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Darwin Correspondence Project

To T. L. Brunton   11 May 1874

Down, | Beckenham, Kent.

May 11 1874

My dear Dr Brunton

Will you add to your former kindness by examining the enclosed slide, gummed to the lid of the box, & then throw it away. It is a minute whitish floculent mass which I want you look at; & the spec. nearest the label is the best. I put minute drop of skimmed milk on the glands of drosera, & it became, as usual, curdled in about 10m. After from 6h.— to 8h a large part of the curds was dissolved & disappeared. It is the remnant scraped off the leaf, with water added, which I want you to look at. I suppose that it consists of oil-globules. I coagulated, for comparison, a small drop of skimmed milk with acetic acid, & found much floculent matter, which I suppose from your book to be Casein. This Casein has all disappeared from the milk subjected for 2 days to the secretion of Drosera, & the oil globules are larger & less regular than in the milk curdled by acetic acid. All that I want to know is, whether I am right in what I have here said.1

There is one other question; I find that if I add a minute drop of Hydrochloric acid of the strength of 1 to 100 of water to the secretion on the leaves, it stops their digesting minute cubes of albumen; but if I add to the secretion minute drops of the strength of 1 to 200 of water, the albumen is all dissolved. Does this different action agree with that of pepsine & H. acid? I presume that artificial digestive fluid fails to digest if it be weakened with a great excess of water2

Pray excuse my troubling you & believe me yours very sincerely | Ch. Darwin

P.S. When I asked you about Urea, I had stupidly forgotten my own observations; I tried it in solution to ascertain whether this refuse product of an animal would act on Drosera; & it did not, any more than gum or sugar   My trial had no relation to digestion.—3

Two specimens of pepsine which I tried seemed to contain some albuminous matter which excited Drosera, but the residual grains were not dissolved.—4


In Insectivorous plants, p. 114, CD described this experiment to ascertain whether casein or albumen in milk excited the digestive secretions of Drosera (sundew), adding: ‘As I was not familiar with the microscopical appearance of milk, I asked Dr. Lauder Brunton to examine the slides, and he tested the globules with ether, and found that they were dissolved. We may, therefore, conclude that the secretion quickly dissolves casein, in the state in which it exists in milk.’
See Insectivorous plants, pp. 95–6.
CD discussed the action of urea on Drosera in Insectivorous plants, p. 124. For Brunton’s reply, see the letter from T. L. Brunton, 4 September 1874; it is incomplete. No previous letter from CD asking about urea has been found.
See Insectivorous plants, pp. 123–4.


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


Encloses, for examination, residue from skim-milk which has been on the glands of Drosera. Asks TLB to confirm his views on action of Drosera secretion on milk. Asks about effects of pepsin and hydrochloric acid in digestive juice.

Please cite as

Darwin Correspondence Project, “Letter no. 9453,” accessed on 26 October 2021,

Also published in The Correspondence of Charles Darwin, vol. 22