More Letters of Charles Darwin — Volume 2

I have got seeds from Dr. King of some Melastomaceae, and will write to Veitch to see if I can get the Melastomaceous genera Monochaetum and Heterocentron or some such name, on which I before experimented. Now, if you can aid me, I know that you will; but if you cannot, do not write and trouble yourself.

2. X.III. CORRESPONDENCE WITH JOHN SCOTT, 1862-1871.

"If he had leisure he would make a wonderful observer, to my judgment; I have come across no one like him." — Letter to J.D. Hooker, May 29th {1863}.

(631/1. The following group of letters to John Scott, of whom some account is given elsewhere (Volume I., Letters 150 and 151, and Index.) deal chiefly with experimental work in the fertilisation of flowers. In addition to their scientific importance, several of the letters are of special interest as illustrating the encouragement and friendly assistance which Darwin gave to his correspondent.)

LETTER 631. JOHN SCOTT TO CHARLES DARWIN. Edinburgh Botanic Gardens, November 11th, 1862.

I take the liberty of addressing you for the purpose of directing your attention to an error in one of your ingenious explanations of the structural adaptations of the Orchidaceae in your late work. This occurs in the genus Acropera, two species of which you assume to be unisexual, and so far as known represented by male individuals only. Theoretically you have no doubt assigned good grounds for this view; nevertheless, experimental observations that I am now making have already convinced me of its fallacy. And I thus hurriedly, and as you may think prematurely, direct your attention to it, before I have seen the final result of my own experiment, that you might have the longer time for reconsidering the structure of this genus for another edition of your interesting book, if indeed it be not already called for. I am furthermore induced to communicate the results of my yet imperfect experiments in the belief that the actuating principle of your late work is the elicitation of truth, and that you will gladly avail yourself of this even at the sacrifice of much ingenious theoretical argumentation.

Since I have had an opportunity of perusing your work on orchid fertilisation, my attention has been particularly directed to the curiously constructed floral organs of Acropera. I unfortunately have as yet had only a few flowers for experimental enquiry, otherwise my remarks might have been clearer and more satisfactory. Such as they are, however, I respectfully lay {them} before you, with a full assurance of their veracity, and I sincerely trust that as such you will receive them.

Your observations seem to have been chiefly directed to the A. luteola, mine to the A. Loddigesii, which, however, as you remark, is in a very similar constructural condition with the former; having the same narrow stigmatic chamber, abnormally developed placenta, etc. In regard to the former point — contraction of stigmatic chamber — I may remark that it does not appear to be absolutely necessary that the pollen-masses penetrate this chamber for effecting fecundation. Thus a raceme was produced upon a plant of A. Loddigesii in the Botanic Gardens here lately; upon this I left only six flowers. These I attempted to fertilise, but with two only of the six have I been successful: I succeeded in forcing a single pollen-mass into the stigmatic chamber of one of the latter, but I failed to do this on the other; however, by inserting a portion of a pedicel with a pollinium attached, I caused the latter to adhere, with a gentle press, to the mouth of the stigmatic chamber. Both of these, as I have already remarked, are nevertheless fertilised; one of them I have cut off for examination, and its condition I will presently describe; the other is still upon the plant, and promises fair to attain maturity. In regard to the other four flowers, I may remark that though similarly fertilised — part having pollinia inserted, others merely attached — they all withered and dropped off without the least swelling of the ovary. Can it be, then, that this is really an {andro-monoecious} species? — part of the flowers male, others truly hermaphrodite.

In making longitudinal sections of the fertilised ovary before mentioned, I found the basal portion entirely destitute of ovules, their place being substituted by transparent cellular ramification of the placentae. As I traced the placentae upwards, the ovules appeared, becoming gradually more abundant towards its apex. A transverse section near the apex of the ovary, however, still exhibited a more than ordinary placental development — i.e. {congenitally?} considered — each end giving off two branches, which meet each other in the centre of the ovary, the ovules being irregularly and sparingly disposed upon their surfaces.

In regard to the mere question of fertilisation, then, I am perfectly satisfied, but there are other points which require further elucidation. Among these I may particularly refer to the contracted stigmatic chamber, and the slight viscidity of its disk. The latter, however, may be a consequence of uncongenial conditions — as you do not mention particularly its examination by any author in its natural habitat. If such be the case, the contracted stigmatic chamber will offer no real difficulty, should the viscous exudations be only sufficient to render the mouth adhesive. For, as I have already shown, the pollen-tubes may be emitted in this condition, and effect fecundation without being in actual contact with the stigmatic surface, as occurs pretty regularly in the fertilisation of the Stapelias, for example. But, indeed, your own discovery of the independent germinative capabilities of the pollen-grains of certain Orchidaceae is sufficiently illustrative of this.

I may also refer to the peculiar abnormal condition that many at least of the ovaries present in a comparative examination of the placentae, and of which I beg to suggest the following explanation, though it is as yet founded on limited observations. In examining certain young ovaries of A. Loddigesii, I found some of them filled with the transparent membranous fringes of more or less distinctly cellular matter, which, from your description of the ovaries of luteola, appears to differ simply in the greater development in the former species. Again, in others I found small mammillary bodies, which appeared to be true ovules, though I could not perfectly satisfy myself as to the existence of the micropyle or nucleus. I unfortunately neglected to apply any chemical test. The fact, however, that in certain of the examined ovaries few or none of the latter bodies occurred — the placenta alone being developed in an irregular membranous form, taken in conjunction with the results of my experiments — before alluded to — on their fertilisation, leads me to infer that two sexual conditions are presented by the flowers of this plant. In short, that many of the ovaries are now normally abortive, though Nature occasionally makes futile efforts for their perfect development, in the production of ovuloid bodies; these then I regard as the male flowers. The others that are still capable of fertilisation, and likewise possessing male organs, are hermaphrodite, and must, I think, from the results of your comparative examinations, present a somewhat different condition; as it can scarcely be supposed that ovules in the condition you describe could ever be fertilised.

