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Darwin Correspondence Project

From Mary Treat   28 July 1873

Vineland, New Jersey.

July 28, 1873.

Mr. Darwin—

Dear Sir,

I write to inform you of the result of my experiments with Drosera filiformis.1 But first allow me to tell you of the immense quantity I found, in its fullest vigor, just coming into bloom.— On the morning of July 7th, I started for the country, some thirty miles distant, and before noon of the same day, was in midst of acres of this beautiful plant, standing so thick as to exclude almost everything else. I had only found it in limited quantities before this, and never such strong, vigorous plants as these. So I wrote you that it caught only small insects, but now found I was mistaken.2 Great Asilus flies3 were held firm prisoners, and innumerable moths and butterflies, were alike held captive until they died; the bright pink flowers and glistening dew-like substance, luring them on to sure death. After the death of the larger insects they fall around the roots of the plants as if to fertilize them, but the smaller insects remain sticking to the leaves. The leaf itself does not coil around the insect, the sticky substance being sufficient to hold it, and the more the fly struggles the more it becomes entangled in the filaments, and the sooner dies.

I carefully removed strong plants away from atmospheric agitation, and found they would bend toward a struggling fly. I pinned living flies within a quarter of an inch of the leaf, in less than an hour the flies legs would become entagled in the filaments. I then tried them three quarters of an inch from the leaves. The leaves bent perceptibly away from the light toward the flies, but did not reach them at this distance. I tried bits of raw beef with the same result. I could see no effect produced upon the glands by rubbing them with a needle; perhaps I did not understand just how you wished it done.4

But the most perfect, active fly-trap among these plants is D. longifolia.5 In less than three hours a vigorous healthy leaf will fold completely around a struggling fly, and bits of raw beef will become so enfolded in the leaf as to be completely hidden from view, while mineral substances—dry bits of chalk, magnesia and pebbles made no impression on the leaves. I wet the chalk in water, the filaments soon began to clasp it, but soon unfolded again, leaving it free on the blade of the leaf.

I also experimented with D. rotundifolia.6 You are aware that the filaments around the edge of this leaf are longer than those on D. longifolia. I placed raw beef on the blade of this leaf, in three hours the inner filaments are curved closely around it, and the longer filaments circling the edge of the leaf were slowly curving upward; in twelve hours all the filaments were clasping the beef, the glands like so many mouths, seeming to absorb nourishment from the meat. While on an equally vigorous leaf a dry bit of chalk made no impression. The filaments slowly curve around raw bits of apple, but not so firmly as around animal substances, and are nearly three times a long in making the movement.

Some days the plants worked much better than others, whether it was owing to the electrical condition of the atmosphere, or whether it was due to the amount of moisture therein contained, I had no means of ascertaining.

I sent Dr. Gray a box of the plants, but he begs me to go on with the experiments and publish results, as he is too much occupied to give them any attention.7

Dionaea does not grow in this state, it has only been found in North Carolina, but I hope sometime to be able to visit its locality, and study it in its native home.8

Yours very respectfully, | Mary Treat.

CD annotations

1.4 was … else. 1.5] scored blue crayon
1.7 Great … died; 1.9] double scored blue crayon
1.12 The leaf … insect,] double scored blue crayon
2.6 I could … it done. 2.8] double scored blue crayon
3.1 In less … home. 7.2] crossed blue crayon
Top of letter: ‘send book, promised’ pencil


Drosera filiformis is the thread-leaved sundew and is found on the eastern seaboard of North America.
Asilus is a genus of robber-flies, which are often over an inch (25.4 mm) long.
CD suggested these experiments in his letter to Treat of 1 January 1873.
Drosera longifolia is now divided into two species: D. anglica (the great sundew) and D. intermedia (the spoonleaf sundew). Treat probably observed D. intermedia, the species native to her home state, New Jersey (see Schnell 2002, pp. 261–2, 268–9, for the distribution of both species in North America).
Drosera rotundifolia is the common or round-leaved sundew.
Treat published the results of her experiments in American Naturalist (Treat 1873b). CD referred to the article in Insectivorous plants, pp. 278 and 281.
CD asked about Dionaea in his letter to Treat of 1 January 1873.


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

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


Reports in detail on her experiments with Drosera. Finds she was mistaken in thinking D. filiformis captured only small insects.

Letter details

Letter no.
Mary Lua Adelia (Mary) Davis/Mary Lua Adelia (Mary) Treat
Charles Robert Darwin
Sent from
Vineland, N.J.
Source of text
DAR 58.1: 30–2
Physical description
ALS 6pp †

Please cite as

Darwin Correspondence Project, “Letter no. 8989,” accessed on 2 March 2024,

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