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Darwin Correspondence Project

From Edward Frankland   10 October 1873

14 Lancaster Gate | Hyde Park

Oct. 10/ 73.

My dear Sir

I have now finished the experiments with the leaves & washings of Drosera.1 The results, I regret to say, are mostly negative, but perhaps not altogether uninteresting on that account. There is no trace of the following acids present either in the leaves or washings:—

Hydrochloric Acid

Sulphuric "

Tartaric "

Oxalic Acid

Formic "

The leaves were macerated in water for 3 or 4 hours, & the solution examined for acids, of which it contained the merest trace. On acidifying with sulphuric acid and distilling the trace of acid or acids did not pass over & was therefore non-volatile. The leaves & product of their maceration emitted no odour resembling pepsin.

The washings in the two bottles were carefully tested for the acids above mentioned. The liquid was acidified with sulphuric acid and then emitted a powerful odour reminding one of pepsin. It was then distilled, a slightly acid liquid condensed, it was neutralised with carbonate of soda & a portion of it tested, (after evaporation nearly to dryness) for formic acid which was, however, not present. The remainder was then acidified with sulphuric acid (after evaporation nearly to dryness) when it evolved volatile acid vapours. These were carefully condensed and digested with carbonate of silver. The weight of the silver salt was only 0.37 grains a quantity much too small for the accurate determination of the molecular weight of the acid. The number obtained however, corresponded nearly with that of propionic acid, and I believe that this, or a mixture of acetic and butyric acids, was present in the liquid. These acids are likely to be produced from the food given to the leaves. The acid seems to me to be present in too minute quantity to be an important agent in the process of digestion, and the digestive liquid probably owes its activity to pepsin or some closely allied compound.2

It would of course be possible to separate and isolate the acid or acids contained in the washings of the leaves, but to do this a very much larger quantity would be required,—the washings of probably not less than 6 or 7 lbs. of the leaves. The acid doubtless belongs to the acetic series none of the members of which (except formic acid) have any well characterised reactions by which small quantities of them can be recognised with certainty.3

I enclose a couple of packets of exceedingly sensitive litmus paper.4 They should be carefully preserved from the air in a dry stoppered bottle.

I shall at all times be most happy to help you in the solution of any chemical problems.

Believe me | Yours sincerely. | E. Frankland.

CD annotations

1.3 There is … Tartaric " 1.7] scored red crayon
1.3 leaves] underl blue crayon
2.4 The leaves … pepsin.] scored red crayon
3.1 The washings … present. 3.5] crossed pencil
3.2 The liquid … pepsin. 3.3] scored red crayon
3.5 The remainder] ‘being larger part’ added pencil
3.5 The remainder … acidified 3.6] ‘then’ del pencil, ‘again?’ interl pencil, underl blue crayon, del pencil
3.8 The weight … grains] scored blue crayon
3.10 corresponded … acids, 3.11] scored red crayon; ‘propionic’ underl blue crayon; ‘acetic and butyric’ underl blue crayon
3.12 to be] ‘by itself’ interl pencil; underl blue crayon
3.12 The acid … certainty. 4.6] crossed pencil
4.3 The acid … series 4.4] scored blue crayon
5.1 I enclose … problems. 6.2] crossed blue crayon


CD had asked Frankland to analyse the fluid secreted by the glands of Drosera (see letter to Edward Frankland, 12 July 1873). He had sent Frankland the washings of some leaves of Drosera and the leaves themselves at the end of September (see letter to Edward Frankland, 29 September 1873).
CD cited Frankland for the information in this letter in Insectivorous plants, p. 88. CD was interested in whether the secretions of insectivorous plants digested food in a similar way to the digestive secretions in the stomachs of animals.
Acetic series: i.e. carboxylic acids. Acetic, butyric, formic, and propionic acid (now usually known as propanoic acid) are carboxylic acids.
CD had requested sensitive litmus paper in his letter to Frankland of 7 October [1873].


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


The results of EF’s tests for acids in the secretion of Drosera are largely negative [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: 44–6
Physical description
6pp †

Please cite as

Darwin Correspondence Project, “Letter no. 9094,” accessed on 18 January 2020,

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