This is at least the most plausible explanation I can offer for the different results in my experiments on the fertilisation of apparently similar morphologically constructed flowers; others may, however, occur to you. Here there is not, as in the Catasetum, any external change visible in the respective unisexual and bisexual flowers. And yet it would appear from your researches that the ovules of Acropera are in a more highly atrophied condition than occurs in Catasetum, though, as you likewise remark, M. Neumann has never succeeded in fertilising C. tridentatum. If there be not, then, an arrangement of the reproductive structures, such as I have indicated, how can the different results in M. Neumann's experiments and mine be accounted for? However, as you have examined many flowers of both A. luteola and Loddigesii, such a difference in the ovulary or placental structures could scarcely have escaped your observation. But, be this as it may, the — to me at least — demonstrated fact still remains, that certain flowers of A. Loddigesii are capable of fertilisation, and that, though there are good grounds for supposing that important physiological changes are going on in the sexual phenomena of this species, there is no evidence whatever for supposing that external morphological changes have so masked certain individuals as to prevent their recognition.

I would now, sir, in conclusion beg you to excuse me for this infringement upon your valuable time, as I have been induced to write you in the belief that you have had negative results from other experimenters, before you ventured to propose your theoretical explanation, and consequently that you have been unknowingly led into error. I will continue, as opportunities present themselves, to examine the many peculiarities you have pointed out in this as well as others of the Orchid family; and at present I am looking forward with anxiety for the maturation of the ovary of A. Loddigesii, which will bear testimony to the veracity of the remarks I have ventured to lay before you.

LETTER 632. TO J.D. HOOKER. Down, 18th {November 1862}.

Strange to say, I have only one little bother for you to-day, and that is to let me know about what month flowers appear in Acropera Loddigesii and luteola; for I want extremely to beg a few more flowers, and if I knew the time I would keep a memorandum to remind you. Why I want these flowers is (and I am much alarmed) that Mr. J. Scott, of Bot. Garden of Edinburgh (do you know anything of him?) has written me a very long and clever letter, in which he confirms most of my observations; but tells me that with much difficulty he managed to get pollen into orifice, or as far as mouth of orifice, of six flowers of A. Loddigesii (the ovarium of which I did not examine), and two pods set; one he gathered, and saw a very few ovules, as he thinks, on the large and mostly rudimentary placenta. I shall be most curious to hear whether the other pod produces a good lot of seed. He says he regrets that he did not test the ovules with chemical agents: does he mean tincture of iodine? He suggests that in a state of nature the viscid matter may come to the very surface of stigmatic chamber, and so pollen-masses need not be inserted. This is possible, but I should think improbable. Altogether the case is very odd, and I am very uneasy, for I cannot hope that A. Loddigesii is hermaphrodite and A. luteola the male of the same species. Whenever I can get Acropera would be a very good time for me to look at Vanda in spirits, which you so kindly preserved for me.

LETTER 633. TO J. SCOTT.

(633/1. The following is Darwin's reply to the above letter from Scott. In the first edition of "Fertilisation of Orchids" (page 209) he assumed that the sexes in Acropera, as in Catasetum, were separate. In the second edition (page 172) he writes: "I was, however, soon convinced of my error by Mr. Scott, who succeeded in artificially fertilising the flowers with their own pollen. A remarkable discovery by Hildebrand (633/2. "Bot. Zeitung," 1863 and 1865.), namely, that in many orchids the ovules are not developed unless the stigma is penetrated by the pollen-tubes...explains the state of the ovarium in Acropera, as observed by me." In regard to this subject see Letter 608.)

Down, November 12th, 1862.

I thank you most sincerely for your kindness in writing to me, and for {your} very interesting letter. Your fact has surprised me greatly, and has alarmed me not a little, for if I am in error about Acropera I may be in error about Catasetum. Yet when I call to mind the state of the placentae in A. luteola, I am astonished that they should produce ovules. You will see in my book that I state that I did not look at the ovarium of A. Loddigesii. Would you have the kindness to send me word which end of the ovarium is meant by apex (that nearest the flower?), for I must try and get this species from Kew and look at its ovarium. I shall be extremely curious to hear whether the fruit, which is now maturing, produces a large number of good and plump seed; perhaps you may have seen the ripe capsules of other Vandeae, and may be able to form some conjecture what it ought to produce. In the young, unfertilised ovaria of many Vandeae there seemed an infinitude of ovules. In desperation it occurs to me as just possible, as almost everything in nature goes by gradation, that a properly male flower might occasionally produce a few seeds, in the same manner as female plants sometimes produce a little pollen. All your remarks seem to me excellent and very interesting, and I again thank you for your kindness in writing to me. I am pleased to observe that my description of the structure of Acropera seems to agree pretty well with what you have observed. Does it not strike you as very difficult to understand how insects remove the pollinia and carry them to the stigmas? Your suggestion that the mouth of the stigmatic cavity may become charged with viscid matter and thus secure the pollinia, and that the pollen-tubes may then protrude, seems very ingenious and new to me; but it would be very anomalous in orchids, i.e. as far as I have seen. No doubt, however, though I tried my best, I shall be proved wrong in many points. Botany is a new subject to me. With respect to the protrusion of pollen-tubes, you might like to hear (if you do not already know the fact) that, as I saw this summer, in the little imperfect flowers of Viola and Oxalis, which never open, the pollen-tubes always come out of the pollen-grain, whilst still in the anthers, and direct themselves in a beautiful manner to the stigma seated at some little distance. I hope that you will continue your very interesting observations.

