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Darwin Correspondence Project

To J. D. Hooker   20 July [1874]1


July 20th

My dear Hooker.

It is grand about Nepenthes. You are heartily welcome to notice in any way all or any of my published or unpublished results,—though I cannot remember anything published.2 I will give an abstract, as far as memory & time or rather strength serves of my chief results. But I do not know what you exactly want. I have a fair copy by copyist of my observations on Dionæa, which I lent Burdon Sanderson;3 & which you could see, but I do not suppose you could want it.— I will give my results higglety-pigglety.4

(1) Organic & inorganic objects placed on discal glands, these transmit influence to marginal tentacles which become inflected; but with this difference that if the object contains soluble nitrogenous matter they remain inflected for a longer period than if none is contained

(2) Immersion in nitrogenous organic fluid induces inflection, not so non-nitrogenous organic fluids.

(3) An organic or inorganic particle placed on a single marginal gland causes that tentacle to bend; & it is a certain & truly wonderful fact that a particle with no soluble matter weighing 180,000 of a grain suffices, although the particle is partly supported by the viscid secretion. I think it certain that the continued pressure of 11,000,000 of a grain would suffice, if wholly resting on the gland: a particle wholly resting on the viscid secretion does not act. Though so sensitive to pressure a gland may be roughly touched with a needle once, or twice, & there is no movement, but if touched thrice or four times there is inflection, for touches seem to act like pressure. All this forms a wonderful contrast to the sensitive filaments of Dionæa, which have been specialised for a touch & not for prolonged gentle pressure.

(4) All salts of Ammonia cause inflection, (& many other salts, but not all, & most acids, but not all) & I shall be bold enough to publish that 120,000,000 of a grain of crystallised phosphate of Ammonia absorbed by one gland suffices to cause it to transmit some influence to the basal & bending part of the tentacles, which sweep through a semicircle of 180o.— Ph. of Ammonia is far more powerful than the Nitrate of Amm., & the Nitrate more powerful than the Carbonate, all in causing inflection: though the latter salt causes aggregation much more quickly than the two former salts.   1140,000 of a grain of the carbonate causes aggregation.—

(5) Water at 120o–125o Fah. quickly causes inflection & aggregation, (not so at higher temperatures)—

(6) You have seen what I call aggregation of the protoplasmic contents of the cells. A multitude of causes which excite inflection induce aggregation; but not all the causes. There may be aggregation without inflection. The process supervenes at the proper rate (or not at all), only when the protoplasm is uninjured (it will not occur in a ruptured cell) & in an oxygenated condition.

The protoplasm seems in so unstable a condition that almost any cause causes the granules to aggregate; for instance a minute particle of glass or hair on a gland.

(7) Objects of any kind placed on the disc not only cause the marginal tentacles to be inflected; but the glands secrete more copiously, & the secretion becomes acid: & according to Frankland the acid belongs to the acetic series & is nearest to Propionic.5

(8) I think the most interesting result is about digestion about which you know. There is now no exception to the rule that whatever substance (& I have tried many) pepsin & hydrochloric acid will digest so will Drosera; & what the former cannot digest, the latter cannot. It is marvellous to see the blade of the leaf convert itself with the overarching tentacles into a temporary stomach & pour out an acid secretion with some ferment so closely analogous to pepsin. I shd have said that B. Sanderson ascertained for me that propionic acid & its several allies can digest with pepsin.6

(9) The action of poisons is remarkable but too long,— strychnine & quinine &c are poisonous; but many deadly poisons to animals, as atropine curare, are quite innocuous

The poison of the cobra (given me by Dr Fayrer)7 is absorbed & seems to act as a pleasant stimulus to the protoplasm, for after an immersion of 50 hours in a strong solution, I never saw before the protoplasm in such vigorous spontaneous movement. What a profound difference between animal & vegetable protoplasm!

(10) Camphor is a stimulant, so that after an immersion of 3m a touch on the glands will cause the tentacles to bend, which otherwise wd not have sufficed.

(11) I dare not say anything about the lines of transmission of the motor influence from the central to the marginal tentacles: I doubt about my old experiments. I can (however) confirm fully Dr Nitschke’s statement that if a minute bit of meat be placed on one side of disc, not only do the surrounding tentacles bend, but they direct themselves to the points where the meat lies. It is very striking to put one atom of meat on one side of disc & another atom on the opposite side, & observe the positions of the inflected tentacles.8

But I shall have wearied you out.— I have no amanuensis to help me, & am very sorry for bad writing.

We go on Saturday for few days to Abinger (T. H Farrer) & thence to W. at Southampton.9

Yours affect | C. Darwin

God Help you reading this letter.—


The year is established by the relationship between this letter and the letter from J. D. Hooker, 18 July 1874.
Hooker wanted to present his experimental results on the digestive ability of the tropical pitcher-plant, Nepenthes, at the meeting of the British Association for the Advancement of Science (see letter from J. D. Hooker, 18 July 1874 and n. 2).
In the summer of 1873, CD had described his work on Drosera (sundew) to John Scott Burdon Sanderson, who proposed testing for electrical changes in the leaves. CD suggested that Dionaea would be more suitable as an experimental subject (see Correspondence vol 21, letter from J. S. Burdon Sanderson, 13 August [1873], and letter to J. S. Burdon Sanderson, 15 August 1873). Burdon Sanderson had lectured on electrical phenomena associated with leaf contraction in Dionaea muscipula (the Venus fly trap) at the Royal Institution of Great Britain (see Burdon Sanderson 1874a, 1874b).
CD published the results of his investigations on Drosera in Insectivorous plants.
Edward Frankland had made the comment in his letter of 10 October 1873 (Correspondence vol 21). Acetic series: i.e. carboxylic acids. Acetic, butyric, formic, and propionic acid (now usually known as propanoic acid) are carboxylic acids.
CD quoted Theodor Nitschke on this point in Insectivorous plants, p. 244 (see also Nitschke 1860, p. 240).
CD was away from home from 25 July until 24 August 1874; the first five days were spent at the home of Thomas Henry Farrer and the remainder at William Erasmus Darwin’s (see ‘Journal’ (Appendix II)).


Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 28 vols to date. Cambridge: Cambridge University Press. 1985–.

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


"It is grand about Nepenthes."

JDH is welcome to notice in any way any of CD’s published or unpublished results with insectivorous plants. Gives an abstract of his observations on Drosera.

Letter details

Letter no.
Charles Robert Darwin
Joseph Dalton Hooker
Sent from
Source of text
Royal Botanic Gardens, Kew (JDH/3/6 Insectivorous plants 1873–8: 32–37)
Physical description
C 6pp

Please cite as

Darwin Correspondence Project, “Letter no. 9555,” accessed on 23 May 2022,

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