My dear Sir

I am much obliged to you for so kindly telling me about the Australian Orchids, (a subject which interests me greatly, & I have now examined nearly all the British kinds);² but I cannot quite understand the description, & without examining the live plants, with reference to visits of insects, I believe their means of fertilisation can never be understood. Even Hooker was led into considerable error, not of facts, but of purpose in his curious description of Listera.³

I want to consult you on one point, as you have so much knowledge: I have consulted Prof. Henslow, but he knows nothing like it.—⁴ The elongated cells in the hairs of Drosera are filled, when expanded, with homogeneous pink fluid: after inflection this fluid always separates into colourless fluid & into thick, viscid, dark red fluid. This latter fluid undergoes a slow, never ceasing, endless changes of form. The enclosed diagrams (1, 2, 3 &c)⁵ represent the same cell full of colourless fluid, with the shaded dark red viscid matter: the outlines were done at the interval of about 1$\frac{1}{2}$ minute; & you will see the changes of form & position of the two original masses; they divide, coalesce, leave threads behind them; these threads change into necklaces, & one or more of the beads swell & enlarge, then often divide, coalesce & so on ad infinitum— I have never seen the same changes twice, they are infinitely varied. It is not due to endosmose, for it takes place in Hair cut off & placed dry between two slips of glass.—   Has anything like this been described? A very weak solution of C. of Ammonia instantly sets the process at work.—

I hope that you will like to hear a little about Dionæ a,—the examination of which I have much enjoyed owing to your kindness.— I have made out but very little. The sensitive Hairs, have no gland, or spiral vessel or stomata, & so differ greatly from the Hairs of Drosera; but I have hardly any doubt that the spirally arranged cells are before the contraction of the leaf filled with homogeneous red fluid; for many cells were in the contracted specimens sent by you thus filled    And I saw the same, but very slow, movements, as in Drosera, & the red viscid matter was much excited by C. of Ammonia. This seems an important point of agreement with Drosera.

On upper or inside of leaf, the whole surface is studded with minute projections on footstalks, just like the back of a tortoise; & in the cells or compartments, there is pink fluid, which with gf/C. of Ammonia acts as the matter in cells of Drosera. From analogy of Drosera I infer that these tortoises are the secreting & absorbing glands.—⁶ One thing surprises me, viz under each footstalk of the tortoise-shells there is what appears a stoma.

These tortoises do not occur on insides of marginal spikes. On back of spikes & on back of leaf, there are many petaloid or star-like little projections on footstalks; & beneath these there are apparently stomata.⁷ The brown matter within these stars is not acted on by C. of Ammonia. The marginal spikes have spiral vessels; & I suspect may be looked at as the extreme marginal hairs of Drosera in a functionless condition.

Now these few observations would lead me to try the following experiments, if I had living plants; but I beg you not to try, unless you feel some little interest on the subject.—⁸

Firstly & chiefly, to gather one more leaf very quietly, (or look at growing leaf) put it under simple microscope & observe whether the sensitive Hairs are uniformly coloured; then touch Hair with needle, & observe whether the colour does not become very soon (how soon?) mottled from the action of segregation, as described, of red viscid matter from red fluid.

Drop weak C. of Ammonia on inside of marginal spikes,—on disc of leaf,, on back of leaf & on sensitive Hairs, but in in latter case so small a drop as not to run down on to the disc, if this be possible.—   These experiments could tell much.—

In how many seconds or minutes does leaf close?

Lastly can you tell me, whether the stomata are ever covered by reticulated cells?

Have stomata & spiral vessels any connection? or relation

First try disc & sensitive Hair; if these do not act, other trials would be useless.—

I have as yet tried only 2 experiments on D. spathulata.—⁹

at 9^o A.M put on one leaf $\frac{1}{2}$ minim of solution of Nitrate of Ammonia (3 gr to 1 oz, so that I gave the leaf $\frac{1}{320}$ of a grain which is much too strong), in 6 hours, hairs incurved; in 7 hours hairs met; in 9 hours edges of the leaf itself much incurved; so that leaf became semi-cylindrical.—

Gum has produced no effect—

I am in complete muddle about effect of various salts; & I am deeply indebted to you (in main part) for having made me try more experiments.—   God forgive me, for having written you so long a letter.—   You will soon find out that I can be troublesome with a vengeance.—

With cordial thanks, Yours very truly | C. Darwin

(It has been ridiculous in me, from my ignorance, taking up this subject; but it was purely accidental.)

Do please touch the sensitive Hair; though my fear is whether with simple microscope you could perceive change from uniform to broken colour: yet I felt sure that colour was broken when I first saw through simple lens the Hairs.— I expect the change to be almost instantaneous. It would be very curious if disc alone was sensitive to Ammonia; & the Hairs only to touch, but Heaven knows how it would be.

Perhaps action of C. of Ammonia on the disc would not be rapid (as from a touch on Hair).—