LETTER 634. TO J. SCOTT. Down, November 19th {1862}.

I am much obliged for your letter, which is full of interesting matter. I shall be very glad to look at the capsule of the Acropera when ripe, and pray present my thanks to Mr. MacNab. (634/1. See Letter 608 (Lindley, December 15th, 1861). Also "Fertilisation of Orchids," Edition II., page 172, for an account of the observations on Acropera which were corrected by Scott.) I should like to keep it till I could get a capsule of some other member of the Vandeae for comparison, but ultimately all the seeds shall be returned, in case you would like to write any notice on the subject. It was, as I said (634/2. Letter 633.), only "in desperation" that I suggested that the flower might be a male and occasionally capable of producing a few seeds. I had forgotten Gartner's remark; in fact, I know only odds and ends of Botany, and you know far more. One point makes the above view more probable in Acropera than in other cases, viz. the presence of rudimentary placentae or testae, for I cannot hear that these have been observed in the male plants. They do not occur in male Lychnis dioica, but next spring I will look to male holly flowers. I fully admit the difficulty of similarity of stigmatic chamber in the two Acroperas. As far as I remember, the blunt end of pollen-mass would not easily even stick in the orifice of the chamber. Your view may be correct about abundance of viscid matter, but seems rather improbable. Your facts about female flowers occurring where males alone ought to occur is new to me; if I do not hear that you object, I will quote the Zea case on your authority in what I am now writing on the varieties of the maize. (634/3. See "Animals and Plants," Edition II., Volume I., page 339: "Mr. Scott has lately observed the rarer case of female flowers on a true male panicle, and likewise hermaphrodite flowers." Scott's paper on the subject is in "Trans. Bot. Soc. Edinburgh," Volume VIII. See Letter 151, Volume I.) I am glad to hear that you are now working on the most curious subject of parthenogenesis. I formerly fancied that I observed female Lychnis dioica seeded without pollen. I send by this post a paper on Primula, which may interest you. (634/4. "Linn. Soc. Journal," 1862.) I am working on the subject, and if you should ever observe any analogous case I should be glad to hear. I have added another very clever pamphlet by Prof. Asa Gray. Have you a copy of my Orchis book? If you have not, and would like one, I should be pleased to send one. I plainly see that you have the true spirit of an experimentalist and good observer. Therefore, I ask whether you have ever made any trials on relative fertility of varieties of plants (like those I quote from Gartner on the varieties of Verbascum). I much want information on this head, and on those marvellous cases (as some Lobelias and Crinum passiflora) in which a plant can be more easily fertilised by the pollen of another species than by its own good pollen. I am compelled to write in haste. With many thanks for your kindness.

LETTER 635. TO J. SCOTT. Down, 20th {1862?}.

What a magnificent capsule, and good Heavens, what a number of seeds! I never before opened pods of larger orchids. It did not signify a few seed being lost, as it would be hopeless to estimate number in comparison with other species. If you sow any, had you not better sow a good many? so I enclose small packet. I have looked at the seeds; I never saw in the British orchids nearly so many empty testae; but this goes for nothing, as unnatural conditions would account for it. I suspect, however, from the variable size and transparency, that a good many of the seeds when dry (and I have put the capsule on my chimney-piece) will shrivel up. So I will wait a month or two till I get the capsule of some large Vandeae for comparison. It is more likely that I have made some dreadful blunder about Acropera than that it should be male yet not a perfect male. May there be some sexual relation between A. Loddigesii and luteola; they seem very close? I should very much like to examine the capsule of the unimpregnated flower of A. Loddigesii. I have got both species from Kew, but whether we shall have skill to flower them I know not. One conjectures that it is imperfect male; I still should incline to think it would produce by seed both sexes. But you are right about Primula (and a very acute thought it was): the long-styled P. sinensis, homomorphically fertilised with own-form pollen, has produced during two successive homomorphic generations only long-styled plants. (635/1. In "Forms of Flowers," Edition II., page 216, a summary of the transmission of forms in the "homomorphic" unions of P. sinensis is given. Darwin afterwards used "illegitimate" for homomorphic, and "legitimate" for "heteromorphic" ("Forms of Flowers," Edition i., page 24).) The short-styled the same, i.e. produced short-styled for two generations with the exception of a single plant. I cannot say about cowslips yet. I should like to hear your case of the Primula: is it certainly propagated by seed?

LETTER 636. TO J. SCOTT. Down, December 3rd, {1862?}.

