My dear Sir

As from an inexhaustible fountain, there come out from your creative genius so many invaluable productions that we can scarcely follow with the expression of our thanks the unintermitted series of your gifts. Still engaged in the study of your book on cross-fertilisation of plants, I am presented with your last work on dimorphic flowers; and now, Mr. Francis Darwin having favoured me with a copy of his paper on Dipsacus, you are kind enough to draw my special attention to the admirable facts discovered by your son and confirmed by yourselves.¹ Immediately after the reception of your very kind letter of July 26, I went to fetch Dipsacus, several species of which grow in our Botanical Garden; and proceeding after your recommendations, I put transverse sections of the cup-like bases of young leaves, or the epidermis of these parts, drawn of carefully from the green parenchyme, into distilled water.² So I had the happiness to witness with my own eyes the most curious discovery of those last week’s. At first I ascertained the anatomical structure of the pearlike glands, which are rather elegant and remarkable: from the basal cell ^(I) rises the stalk-cell^(II); in the second story there are two^(III), in the third four^(IV), and in the uppermost eight cells^(V)., cuneiform, all converging to the centre.

But you may conceive how much I was surprised, by seeing the filiform protuberances protruded from out the apex of the glands. It was quite perplexing a spectacle: the filaments are in their refrangibility very like to the pseudopodia of some Rhizopods (Arcella or Difflugia f.i).³ I followed their changes through some time, and I did remark quite definitely as I find exposed in the paper of Mr. Francis: how the protuberaces slowly prolong, crook themselves, hooklike or winding, then extend again straight on, get knopped on their summit or midway; I saw the knops or beads glide down along the thread and lastly be sucked into a globulous mass adhering to the gland. I saw the protuberances always rise between the septa of two or more adjoining cells, but nearly as frequently between the lateral septa as on the apical centrum; generally there were many protuberances on the same glands pressed forward out of different spots; sometimes I saw two diverging branches proceed from the same point like to a pair of compasses, each behaving particularly in its changes. But the most curious appearance in those protuberances was a constant waving undulation along their extension, sometimes slower and difficultly perceivable, sometimes vigourous and quicker—but never ceasing; more delicate filaments appeared to me very like to Vibrio, or to the vibratory flagella of some Infusoria.⁴ Not finding a special description of the waving movements of the filaments in your son’s paper, I asked some of my pupils, if they saw something remarkable on the filaments, without indicating what? but they all had the same impression as myself. The only facts I could not yet witness of your son’s discoveries, are Fig 6, 14, 15 and the moniliform contractions; nor did I find time hitherto to apply chemical reagents of which Mr. Francis D. has made so admirable a use.⁵

Of course I am not able after two day’s inspection to form a definite judgment about the true nature of the filiform protuberances. Putting aside the hypothesis of a parasitic Rhizopod, there are two probabilities which still balance in my mind, as Mr. Francis D. has already exposed with so great discernment: 1). The protuberances are secretions of some colloidal matter, absorbing water but insoluble in it; the movements are physical, not vital ones, the prolonging depending upon the imbibition, the contraction upon the withdrawing of water by different reagents. There are such substances, f.i myeline which shows rather similar changes in water.⁶ Please also to repeat the experiments I produced in the meeting of the British Association of last year: Into a cylindrical glas containing soluble silicate of alcali (Wasserglas) diluted by water, half and half, put a small piece of crystallised chloride of iron; from the fragment there rises a hollow reddish tube, growing upwards and moving very quickly, like an Entermorpha.⁷ But if you put into the diluted Silicate some Protochloride of iron (the latter mostly powderlike but easily brought by gentle pressure of the fingers into greater crumbs) then from the lump there arise innumerable filaments, very delicate and transparent, very like to the glas threads of Hyalonema, which grow in fascicles vertically till they reach the superficies of the fluid.⁸

But I cannot deny, the general impression of Dipsacus does not contradict the hypothesis that the changes of the filaments are a vital phenomenon of protoplasmatic pseudopodium’s. A French biologist, whose name I could not yet make out, has proved many years ago, (I suppose, in some elder publication of the Bulletins de la Société Botanique de France) the water in the cups of Dipsacus not to be a simple collection of rain in a gutter, but a secretion of the leaf-bases.⁹ If this truly be the case, then it is quite probable that the glands may have a special adaptation for this purpose. Indeed I should not hesitate to agree with the vital theory if there were any analogy known in plants. But pursued study of the phenomenon and the repetition of the chemical reactions which Mr. Francis D. has already indicated, will, I hope, in short time enable me to a more decided judgment in this perplexing dilemma.

In meantime I am happy to congratulate Mr. Francis Darwin and yourselves on account of the extraordinary discovery he has made, and the truly scientific and sagacious paper in which he has elaborated it, and which has added a series of quite unexpected facts to the physiology of plants.

Minding to write again after some time when I shall have completed my repetition of the experiments on Dipsacus, I beg to send a Report on the proceedings of the Botanical Section of the Silesion society of last year which contains a short communication on Lathraea, which plant I suspected to be carnivorous without being able to discover a confirming fact¹⁰

Mrs Cohn presents her kindest regards to Mrs Darwin and your family; we shall never forget the happy hours we past last year in your house.¹¹ | Believe me dear Sir | Yours very faithfully | Ferdinand Cohn

Is Mr. Francis Darwin living in Down? Not knowing his address I am obliged to beg the favour of you to express to him in my name my best thanks for the many presents he has bestowed upon me with his papers.¹²

Perhaps it will interest you that a pupil of our university Dr. Penzig will publish in short time an inaugural dissertation on the anatomy of Drosophyllum lusitanicum.¹³