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Darwin Correspondence Project

To Mary Treat   1 January 1873

Down, | Beckenham, Kent.

Jan 1. 73

Dear Madam,

I am very much obliged for your kind letter;1 & should esteem it a great favour if during warm weather next summer you will observe two points for me in Drosera Filiformis.2 Namely to place some flies within quarter of an inch of the apex of the leaf & observe whether it bends at all after an interval of a day or two. Secondly to rub with a clean needle a few of the glands with some little force, and to touch each gland half a dozen times; & then observe whether in the course of an hour or two the hairs or filaments bearing these glands become incurved. I am glad to hear that D. filiformis catches only small insects, as I suspected this. I have observed with care several other species of Drosera.3 Does the Dionæa4 grow in your neighbourhood? If so I much wish to learn what sort of insects it commonly catches, more especially whether large or small kinds. I have sometimes suspected that its structure & movements favour the escape of small insects.

Dr. Gray has given a rather free translation of what I said to him about nerves; and this related only to Drosera.5 I have found that by pricking a particular point in the leaf I can paralyse half of it; but I must make many more trials next summer before coming to any final conclusion.

With my best thanks | I remain Dear Madam | Yours very faithfully | Charles Darwin

P.S. I subscribe to the American Naturalist, so I am glad to say that I shall see your article—6


Drosera filiformis (the threadleaf sundew) is a North American species.
CD had worked on Drosera between 1860 and 1862 (see Correspondence vols. 8–10); he resumed his experiments in 1872, after finishing work on the proofs of Expression (Correspondence vol. 20, Appendix II). For the species he worked on, see Insectivorous plants.
Dionaea muscipula (Dionaea is a monospecific genus) is the Venus fly trap.
In her letter of 13 December 1872 (Correspondence vol. 20), Treat had written, ‘Prof. Gray writes me that you have found the nerves in Dionæa.’ CD had written in his letter to Asa Gray of 22 October 1872 (ibid.), ‘The point which has interested me most is tracing the nerves!!! which follow the vascular bundles. By a prick with a sharp lancet at a certain point, I can paralyse 12 the leaf, so that a stimulus to the other half causes no movement.’
Treat’s article ‘Controlling sex in butterflies’ was published in the American Naturalist of March 1873 (Treat 1873a). CD’s copies of the American Naturalist are in the Darwin Archive–CUL.


Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 29 vols to date. Cambridge: Cambridge University Press. 1985–.

Expression: The expression of the emotions in man and animals. By Charles Darwin. London: John Murray. 1872.

Insectivorous plants. By Charles Darwin. London: John Murray. 1875.


Asks for certain observations to be made on Drosera and Dionaea.

Letter details

Letter no.
Charles Robert Darwin
Mary Lua Adelia (Mary) Davis/Mary Lua Adelia (Mary) Treat
Sent from
Source of text
Amy Nagashima (private collection)
Physical description
LS(A) 2pp

Please cite as

Darwin Correspondence Project, “Letter no. 8719,” accessed on 21 June 2024,

Also published in The Correspondence of Charles Darwin, vol. 21