My dearest Sir

I can not let finish the year before I did send you my kindest thanks for the admirable present you forwarded to the whole scientific world as well as to myself.¹ It is a fresh leaf to the wreath you have gained in the battle of science, evergreen as all the other’s which adorn your brow. Immediately after having received your book, I went about studying and repeating the principal experiments upon which you have founded your theory of circumnutation; of course I succeeded in ascertaining the curious curves the apex of a growing plant describes; as for your experiments about the sensitive qualities of the top of the radicle, most of your results were not foreign to me as I was engaged many years ago with studying the germination of seeds (my inaugural dissertation treats a “symbola ad seminis physiologiam, and the researches of Ciesielksy were made in my laboratory,);² several important facts you did discover, I am about of ascertaining by repetition of your experiments. I wish, I could personaly discuss with you the many questions your book arouses, for to write about requires rather a book than a letter. In the whole I agree totally with you; I am quite sure that in plants exist tissues—which are only sensitive and transmit a stimulus to other tissues which are not sensitive but contract or swell by irritation. But I am not convinced that the theory of alternative turgenscence which you adopt for the circumnutation and other movements, does touch the truth, as you and most German physiologists accept.³ If I am not mistaken the force of movements does not dwell upon the quantity of water but upon the quality of protoplasm; the latter, in plants not less than in animals, is the truly contractile substance; and all theories which propose an essential difference between the movements of plants and animal’s cells, walk out of the right way.⁴ As far as my own observations go, which however, I confess, are not yet concluding, the movements of vegetable tissues (circumnutation, heliotropism etc) depend upon changes in the shape of the protoplasmic bodies of the cells (they become longer but thinner etc.) without change of volume (conservative of turgescence) or the changes of turgensie are only secondary ones. The changes of shape are conformable to some purpose or useful to the plant and therefore acquired by heredity:— But this is a theme for a long dissertation and so I refrain from; I may only add that my views repose mostly upon the study of unicellular plants where the biological facts are more palpable than in the higher and more complicated classes (cf. Oscillaria, a very fine speciman of circumnutation).⁵

I don’t know if I should dare to express how much I admire in your last book as much as in your former, all the qualities of a great biologist and philosopher

May to you be reserved a long series of happy years for the benefit of human knowledge. So I send you my kindest congratulations for the coming new year | and many happy returns of this day, | Truly yours Ferdinand Cohn