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Darwin Correspondence Project

From Edward Frankland   22 September 1873

14 Lancaster Gate | Hyde Park W.

Sep. 22/73

My dear Sir

Your discovery of the acidity of the Drosera secretion under stimulus is very interesting, and your experiments altogether certainly afford considerable ground for believing that pepsin or some substance analogous to it, and hydrochloric acid are secreted by the leaves.1 If the acid be really hydrochloric, there would be no difficulty in detecting it in, say, the washings of a dozen leaves. It will be necessary however to avoid the presence of chlorides in the stimulant and I would therefore advise you to use either casein perfectly free from salt or better, fibrin thoroughly macerated & washed in several successive quantities of distilled water.

As the plant seems to make an effort to neutralize carbonate of soda in the presence of food, you could perhaps considerably increase the quantity of acid secreted by continually neutralizing it with carbonate of soda. I fear that no pepsin would be free from hydrochloric acid or chlorides.2

There is no necessity to assume that the materials for the hydrochloric acid are obtained from the bodies of insects: all soils contain chloride of sodium & this salt is never absent from the rain water & even the air of this country. It is quite conceivable that by a kind of osmotic action aided perhaps by chemical affinity, the plant may decompose a solution of chloride of sodium into soda which passes inwards & free hydrochloric acid which exudes from the tentacles. The presence of fragments of pure casein or fibrin or of tentacles in the washings will not interfere with the detection of hydrochloric acid, but I must ask you to be very careful that the washing water does not touch your fingers, & also that all the vessels &c used be carefully washed out with distilled water.

I regret that I have no mucin or vegetable casein3 in my possession neither do I know where to procure any; but, if that will not be too late, I shall be glad to prepare you some when my assistants return on the 1st. ult.

I do not think that anything can be inferred from the gold experiment.4 Gold is precipitated from its chloride by many animal, vegetable & other substances & even by light alone.

I am ready at any time to test the liquid for hydrochloric acid.

Yours sincerely | E. Frankland.

Charles Darwin Esq. F.R.S.

CD annotations

1.1 Your … 1st. ult. 4.3] crossed blue crayon
1.2 and … the leaves. 1.4] double scored blue crayon
3.1 There … country. 3.3] scored blue crayon
3.9 & also … water. 3.10] scored red crayon
5.1 I … acid. 6.1] scored blue crayon
Top of letter: ‘Last Page Keepblue crayon


Pepsin is only active in a moderately acid environment and is more soluble in the presence of salt (sodium chloride). Salt acts as a buffer to maintain a stable pH level when small amounts of either acidic or alkaline substances are added to a solution.
Vegetable casein is another term for legumin, a globulin protein derived from leguminous seeds. Many chemists at this time considered it to be chemically identical to casein derived from milk.


CD’s discovery of acidity of Drosera secretion is interesting. EF explains how hydrochloric acid can be detected and identified. [See Insectivorous plants, p. 88.]

Letter details

Letter no.
Edward Frankland
Charles Robert Darwin
Sent from
London, Lancaster Gate, 14
Source of text
DAR 58.1: 38–9
Physical description
4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 9062,” accessed on 18 April 2021,

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