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