What a capital observer you are! and how well you have worked the primulas. All your facts are new to me. It is likely that I overrate the interest of the subject; but it seems to me that you ought to publish a paper on the subject. It would, however, greatly add to the value if you were to cover up any of the forms having pistil and anther of the same height, and prove that they were fully self-fertile. The occurrence of dimorphic and non-dimorphic species in the same genus is quite the same as I find in Linum. (636/1. Darwin finished his paper on Linum in December 1862, and it was published in the "Linn. Soc. Journal" in 1863.) Have any of the forms of Primula, which are non-dimorphic, been propagated for some little time by seed in garden? I suppose not. I ask because I find in P. sinensis a third rather fluctuating form, apparently due to culture, with stigma and anthers of same height. I have been working successive generations homomorphically of this Primula, and think I am getting curious results; I shall probably publish next autumn; and if you do not (but I hope you will) publish yourself previously, I should be glad to quote in abstract some of your facts. But I repeat that I hope you will yourself publish. Hottonia is dimorphic, with pollen of very different sizes in the two forms. I think you are mistaken about Siphocampylus, but I feel rather doubtful in saying this to so good an observer. In Lobelia the closed pistil grows rapidly, and pushes out the pollen and then the stigma expands, and the flower in function is monoecious; from appearance I believe this is the case with your plant. I hope it is so, for this plant can hardly require a cross, being in function monoecious; so that dimorphism in such a case would be a heavy blow to understanding its nature or good in all other cases. I see few periodicals: when have you published on Clivia? I suppose that you did not actually count the seeds in the hybrids in comparison with those of the parent-forms; but this is almost necessary after Gartner's observations. I very much hope you will make a good series of comparative trials on the same plant of Tacsonia. (636/2. See Scott in "Linn. Soc. Journal," VIII.) I have raised 700-800 seedlings from cowslips, artificially fertilised with care; and they presented not a hair's-breadth approach to oxlips. I have now seed in pots of cowslip fertilised by pollen of primrose, and I hope they will grow; I have also got fine seedlings from seed of wild oxlips; so I hope to make out the case. You speak of difficulties on Natural Selection: there are indeed plenty; if ever you have spare time (which is not likely, as I am sure you must be a hard worker) I should be very glad to hear difficulties from one who has observed so much as you have. The majority of criticisms on the "Origin" are, in my opinion, not worth the paper they are printed on. Sir C. Lyell is coming out with what, I expect, will prove really good remarks. (636/3. Lyell's "Antiquity of Man" was published in the spring of 1863. In the "Life and Letters," Volume III., pages 8, 11, Darwin's correspondence shows his deep disappointment at what he thought Lyell's half-heartedness in regard to evolution. See Letter 164, Volume I.) Pray do not think me intrusive; but if you would like to have any book I have published, such as my "Journal of Researches" or the "Origin," I should esteem it a compliment to be allowed to send it. Will you permit me to suggest one experiment, which I should much like to see tried, and which I now wish the more from an extraordinary observation by Asa Gray on Gymnadenia tridentata (in number just out of Silliman's N. American Journal) (636/4. In Gymnadenia tridentata, according to Asa Gray, the anther opens in the bud, and the pollen being somewhat coherent falls on the stigma and on the rostellum which latter is penetrated by the pollen-tubes. "Fertilisation of Orchids," Edition II., page 68. Asa Gray's papers are in "American Journal of Science," Volume XXXIV., 1862, and XXXVI., 1863.); namely, to split the labellum of a Cattleya, or of some allied orchis, remove caudicle from pollen-mass (so that no loose grains are about) and put it carefully into the large tongue-like rostellum, and see if pollen-tubes will penetrate, or better, see if capsule will swell. Similar pollen-masses ought to be put on true stigmas of two or three other flowers of same plants for comparison. It is to discover whether rostellum yet retains some of its primordial function of being penetrated by pollen-tubes. You will be sorry that you ever entered into correspondence with me. But do not answer till at leisure, and as briefly as you like. My handwriting, I know, is dreadfully bad. Excuse this scribbling paper, as I can write faster on it, and I have a rather large correspondence to keep up.

LETTER 637. TO J. SCOTT. Down, January 21st, 1863.

I thank you for your very interesting letter; I must answer as briefly as I can, for I have a heap of other letters to answer. I strongly advise you to follow up and publish your observations on the pollen-tubes of orchids; they promise to be very interesting. If you could prove what I only conjectured (from state of utriculi in rostellum and in stigma of Catasetum and Acropera) that the utriculi somehow induce, or are correlated with, penetration of pollen-tubes you will make an important physiological discovery. I will mention, as worth your attention (and what I have anxiously wished to observe, if time had permitted, and still hope to do) — viz., the state of tissues or cells of stigma in an utterly sterile hybrid, in comparison with the same in fertile parent species; to test these cells, immerse stigmas for 48 hours in spirits of wine. I should expect in hybrids that the cells would not show coagulated contents. It would be an interesting discovery to show difference in female organs of hybrids and pure species. Anyhow, it is worth trial, and I recommend you to make it, and publish if you do. The pollen-tubes directing themselves to stigma is also very curious, though not quite so new, but well worth investigation when you get Cattleya, etc., in flower. I say not so new, for remember small flowers of Viola and Oxalis; or better, see Bibliography in "Natural History Review," No. VIII., page 419 (October, 1862) for quotation from M. Baillon on pollen-tubes finding way from anthers to stigma in Helianthemum. I should doubt gum getting solid from {i.e. because of} continued secretion. Why not sprinkle fresh plaster of Paris and make impenetrable crust? (637/1. The suggestion that the stigma should be covered with a crust of plaster of Paris, pierced by a hole to allow the pollen-tubes to enter, bears a resemblance to Miyoshi's experiments with germinating pollen and fungal spores. See "Pringsheim's Jahrbucher," 1895; "Flora," 1894.) You might modify experiment by making little hole in one lower corner, and see if tubes find it out. See in my future paper on Linum pollen and stigma recognising each other. If you will tell me that pollen smells the stigma I will try and believe you; but I will not believe the Frenchman (I forget who) who says that stigma of Vanilla actually attracts mechanically, by some unknown force, the solid pollen-masses to it! Read Asa Gray in 2nd Review of my Orchis book on pollen of Gymnadenia penetrating rostellum. I can, if you like, lend you these Reviews; but they must be returned. R. Brown, I remember, says pollen-tubes separate from grains before the lower ends of tubes reach ovules. I saw, and was interested by, abstract of your Drosera paper (637/2. A short note on the irritability of Drosera in the "Trans. Bot. Soc. Edin." Volume VII.); we have been at very much the same work.

