwin? What do you think these books can tell us about the progress of Darwin's theory of natural selection?

2. How do you think the collaboration between Francis Darwin and Darwin functioned? What type of fatherly advice does Darwin offer to Francis?

3. Does it change your perception of Darwin to know that he collaborated with his children and other family members? Do you think learning about his family life is important to understanding Darwin's science?

4. What is the tone of the beginning of Chapter 9? Why do you think that Darwin and his collaborators use this mode of argumentation?

EXPERIMENT

An example of classroom activity performed at Harvard:

In this week of the course the students replicated some of Darwin’s work on heliotropism, which is the movement of plants towards the sun. Darwin considered heliotropism worth investigating because “it is no doubt a great advantage to [seedlings] in their struggle for life to expose their cotyledons to the light as quickly and as fully as possible, for the sake of obtaining carbon.”[1] Darwin was assisted by his son Francis (1848-1925) in this work. Their work resulted in the publication of The Power of Movement in Plants in 1880.

After reading Chapter Nine of The Power of Movement in Plants the class was able to learn more about Francis and Charles Darwin’s work on plant movement by replicating one of the Darwins experiments. For the first step of the experiment, the class grew various seedlings (canary grass, oats, corn) in the dark. Once the seedlings sprouted and were each a few centimetres tall, the students clipped about 5mm off of the top of half of the plants. After clipping half of the plants, the students exposed all of the seedlings to unidirectional light for two hours.

Once the two hours elapsed, the students observed their plants. The result was clear: the plants that had been clipped failed to bend towards the light while the plants that remained intact bent notably toward the light. This is significant: it illustrates that the mechanism responsible for heliotropism in these plants is located at the tip of the plant. Francis and Charles Darwin’s work on heliotropism constituted some of the first work on plant hormones. It wasn’t until 1928 that the mechanism responsible for heliotropism (the auxin plant hormones) was identified by Fritz Went. As we now know, auxin is a growth-inducing hormone that is located at the tip of these seedlings. When exposed to sunlight, the auxin in plants moves away from the sun – thereby inducing growth on the side of the plant that doesn’t face the sun. This is what leads the plant to bend towards the sun.

 

 

Learning objectives

1. Learn more about Dariwn’s later work and understand the developments of and additions to his theory of evolution by natural selection. 

2. Understand the relationship between Darwin’s family life and his scientific work. 

3. Discuss Darwin’s sustained interest in botanical subjects and understand his experimental methods.