To F. J. Cohn   2 September 1875

Down, | Beckenham, Kent.

Sep 2. 1875

My dear Sir

I had resolved not to trouble you with another letter, but I cannot resist thanking you for your last note. When you have watched the aggregated masses in the cells of the tentacles for some little time, under a high power, you will be able to judge whether any other substance except protoplasm undergoes such movements.1 Your remark about the acid passing thro’ the cell-membrane of the glands when irritated has led me to think that this probably is the explanation of the process of aggregation; for, as I have attempted to show (supported as my view is by Gorup-Besanez p.362 of my book) that protoplasm is held in solution by a ferment (the same which gives to Drosera the power of digestion) together with an acid; consequently as soon as the acid passes outwards, the protoplasm is precipitated, & then displays its characteristic spontaneous movements.2 If this view is correct, it would have to be extended to the roots of Drosera, Euphorbia &c (p. 64— & 141) with this difference, that in these cases the Carbonate of Ammonia is absorbed by the roots & unites with some free acid, causing the precipitation of the protoplasm.3 I am however aware that I ought to have worked more on the roots of plants, & on the action of Carb. of Ammonia on the grains of Chlorophyll but time & strength failed me. You are very good to speak so encouragingly about my observations on the so-called process of aggregation.4

I have addressed this note to Breslau, from not knowing how long you will remain at your present residence.5

My dear Sir | yours sincerely | Charles Darwin

LS(A)

1
Cohn had written to CD about his uncertainty regarding the protoplasmic nature of aggregated masses in cells of the tentacles of Drosera rotundifolia (the common or round-leaved sundew); see letters from F. J. Cohn, 21 August 1875 and 28 August 1875, and letter to F. J. Cohn, 24 August 1875.
2
In Insectivorous plants, p 362, CD pointed out that all plants had the power of digesting ‘albuminous or proteid substances’, and speculated that this was effected by a solvent, consisting of ‘a ferment together with an acid’. He cited a paper by Eugen Franz von Gorup-Besanez, who had discovered that vetch seeds contained a ferment that dissolved albuminous substances (Gorup-Besanez 1874).
3
In Insectivorous plants, p. 64, CD noted that he had observed a process like aggregation in roots of Euphorbia peplus (petty spurge) placed in solution of carbonate of ammonia; in ibid., p. 141, he described the effect of carbonate of ammonia on Drosera roots.
4
Insectivorous plants, pp. 38–65. In his letter of 28 August 1875, Cohn described CD’s discovery as ‘the most important in biology of our time’.
5
Breslau in Prussia (Germany) is now Wrocław in Poland. Cohn had written from Liebwerda in Bohemia (now the Czech Republic; letter from F. J. Cohn, 28 August 1875).

Manuscript Alterations and Comments

1.5 when irritated] interl in CD’s hand
1.15 on … Chlorophyll 1.16] interl in CD’s hand

Please cite as “DCP-LETT-10150A,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-10150A