LETTER 638. TO J. SCOTT. Down, February 16th {1863}.

Absence from home has prevented me from answering you sooner. I should think that the capsule of Acropera had better be left till it shows some signs of opening, as our object is to judge whether the seeds are good; but I should prefer trusting to your better judgment. I am interested about the Gongora, which I hope hereafter to try myself, as I have just built a small hot-house.

Asa Gray's observations on the rostellum of Gymnadenia are very imperfect, yet worth looking at. Your case of Imatophyllum is most interesting (638/1. A sucker of Imatophyllum minatum threw up a shoot in which the leaves were "two-ranked instead of four-ranked," and showed other differences from the normal. — "Animals and Plants," Edition II., Volume I., page 411.); even if the sport does not flower it will be worth my giving. I did not understand, or I had forgotten, that a single frond on a fern will vary; I now see that the case does come under bud-variation, and must be given by me. I had thought of it only as proof {of} inheritance in cryptogams; I am much obliged for your correction, and will consult again your paper and Mr. Bridgeman's. (638/2. The facts are given in "Animals and Plants," Edition II., Volume I., page 408.) I enclose varieties of maize from Asa Gray. Pray do not thank me for trusting you; the thanks ought to go the other way. I felt a conviction after your first letter that you were a real lover of Natural History.

If you can advance good evidence showing that bisexual plants are more variable than unisexual, it will be interesting. I shall be very glad to read the discussion which you are preparing. I admit as fully as any one can do that cross-impregnation is the great check to endless variability; but I am not sure that I understand your view. I do not believe that the structure of Primula has any necessary relation to a tendency to a dioecious structure, but seeing the difference in the fertility of the two forms, I felt bound unwillingly to admit that they might be a step towards dioeciousness; I allude to this subject in my Linum paper. (638/3. "Linn. Soc. Journal," 1863.) Thanks for your answers to my other queries. I forgot to say that I was at Kew the other day, and I find that they can give me capsules of several Vandeae.

LETTER 639. TO J. SCOTT. Down, March 24th {1863}.

Your letter, as every one you have written, has greatly interested me. If you can show that certain individual Passifloras, under certain known or unknown conditions of life, have stigmas capable of fertilisation by pollen from another species, or from another individual of its own species, yet not by its own individual pollen (its own individual pollen being proved to be good by its action on some other species), you will add a case of great interest to me; and which in my opinion would be quite worth your publication. (639/1. Cases nearly similar to those observed by Scott were recorded by Gartner and Kolreuter, but in these instances only certain individuals were self-impotent. In "Animals and Plants," Edition II., Volume II., page 114, where the phenomenon is fully discussed, Scott's observations ("Trans. Bot. Soc. Edin." 1863) are given as the earliest, except for one case recorded by Lecoq ("Fecondation," 1862). Interesting work was afterwards done by Hildebrand and Fritz Muller, as illustrated in many of the letters addressed to the latter.) I always imagined that such recorded cases must be due to unnatural conditions of life; and think I said so in the "Origin." (639/2. See "Origin of Species," Edition I., page 251, for Herbert's observations on self-impotence in Hippeastrum. In spite of the uniformness of the results obtained in many successive years, Darwin inferred that the plants must have been in an "unnatural state.") I am not sure that I understand your result, {nor} whether it means what I have above obscurely expressed. If you can prove the above, do publish; but if you will not publish I earnestly beg you to let me have the facts in detail; but you ought to publish, for I may not use the facts for years. I have been much interested by what you say on the rostellum exciting pollen to protrude tubes; but are you sure that the rostellum does excite them? Would not tubes protrude if placed on parts of column or base of petals, etc., near to the stigma? Please look at the "Cottage Gardener" (or "Journal of Horticulture") (639/3. "Journal of Horticulture" and "Cottage Gardener," March 31st, 1863. A short note describing Cruger's discovery of self-fertilisation in Cattleya, Epidendrum, etc., and referring to the work of "an excellent observer, Mr. J. Scott." Darwin adds that he is convinced that he has underrated the power of tropical orchids occasionally to produce seeds without the aid of insects.) to be published to-morrow week for letter of mine, in which I venture to quote you, and in which you will see a curious fact about unopened orchid flowers setting seed in West Indies. Dr. Cruger attributes protrusion of tubes to ants carrying stigmatic secretion to pollen (639/4. In Cruger's paper ("Linn. Soc. Journ." VIII., 1865; read March 3rd 1864) he speaks of the pollen-masses in situ being acted on by the stigmatic secretion, but no mention is made of the agency of ants. He describes the pollen-tubes descending "from the {pollen} masses still in situ down into the ovarian canal."); but this is mere hypothesis. Remember, pollen-tubes protrude within anther in Neottia nidus-avis. I did think it possible or probable that perfect fertilisation might have been effected through rostellum. What a curious case your Gongora must be: could you spare me one of the largest capsules? I want to estimate the number of seed, and try my hand if I can make them grow. This, however, is a foolish attempt, for Dr. Hooker, who was here a day or two ago, says they cannot at Calcutta, and yet imported species have seeded and have naturally spread on to the adjoining trees! Dr. Cruger thinks I am wrong about Catasetum: but I cannot understand his letter. He admits there are three forms in two species; and he speaks as if the sexes were separate in some and that others were hermaphrodites (639/5. Cruger ("Linn. Soc. Journal," VIII., page 127) says that the apparently hermaphrodite form is always sterile in Trinidad. Darwin modified his account in the second edition of the orchid book.); but I cannot understand what he means. He has seen lots of great humble-bees buzzing about the flowers with the pollinia sticking to their backs! Happy man!! I have the promise, but not yet surety, of some curious results with my homomorphic seedling cowslips: these have not followed the rule of Chinese Primula; homomorphic seedlings from short-styled parent have presented both forms, which disgusts me.

