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Darwin Correspondence Project

To Edward Frankland   21 September [1873]

Down, | Beckenham, Kent.

Sept 21st

My dear Sir

I hope that you will excuse me for troubling you with a very long letter, but which will be easily read. In order to try for pepsine, as you suggested, in the secretion of Drosera, I washed a large number of leaves, but the solution was so dirty & so weak that I could do nothing with it.1 This however led me to make a curious observation: the viscid secretion of the tentacles, when expanded, is not in the least acid, or in old leaves just perceptibly so; but if the same leaf is stimulated by any thing placed on the disk, the secretion, as soon as the tentacles become much inflected over the object, & before they have touched it, becomes plainly acid. So that the glands, as far as acid is concerned, act just as in the stomach of a mammal. My failure with the washing led me to place little cubes of the white of boiled egg on some leaves; adding little drops of water to make the trial fair. On other leaves I added to the cubes minute drops of weak hydro-chloric acid (1 to 437 of water). On other leaves I added to the cubes minute drops of carb. of soda (1 to 437 water) but I had to renew the solution after a few hours as the secretion soon again became acid. Consequently I renewed water to the first lot, & acid to the second lot; so that all were treated alike as far as fluid was concerned. The result was that those leaves to which I gave the acid, dissolved the albumen considerably before those which were left to nature (with the exception of the little drops of water). On the other hand, the cubes of albumen to which I had added the Carb. of Soda, had their angles as sharp as ever: I then added to these latter cubes little drops of hydrochloric a., so as to neutralize the soda & to make the secretion slightly acid; & then the cubes were dissolved as quickly as in the former cases. Now I judge from your letter, which has been of the greatest value to me, that the above results indicate the presence of some substance in the secretion analogous to pepsin. Is not this the case? I may add that I put cubes in the same weak Hydrochloric A, at the same time, & they were not in the least acted on.2

Now I am more anxious than ever to know what acid Drosera secretes; but I am in great perplexity now to obtain it. From what I have said above, I can only do so by giving the leaves albumen or, fibrin’. Casein wd be the best of all, for this is acted on very slowly, but excites more copious acid secretion than any other substance which I have tried. Pepsin wd do excellently, but then I know not whether this substance, procured from a good druggist, is really pure.

Here however comes the question, whether when I wash the leaves with fibrin, Casein or pepsin on them, in the sol. of Carb. of Soda, you wd. be able to discover the nature of the acid.3 Even if I wash 50 leaves there wd be very little acid. I may add that pure gelatine will not serve, as it excites very little acid secretion.

Lastly, I have tried various very weak acids of the same strength, (viz 1. to 437 of water). Most of them killed the leaves, even oxalic, acetic & benzoic. Tartaric & citric & some few others produced no sort of effect. Hydro-chloric stimulates them, for they soon become inflected, but afterwards in water, they expand & seem uninjured.4 Again I find that 9 salts of soda, including the chloride, cause inflection, but do not injure the leaves. Whereas the 9 corresponding salts of potash cause no inflection & often seem injurious. From these 2 sets of facts I cannot help suspecting that the acid will prove hydrochloric, obtained by decomposing the chloride in the bodies of the insects which they capture & digest. If this shd be so, the digestion by the leaves of Drosera will be curiously analogous to that by the stomach of a mammal; & the subject seems to me well worth investigating.

I beg you forgive me for the inordinate length of this letter & I remain with cordial thanks | yours sincerely | Charles Darwin

P.S. I have heard indirectly thro’ Prof. Noad, that Dr Moore of 113 Brixton Road,5 may be trusted to prepare animal substances pure; & he has sent me some Casein; so that I am prepared to stimulate & wash the leaves, as soon as ever I can hear from you. Hoping, however, that the grains of Casein & the glands of Drosera, many of which will surely be torn off, & both mingled in the fluid, will not prevent your ascertaining what acid is secreted.

By any chance do you possess, & can you give me a few grains of dried Mucin, as I much wish to try this substance.6

P.S. 2d When the leaves of Drosera are put into weak Chloride of Gold, 1 gr to 1.oz of water metallic gold is precipitated & floats on the surface; does this not look as if acid was Oxalic; I gather so from one book.7

Excuse erasures as this letter was written a week ago.8


See letter from Edward Frankland, 17 July 1873. Frankland had warned CD that the identification would be difficult, since the chemical composition of pepsin was not known. Although pepsin had been discovered in 1836 by Theodor Schwann, its composition was not determined until the twentieth century (NDB s.v. Schwann, Theodor; Northrop 1939).
CD’s experiments were carried out on Drosera rotundifolia (the common or round-leaved sundew; see Insectivorous plants, pp. 86–8).
CD hoped to get pure forms of fibrin, casein, and other substances for his experiments (see letter to J. S. Burdon Sanderson, 14 September [1873] and nn. 5 and 9). Carbonate of soda is now more commonly known as sodium carbonate.
In Insectivorous plants, p. 189, CD gave a table of the results produced by several acids dropped on leaves of Drosera rotundifolia.
CD had also asked John Scott Burdon Sanderson for a source of small quantities of mucin (see letter to J. S. Burdon Sanderson, 14 September [1873] and n. 5).
Oxalic acid is commonly used as a reducing agent to obtain gold from gold (auric) chloride. CD owned a dictionary of chemistry (see letter to Francis Darwin, [4 September 1873], n. 1) that described the process.
Frankland had informed CD that he planned to be in Switzerland until the beginning of October (see letter from Edward Frankland, 17 July 1873). For the erasures, see Manuscript alterations and comments.


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

NDB: Neue deutsche Biographie. Under the auspices of the Historical Commission of the Bavarian Academy of Sciences. 27 vols. (A–Wettiner) to date. Berlin: Duncker & Humblot. 1953–.

Northrop, John Howard. 1939. Crystalline enzymes: the chemistry of pepsin, trypsin, and bacteriophage. New York: Columbia University Press.


Although CD’s experiments with pepsin were unsuccessful, he observed that the glands [of Drosera] as far as acid is concerned act just as the stomach of a mammal. Further experiments detailed. The secretion must contain something analogous to pepsin. [See 9062.]

Letter details

Letter no.
Charles Robert Darwin
Edward Frankland
Sent from
SP 22 73
Source of text
The John Rylands Library, The University of Manchester
Physical description
LS(A) 9pp

Please cite as

Darwin Correspondence Project, “Letter no. 9061A,” accessed on 12 September 2023,

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