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Darwin Correspondence Project

From F. J. Cohn   28 August 1875

Liebwerda Bohemia

Aug. 28th. 1875

Dear Sir

By recalling in mind what I have written to you some days ago, I fear, not to have expressed clearly enough my thoughts about aggregation.1 I beg permission to explain them more explicitly.

The cells of the glands and tentacles of Drosera consist

1) of delicate, non incrassated cell-membranes,

2) of a coating of colourless protoplasma, in which granules of chlorophylle—without amylon2—are imbedded.

3) of a central cell-fluid, clear and transparent.

In the living tentacles, there are dissolved in the cell-fluid, besides other substances a) erythrophylle3 b.) mucilago c.) an acid—(also in the non irritated glands.) d). a substance, which is only soluble in the acid fluid.

By neutralising the acid (adding a strong solution of carbonate of ammonia) the later substance is quickly precipitated in shape of greater or smaller drops or granules which principally are tinged with erythrophylle, but soon by exosmosis of the pigment, become colourless or black, and render the cells non-transparent.

The cell-membrane and protoplasma-coating of the living, but non irritated tentacles let diffuse by exosmosis a viscous fluid (mucilago?), but neither erythrophylle nor the acid.

By irritation the molecular arrangement of the cell-membrane and protoplasma-coating are changed in as much as a part of the acid exsudates, and the viscous secretion thus becomes acid.

By killing the cells, this change in the molecular arrangement of their cell-membranes and protoplasma-coatings proceeds so far as to permit also the exosmosis of erythrophylle; thus the tissues loose their red colour.

Aggregation seems to me a process of partial precipitation of certain substances dissolved in the acid cell-fluid, in consequence of the exosmosis of the acid. The changes in the shape of the aggregated masses seem to me analogous to those of clouds which continually change their shapes by partial precipitation and redissolution of aqueous vapours. But very probably other causes may also determine the cloud-like precipitation of the dissolved substance which is intimately united with erythrophylle, if present, which, however, I consider not as protoplasma.—

The lines above were written before I did receive your kind letter of Aug. 24th.. I am very proud of the expression of kindness with wich you did favour me; such words as yours are the highest honour a man of science may aspire at.4 You are quite right, that chlorophylle is dissolved in or mixed with protoplasma; but erythrophylle, as much as I know, behaves different, and is never united with protoplasma. But the remarks I dare to submit to your consideration, are only the first impressions got from the repetition of your observations with a quite insufficient microscope and without the necessary completion of microchemic reagents.

Perhaps I shall be happy enough, after returning home next week, to ascertain the value of my interpretation of your discovery, the most important in biology of our time.5 Believe me dear Sir

yours sincerely | Ferdinand Cohn


See letter from F. J. Cohn, 21 August 1875. Cohn refers to the process of aggregation in tentacle cells of Drosera rotundifolia (common or round-leaved sundew) first described by CD. In his essay review of Insectivorous plants (Cohn 1876, pp. 454–5), Cohn remained sceptical of the protoplasmic nature of the aggregated material, referring instead to the aggregation of particles in the red cell sap.
Amylon: amylum, or starch (Greek).
Erythrophyll was the name given to a common red pigment found in plants (see letter from F. J. Cohn, 21 August 1875 and n. 6).
Cohn credited CD with discovering carnivory in a number of new plants, with providing the first evidence that the leaves of Drosera not only caught and dissolved organic matter but digested it in the same way as a human stomach, and with being the first to establish a reflex response in its tentacles (‘aggregation’) analagous to that of a nervous system (Cohn 1876, pp. 454–5, especially pp. 450 and 452). For another contemporary assessment of CD’s contribution, see Leland 1876.


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

Leland, E. R. 1876. Insectivorous plants. Popular Science Monthly 8 (1875–6): 45–60.


Clarifies his thoughts on "aggregation" in Drosera.

Letter details

Letter no.
Ferdinand Julius Cohn
Charles Robert Darwin
Sent from
Liebwerda (Hejnice)
Source of text
DAR 86: B3–4
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 10137,” accessed on 2 December 2020,

Also published in The Correspondence of Charles Darwin, vol. 23