You will see that I am better; but still I greatly fear that I must have a compulsory holiday. With sincere thanks and hearty admiration at your powers of observation...

My poor P. scotica looks very sick which you so kindly sent me. (639/6. Sent by Scott, January 6th, 1863.)

LETTER 640. TO J. SCOTT. April 12th {1863}.

I really hardly know how to thank you enough for your very interesting letter. I shall certainly use all the facts which you have given me (in a condensed form) on the sterility of orchids in the work which I am now slowly preparing for publication. But why do you not publish these facts in a separate little paper? (640/1. See Letter 642, note, for reference to Scott's paper.) They seem to me well worth it, and you really ought to get your name known. I could equally well use them in my book. I earnestly hope that you will experiment on Passiflora, and let me give your results. Dr. A. Gray's observations were made loosely; he said in a letter he would attend this summer further to the case, which clearly surprised him much. I will say nothing about the rostellum, stigmatic utriculi, fertility of Acropera and Catasetum, for I am completely bewildered: it will rest with you to settle these points by your excellent observations and experiments. I must own I never could help doubting Dr. Hooker's case of the poppy. You may like to hear what I have seen this morning: I found (640/2. See Letter 658.) a primrose plant with flowers having three pistils, which when pulled asunder, without any tearing, allowed pollen to be placed on ovules. This I did with three flowers — pollen-tubes did not protrude after several days. But this day, the sixteenth (N.B. — primulas seem naturally slowly fertilised), I found many tubes protruded, and, what is very odd, they certainly seemed to have penetrated the coats of the ovules, but in no one instance the foramen of the ovule!! I mention this because it directly bears on your explanation of Dr. Cruger's case. (640/3. Cruger's case here referred to is doubtless the cleistogamic fertilisation of Epidendrum, etc. Scott discusses the question of self-fertilisation at great length in a letter to Darwin dated April, and obviously written in 1863. In Epidendrum he observed a viscid matter extending from the stigmatic chamber to the anther: pollen-tubes had protruded from the anther not only where it was in contact with the viscid matter, but also from the central part, and these spread "over the anterior surface of the rostellum downward into the stigma." Cruger believed the viscid matter reaching the anther was a necessary condition for the germination of the pollen-grains. Scott points out that the viscid matter is produced in large quantity only after the pollen-grains have penetrated the stigma, and that it is, in fact, a consequence, not a preliminary to fertilisation. He finally explains Cruger's case thus: "The greater humidity and equability of temperature consequent on such conditions {i.e. on the flowers being closed} is, I believe, the probable cause of these abnormally conditioned flowers so frequently fertilising themselves." Scott also calls attention to the danger of being deceived by fungal hyphae in observations on germination of pollen.) I believe that your explanation is right; I should never have thought of it; yet this was stupid of me, for I remember thinking that the almost closed imperfect flowers of Viola and Oxalis were related to the protrusion of the pollen-tubes. My case of the Aceras with the aborted labellum squeezed against stigma supports your view. (640/4. See "Fertilisation of Orchids," Edition II., page 258: the pollen germinated within the anther of a monstrous flower.) Dr. Cruger's notion about the ants was a simple conjecture. About cryptogamic filaments, remember Dr. C. says that the unopened flowers habitually set fruit. I think that you will change your views on the imperfect flowers of Viola and Oxalis...

LETTER 641. (?)

LETTER 642. TO J. SCOTT. May 2nd {1863}.

