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
Footnotes
Bibliography
Insectivorous plants. By Charles Darwin. London: John Murray. 1875.
Leland, E. R. 1876. Insectivorous plants. Popular Science Monthly 8 (1875–6): 45–60.
Summary
Clarifies his thoughts on "aggregation" in Drosera.
Letter details
- Letter no.
- DCP-LETT-10137
- From
- Ferdinand Julius Cohn
- To
- Charles Robert Darwin
- Sent from
- Liebwerda (Hejnice)
- Source of text
- DAR 86: B3–4
- Physical description
- ALS 4pp
Please cite as
Darwin Correspondence Project, “Letter no. 10137,” accessed on 28 March 2024, https://www.darwinproject.ac.uk/letter/?docId=letters/DCP-LETT-10137.xml
Also published in The Correspondence of Charles Darwin, vol. 23