Dear Cresy

I have received in a parcel from Down your letter and most interesting enclosures and Taylor’s pamphet.² Good heavens, what trouble you have taken for me! I am sure I am very much obliged; and the facts are most interesting to me, but I have not yet digested them.—   Since writing last we have had a terrible week with my poor girl on the point of death; but she has rallied surprisingly. When we shall be able to move her home we know not as yet at all.—   You ask how my experiments were tried; but they have been tried in so many manners that I hardly know how to answer. I will before very long (but all my work has been utterly stopped for a fortnight by this miserable illness) publish an account.—

When I saw you I had tried only putting a minute drop on the leaves on growing plants and observing whether or not they contracted as if over a fly.³ From many reasons I inferred that the leaves absorbed some nitrogenous element, probably some form of Ammonia; so I thought I would try under the microscope the effect of C. of Ammonia; and several other salts and substances, and it is truly wonderful how quickly a minute dose acts and produces marvellous changes in the absorbing glands and in the adjoining cells. I have tried plain water over and over again with no effect. I have over and over again kept a leaf for 1, 2, 3, 4, and 5 hours in water with no effect, and then put these same leaves in a few measured drops of very weak solutions of C. of Ammonia (all made, and re-made by myself) and the same peculiar effects were produced in one hour or 1$\frac{1}{2}$ hour as is produced instantaneously by a stronger solution. Generally I have scrutinized every gland and and hair on leaf before experimenting; but it occurred to me that I might in some way affect the leaf; though this is almost impossible, as I scrutinized with equal care those that I put into distilled water (the same water being used for dissolving the C. of Ammonia). I then cut off 4 leaves (not touching them with fingers) and put in plain water and 4 other leaves into the weak solution and after leaving them for 1$\frac{1}{2}$ hours I examined every hair on all 8 leaves; no changes on the 4 in water    every gland and hair affected in those of Ammonia.

I had measured the quantity of weak solution and I counted the glands which had absorbed the ammonia and were plainly affected; the result convinced me that each gland could not have absorbed more than ^1/64,000 or $\frac{1}{65000}$ of a grain.— I have tried numbers of other experiments all pointing to the same result. Some experiments lead me to believe that very sensitive leaves are acted on by much smaller doses. Reflect how little Ammonia a plant can get growing on poor soil—yet it is nourished. The really surprising part seems to me that the effect should be visible and not under very high power; for after trying high power, I thought it would be safer not to consider any effect which was not plainly visible under $\frac{2}{3}$ object glass and middle eye-piece. The effect which the C. of Ammonia produces is the segregation of the homogeneous fluid in the cells into (first) a cloud of granules and colourless fluid; and subsequently the granules coalesce into larger masses and for hours have the oddest movements coalescing, dividing, coalescing ad infinitum.⁴ I do not know whether you will care for these ill-written details; but as you asked I am sure I am bound to comply after all the very kind and great trouble which you have taken.

Yours very truly | C. Darwin