I have left home for a fortnight to see if I can, with little hope, improve my health. The parcel of orchid pods, which you have so kindly sent me, has followed me. I am sure you will forgive the liberty which I take in returning you the postage stamps. I never heard of such a scheme as that you were compelled to practise to fertilise the Gongora! (642/1. See "Fertilisation of Orchids," Edition, II., page 169. "Mr. Scott tried repeatedly, but in vain, to force the pollen-masses into the stigma of Gongora atro-purpurea and truncata; but he readily fertilised them by cutting off the clinandrum and placing pollen-masses on the now exposed stigma.") It is a most curious problem what plan Nature follows in this genus and Acropera. (642/2. In the "Fertilisation of Orchids," Edition II., page 169, Darwin speculates as to the possible fertilisation of Acropera by an insect with pollen-masses adhering to the extremity of its abdomen. It would appear that this guess (which does not occur in the first edition) was made before he heard of Cruger's observation on the allied genus Gongora, which is visited by a bee with a long tongue, which projects, when not in use, beyond and above the tip of the abdomen. Cruger believes that this tongue is the pollinating agent. Cruger's account is in the "Journal of the Linn. Soc." VIII., 1865, page 130.) Some day I will try and estimate how many seeds there are in Gongora. I suppose and hope you have kept notes on all your observations on orchids, for, with my broken health and many other subjects, I do not know whether I shall ever have time to publish again; though I have a large collection of notes and facts ready. I think you show your wisdom in not wishing to publish too soon; a young author who publishes every trifle gets, sometimes unjustly, to be disregarded. I do not pretend to be much of a judge; but I can conscientiously say that I have never written one word to you on the merit of your letters that I do not fully believe in. Please remember that I should very much wish for a copy of your paper on sterility of individual orchids (642/3. "On the Individual Sterility and Cross-Impregnation of Certain Species of Oncidium." {Read June 2nd, 1864.} "Linn. Soc. Journal," VIII., 1865. This paper gives a full account of the self-sterility of Oncidium in cases where the pollen was efficient in fertilising other individuals of the same species and of distinct species. Some of the facts were given in Scott's paper, "Experiments on the Fertilisation of Orchids in the Royal Botanic Garden of Edinburgh," published in the "Proc. Bot. Soc. Edinb." 1863. It is probably to the latter paper that Darwin refers.) and on Drosera. (642/4. "Trans. Bot. Soc. Edinburgh," Volume VII.) Thanks for {note} about Campanula perfoliata. I have asked Asa Gray for seeds, to whom I have mentioned your observations on rostellum, and asked him to look closer to the case of Gymnadenia. (642/5. See "Fertilisation of Orchids," Edition II., page 68.) Let me hear about the sporting Imatophyllum if it flowers. Perhaps I have blundered about Primula; but certainly not about mere protrusion of pollen-tubes. I have been idly watching bees of several genera and diptera fertilising O. morio at this place, and it is a very pretty sight. I have confirmed in several ways the entire truth of my statement that there is no vestige of nectar in the spur; but the insects perforate the inner coat. This seems to me a curious little fact, which none of my reviewers have noticed.

LETTER 643. TO J.D. HOOKER. Down, May 23rd {1863}.

You can confer a real service on a good man, John Scott, the writer of the enclosed letter, by reading it and giving me your opinion. I assure {you} John Scott is a truly remarkable man. The part struck out is merely that he is not comfortable under Mr. McNab, and this part must be considered as private. Now the question is, what think you of the offer? Is expense of living high at Darjeeling? May I say it is healthy? Will he find the opportunity for experimental observations, which are a passion with him? It seems to me rather low pay. Will you advise me for him? I shall say that as far as experiments in hand at the Botanical Garden in Edinburgh are concerned, it would be a pity to hesitate to accept the offer.

J. Scott is head of the propagating department. I know you will not grudge aiding by your advice a good man. I shall tell him that I have not the slightest power to aid him in any way for the appointment. I should think voyage out and home ought to be paid for?

LETTER 644. TO JOHN SCOTT. Down, May 25th, 1863.

Now for a few words on science. I do not think I could be mistaken about the stigma of Bolbophyllum (644/1. Bolbophyllum is remarkable for the closure of the stigmatic cavity which comes on after the flower has been open a little while, instead of after fertilisation, as in other genera. Darwin connects the fact with the "exposed condition of the whole flower." — "Fertilisation of Orchids," Edition II., page 137.); I had the plant alive from Kew, and watched many flowers. That is a most remarkable observation on foreign pollen emitting tubes, but not causing orifice to close (644/2. See Scott, "Bot. Soc. Edin." 1863, page 546, note. He applied pollinia from Cypripedium and Asclepias to flowers of Tricopilia tortilis; and though the pollen germinated, the stigmatic chamber remained open, yet it invariably closes eighteen hours after the application of its own pollen.); it would have been interesting to have observed how close an alliance of form would have acted on the orifice of the stigma. It will probably be so many years, if ever, {before} I work up my observations on Drosera, that I will not trouble you to send your paper, for I could not now find time to read it. If you have spare copy of your Orchid paper, please send it, but do not get a copy of the journal, for I can get one, and you must often want to buy books. Let me know when it is published. I have been glad to hear about Mercurialis, but I will not accept your offer of seed on account of time, time, time, and weak health. For the same reason I must give up Primula mollis. What a wonderful, indefatigable worker you are! You seem to have made a famous lot of interesting experiments. D. Beaton once wrote that no man could cross any species of Primula. You have apparently proved the contrary with a vengeance. Your numerous experiments seem very well selected, and you will exhaust the subject. Now when you have completed your work you should draw up a paper, well worth publishing, and give a list of all the dimorphic and non-dimorphic forms. I can give you, on the authority of Prof. Treviranus in "Bot. Zeitung," case of P. longiflora non-dimorphic. I am surprised at your cowslips in this state. Is it a common yellow cowslip? I have seen oxlips (which from some experiments I now look at as certainly natural hybrids) in same state. If you think the Botanical Society of Edinburgh would not do justice and publish your paper, send it to me to be communicated to the Linnean Society. I will delay my paper on successive dimorphic generations in Primula (644/3. Published in the "Journ. Linn. Soc." X., 1869 {1868}.) till yours appears, so as in no way to interfere with your paper. Possibly my results may be hardly worth publishing, but I think they will; the seedlings from two successive homomorphic generations seem excessively sterile. I will keep this letter till I hear from Dr. Hooker. I shall be very glad if you try Passiflora. Your experiments on Primula seem so well chosen that whatever the result is they will be of value. But always remember that not one naturalist out of a dozen cares for really philosophical experiments.

