Research assistants with the Darwin Correspondence Project joined the teaching team for a new Freshman Seminar at Harvard College called “Getting to Know Darwin.” The class was composed of first year undergraduates and was taught by Ned Friedman, Director of the Arnold Arboretum at Harvard University and Professor in the Department of Organismic and Evolutionary Biology. Here is a link to a feature about the course.
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.” 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.
We are currently working on a video of this experiment – check back soon to see it!
 Charles Darwin, The Power of Movement in Plants (London: John Murray, 1880), 454.