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Darwin Correspondence Project

From F. J. Cohn   21 August 1875

Pflanzenphysiologisches Institut | der | K. Universität Breslau. | Liebwerda, Bohemia

Aug. 21st. 1875

Dear Sir

Believe me not ungrateful because I did not acknowledge till today the receipt of so precious a gift as your book on Insectivorous plants is valued by me.1 I found it impossible to limit myself to a simple expression of thanks; on the contrary I felt obliged to bestow the most sedulous study into a work affording an incredible amount of new facts, and a true model of inductive method which cautiously proceeding step by step, conducts to the most surprising biological revelations. So I took with me your book into this little place at the utmost corner of North-Bohemia on the foot of the Iser-mountains, where I am spending some weeks in quiet seclusion amidst forests and hills;2 soon I happened to gather a supply of living Drosera rotundifolia from the moors of this country which did forward me the opportunity to repeat at least the most striking of your admirable experiments, in as much the total want of scientific apparatus did allow; and a little pocket-microscope, which I happily brought there in my carpet bag, permits me to observe your wonderful discovery of aggregation.3 Many years before I had bestowed much time to the observation of Drosera, Mr. Nitschke being a pupil of mine and his principal researches about Drosera being made in my then-private laboratory under my direction for the purpose of an inaugural dissertation.4 But your book shows the whole question quite under a new light, and I congratulate you most sincerely to this last but by no means least contribution to the Advancement of science our generation is owing to your genius.

Will you kindly permit me to express candidly a scrupule I was struck with, by perusing your book attentively? By descriving the aggregation in the cells of the tentacles, you always take it granted that the red masses, in incessant changes of form coalescent and again separating, are protoplasma.5 But did you really prove it? To tell the truth, I have some doubts about the protoplasmatic nature of the aggregated masses. My principal reason is their colour. By its general proprieties as well as after the spectroscopic examination of Mr. Sorby the red pigment seems not different from the common erythrophylle of red leaves, petala, fruits, which is not wanting in the tissues of stems and roots; if red, it shows always an acid reaction: if neutral, it changes in violet; if alcaline, it becomes blue or green; the blue colour is generally called anthocyane.6 Perhaps there are different pigments confound under these denominations; but they all agree: they are always dissolved in the watery fluid which fills out the inner cavity of the cells, and they are insoluble in protoplasma. Compare for instance the blue hair-cells at the stamens of Tradescantia and you will well distinguish the colourless circulating protoplasma adhering to the inner cell-wall, and the blue watery fluid of the central cavity.7 I guess, that in Drosera the true circulating Protoplasma is also colourless, and that the purple substance is not protoplasma. But how explain the wonderful phenomenon of aggregation discovered by you? To my greatest regret the microscope here in my possession does not suffice to solve the question; perhaps I shall be happier after my return at Breslau. But I dare to indicate an analogy, first described by Naegeli; put red or blue petala in a denser fluid; by exosmosis water quickley exsudates from out the cells, through their membranes; the membranes withdraw it from the colourless protoplasma and the protoplasma from the coloured watery fluid of the cells, in such a degree, that the pigment looses the necessary quantity of water for its dissolution and is reduced in dense and intensively coloured drops, smaller or larger; by adding fresh distilled water, the later enters by endosmosis into the cells, and the coloured drops dissolve in the cell-fluid once more.8 Perhaps there is the clue, for explaining the aggregation by the water-absorbing power of the protoplasma.

Please to accept these remarks as a token of the deepest interest I take on your admirable researches and believe me

Truly yours | Ferdinand Cohn


Cohn’s name is on CD’s presentation list for Insectivorous plants (see Appendix IV).
The health resort where Cohn stayed is now known as Lazne Libverda in the Czech Republic and the mountain range is the Jizera mountains (Columbia gazetteer of the world).
Most of CD’s experimental work was done with Drosera rotundifolia (common or round-leaved sundew; see Insectivorous plants, pp. 1–277). He described the aggregation of matter within cells of the tentacles and concluded, from the movement and consistency of the masses, that they consisted of protoplasm (ibid., p. 40).
Theodor Nitschke’s doctoral dissertation was titled Commentatio anatomico-physiologica de Droserae rotundifoliae (L.) irritabilitate (Nitschke 1858). Nitschke published articles on the growth of round-leaved sundew, on the sensitivity of its leaves, and on its morphology and anatomy (Nitschke 1860a, 1860b, 1861a, 1861b). CD referred frequently to these papers in Insectivorous plants.
See n. 3, above.
Henry Clifton Sorby, who had invented a spectrum microscope, had studied the nature of pigments in several organic and inorganic substances. CD had learned from Sorby that the red pigment in hairs of Drosera rotundifolia was erythrophyll, a common red pigment found in leaves, fruit, and flowers (see Correspondence vol. 22, enclosure to letter from J. D. Hooker, 15 September 1874; see also Insectivorous plants, p. 5 n.). The term erythrophyll is no longer generally used in scientific terminology, but the substance belongs to a family of pigments that vary in appearance depending on the acidity or alkalinity of the environment, and are now known as anthocyanins.
The stamen hairs in Tradescantia (spiderwort) are usually blue; each hair consists of a single row of connected bead-like cells, in which the blue pigment is located in the central vacuoles, while the clear protoplasm is near the outer cell wall.
Carl Wilhelm von Nägeli described this process in 1855 (see Nägeli and Cramer 1855–8, part 1, pp. 5–7). CD’s copies of the first and fourth parts of Nägeli and Cramer 1855–8 (the first and second parts were written by Nägeli) are in the Darwin Pamphlet Collection–CUL.


Columbia gazetteer of the world: The Columbia gazetteer of the world. Edited by Saul B. Cohen. 3 vols. New York: Columbia University Press. 1998.

Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 27 vols to date. Cambridge: Cambridge University Press. 1985–.

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

Nitschke, Theodor Rudolf Joseph. 1858. Commentatio anatomico-physiologica de Droserae rotundifoliae (L.) irritabilitate. Part I. Physiologica. Breslau University dissertation.


Acknowledges presentation copy of Insectivorous plants.

Studying Drosera on vacation in Bohemia. Thinks CD has erred in considering "aggregation" to have occurred in the protoplasm. Suggests it is result of exosmosis of vacuole.

Letter details

Letter no.
Ferdinand Julius Cohn
Charles Robert Darwin
Sent from
Liebwerda (Hejnice)
Source of text
DAR 161: 200
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 10131,” accessed on 20 January 2021,

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