LETTER 645. TO J. SCOTT. Down, May 31st {1863}.

I am unwell, and must write briefly. I am very much obliged for the "Courant." (645/1. The Edinburgh "Evening Courant" used to publish notices of the papers read at the Botanical Society of Edinburgh. The paper referred to here was Scott's on Oncidium.) The facts will be of highest use to me. I feel convinced that your paper will have permanent value. Your case seems excellently and carefully worked out. I agree that the alteration of title was unfortunate, but, after all, title does not signify very much. So few have attended to such points that I do not expect any criticism; but if so, I should think you had much better reply, but I could if you wished it much. I quite understand about the cases being individual sterility; so Gartner states it was with him. Would it be worth while to send a corrected copy of the "Courant" to the "Gardeners' Chronicle?" (645/2. An account of Scott's work appeared in the "Gardeners' Chronicle," June 13th, 1863, which is, at least partly, a reprint of the "Courant," since it contains the awkward sentence criticised by Darwin and referred to below. The title is "On the Fertilisation of Orchids," which was no doubt considered unfortunate as not suggesting the subject of the paper, and as being the same as that of Darwin's book.) I did not know that you had tried Lobelia fulgens: can you give me any particulars on the number of plants and kinds used, etc., that I may quote, as in a few days I shall be writing on this whole subject? No one will ever convince me that it is not a very important subject to philosophical naturalists. The Hibiscus seems a very curious case, and I agree with your remarks. You say that you are glad of criticisms (by the way avoid "former and latter," the reader is always forced to go back to look). I think you would have made the case more striking if you had first showed that the pollen of Oncidium sphacelatum was good; secondly, that the ovule was capable of fertilisation; and lastly, shown that the plant was impotent with its own pollen. "Impotence of organs capable of elimination" — capable here strictly refers to organs; you mean to impotence. To eliminate impotence is a curious expression; it is removing a non-existent quality. But style is a trifle compared with facts, and you are capable of writing well. I find it a good rule to imagine that I want to explain the case in as few and simple words as possible to one who knows nothing of the subject. (645/3. See Letter 151, Volume I.) I am tired. In my opinion you are an excellent observer.

LETTER 646. TO J. SCOTT. Down, June 6th, 1863.

I fear that you think that I have done more than I have with respect to Dr. Hooker. I did not feel that I had any right to ask him to remember you for a colonial appointment: all that I have done is to speak most highly of your scientific merits. Of course this may hereafter fructify. I really think you cannot go on better, for educational purposes, than you are now doing, — observing, thinking, and some reading beat, in my opinion, all systematic education. Do not despair about your style; your letters are excellently written, your scientific style is a little too ambitious. I never study style; all that I do is to try to get the subject as clear as I can in my own head, and express it in the commonest language which occurs to me. But I generally have to think a good deal before the simplest arrangement and words occur to me. Even with most of our best English writers, writing is slow work; it is a great evil, but there is no help for it. I am sure you have no cause to despair. I hope and suppose your sending a paper to the Linnean Society will not offend your Edinburgh friends; you might truly say that you sent the paper to me, and that (if it turns out so) I thought it worth communicating to the Linnean Society. I shall feel great interest in studying all your facts on Primula, when they are worked out and the seed counted. Size of capsules is often very deceptive. I am astonished how you can find time to make so many experiments. If you like to send me your paper tolerably well written, I would look it over and suggest any criticisms; but then this would cause you extra copying. Remember, however, that Lord Brougham habitually wrote everything important three times over. The cases of the Primulae which lose by variation their dimorphic characters seem to me very interesting. I find that the mid-styled (by variation) P. sinensis is more fertile with own pollen, even, than a heteromorphic union! If you have time it will be very good to experiment on Linum Lewisii. I wrote formerly to Asa Gray begging for seed. If you have time, I think experiments on any peloric flowers would be useful. I shall be sorry (and I am certain it is a mistake on the part of the Society) if your orchid paper is not printed in extenso. I am now at work compiling all such cases, and shall give a very full abstract of all your observations. I hope to add in autumn some from you on Passiflora. I would suggest to you the advantage, at present, of being very sparing in introducing theory in your papers (I formerly erred much in Geology in that way): LET THEORY GUIDE YOUR OBSERVATIONS, but till your reputation is well established be sparing in publishing theory. It makes persons doubt your observations. How rarely R. Brown ever indulged in theory: too seldom perhaps! Do not work too hard, and do not be discouraged because your work is not appreciated by the majority.

LETTER 647. TO J. SCOTT. July 2nd {1863?}