Plants that consume insects
Darwin began his work with insectivorous plants in the mid 1860s, though his findings would not be published until 1875. In his autobiography Darwin reflected on the delay that allowed him to refine his work: "The delay in this case, as with all my other books, has been a great advantage to me; for a man after a long interval can criticise his own work, almost as well as if it were that of another person." Several decades after he began this work, Darwin was clearly able to see that a plant "should secrete" a "fluid... closely analogous to the digestive fluid of an an animal, was certainly a remarkable discovery."1 The resulting volume, Insectivorous Plants (1875), was one in a series of works in which Darwin explored the reaches of natural selection. This work allowed Darwin to focus on the features of insectivorous plants that allowed them to survive in difficult environments. Darwin used several experiments to stimulate the plants' trap mechanisms, including feeding them meat, blowing on them, and stimulating them with hair. Through his work he concluded that the plants would only react to the movements of 'prey'; Darwin believed that this was a wonderful adaptation for the plants as it enabled them to ignore unhelpful stimuli.
The monograph is an excellent example of Darwin's interest in scientific collaboration with other naturalists and experts; in particular he worked with Professor Edward Frankland of the Royal College of Chemistry in designing and executing some of the key experiments. Darwin's sons George and Francis helped him with the illustrations for the volume, each working up illustrations for the key species Darwin was most interested in: Drosera, Dionaea, Aldrovanda, and Utricularia. He also corresponded with the New Jersey naturalist, Mary Treat, about carnivorous plants. Darwin and Treat exchanged fifteen letters from 1871-1876 about the behavior and mechanisms of these plants. Darwin corresponded more with Treat than with any other female naturalist.
1. Darwin, F. Ed. 1887. The life and letters of Charles Darwin, including an autobiographical chapter. London; John. Murray.
You can download the entire packet of reading materials here: Insectivorous Plants Source Packet.
Darwin, Charles. 1875. Insectivorous Plants. London: John Murray. Chapters 17 and 18
Letter 3853- Charles Darwin to John Scott, 11 December 1862
This is a lengthy letter from Darwin to John Scott, a Scottish Botanist who emigrated to India in 1864 through Darwin's patronage. In this letter Darwin remarks that he and Scott often seem to work on the same subjects; Darwin remarks that he has a wealth of material on Drosera and Dionaea that he may publish someday if he takes the time to work up his material. He does not understand Scott's objection to Natural Selection.
Letter 2951- Charles Darwin to Daniel Oliver, 17 Oct 
Darwin thanks Daniel Oliver, a professor of botany at University College, London, for Oliver's information and extracts on Dionea. Darwin remarks that the only full account of the plant he has seen is in the Penny Encyclopaedia. He details the experiments he is conducting using carbonate of ammonia as a substitute for flies on Drosera.
Letter 2932- Charles Darwin to J.S. Henslow, 28 September 1860
Darwin writes to his friend John Stevens Henslow about his observations of Drosera. Henslow was a professor of mineralogy and had been Darwin's teacher during Darwin's students days at Cambridge. In this letter Darwin asks Henslow whether the motion he observes pouring out from the hairs on the Drosera is a known or common phenomenon.
Letter 8113- Mary Treat to Charles Darwin, 20 December 1871
In this long letter Treat describes her observations of the fly-catching Drosera longifolia. She remarks that these observations "have escaped the notice of botanists". She offers up her observations to Darwin, remarking that she owes him such experiments and observations in gratitude for Darwin's own work.
Letter 9005- Charles Darwin to Mary Treat, 12 August 1873
Darwin writes to Treat to thank her for her observations and information on Drosera filiformis. He warns her against publishing statements about Drosera bending toward flies or meat that they have not touched. He declares that he would not publish such a claim himself unless he had "tried the experiment many times, under the most rigorous precautions".
1. How does Darwin utilize his correspondence network to advance his experimental interests? Does he make requests of his correspondents? Do you think these requests are fulfilled?
2. What sorts of observations does Darwin make of Utricularia in Chapter 17 of his volume? What does he suggest is the mechanism of absorption? Why does he find this significant?
3. Does Darwin address Mary Treat differently from his other scientific correspondents? What do you think of the advice he gives her regarding her observations of Drosera?
An example of a classroom activity performed at Harvard:
To learn about Darwin’s work on carnivorous plants, the class observed a variety of carnivorous plants: sundews, venus fly traps, utricularia, and pitcher plants. Each plant employs its own mechanism to trap its prey, and the students enjoyed seeing, firsthand, how these mechanisms work.
To get a closer look at the carnivorous plants the students used microscopes to examine them. The students particularly enjoyed learning more about how utricularia capture their prey. After examining utricularia, the students were better able to appreciate the awe of utricularia expressed by Mary Treat in an 1874 letter to Charles Darwin:
I have been studying the bladder-bearing species of Utricularia off and on the last year, and am now fully satisfied that they are the most wonderful carnivorous plants that I have yet seen.
If you don’t have access to utricularia or microscopes, this video provides a good alternative.