More Letters of Charles Darwin — Volume 2

Do you remember calling my attention to certain flowers in the truss of Pelargoniums not being true, or not having the dark shade on the two upper petals? I believe it was Lady Lubbock's observation. I find, as I expected, it is always the central or sub-central flower; but what is far more curious, the nectary, which is blended with the peduncle of the flowers, gradually lessens and quite disappears (588/1. This fact is mentioned in Maxwell Masters' "Vegetable Teratology" (Ray Society's Publications), 1869, page 221.), as the dark shade on the two upper petals disappears. Compare the stalk in the two enclosed parcels, in each of which there is a perfect flower.

Now, if your gardener will not be outrageous, do look over your geraniums and send me a few trusses, if you can find any, having the flowers without the marks, sending me some perfect flowers on same truss. The case seems to me rather a pretty one of correlation of growth; for the calyx also becomes slightly modified in the flowers without marks.

LETTER 589. TO MAXWELL MASTERS. Down, April 7th {1860}.

I hope that you will excuse the liberty which I take in writing to you and begging a favour. I have been very much interested by the abstract (too brief) of your lecture at the Royal Institution. Many of the facts alluded to are full of interest for me. But on one point I should be infinitely obliged if you could procure me any information: namely, with respect to sweet-peas. I am a great believer in the natural crossing of individuals of the same species. But I have been assured by Mr. Cattell (589/1. The nurseryman he generally dealt with.), of Westerham, that the several varieties of sweet-pea can be raised close together for a number of years without intercrossing. But on the other hand he stated that they go over the beds, and pull up any false plant, which they very naturally attribute to wrong seeds getting mixed in the lot. After many failures, I succeeded in artificially crossing two varieties, and the offspring out of the same pod, instead of being intermediate, was very nearly like the two pure parents; yet in one, there was a trace of the cross, and these crossed peas in the next generation showed still more plainly their mongrel origin. Now, what I want to know is, whether there is much variation in sweet-peas which might be owing to natural crosses. What I should expect would be that they would keep true for many years, but that occasionally, perhaps at long intervals, there would be a considerable amount of crossing of the varieties grown close together. Can you give, or obtain from your father, any information on this head, and allow me to quote your authority? It would really be a very great favour and kindness.

LETTER 590. TO J.D. HOOKER.

(590/1. The genera Scaevola and Leschenaultia, to which the following letter refers, belong to the Goodeniaceae (Goodenovieae, Bentham & Hooker), an order allied to the Lobeliaceae, although the mechanism of fertilisation resembles rather more nearly that of Campanula. The characteristic feature of the flower in this order is the indusium, or, as Delpino (590/2. Delpino's observations on Dichogamy, summarised by Hildebrand in "Bot. Zeitung," 1870, page 634.) calls it, the "collecting cup": this cuplike organ is a development of the style, and serves the same function as the hairs on the style of Campanula, namely, that of taking the pollen from the anthers and presenting it to the visiting insect. During this stage the immature stigma is at the bottom of the cup, and though surrounded by pollen is incapable of being pollinated. In most genera of the order the pollen is pushed out of the indusium by the growth of the style or stigma, very much as occurs in Lobelia or the Compositae. Finally the style emerges from the indusium (590/3. According to Hamilton ("Proc. Linn. Soc. N. S. Wales," X., 1895, page 361) the stigma rarely grows beyond the indusium in Dampiera. In the same journal (1885-6, page 157, and IX., 1894, page 201) Hamilton has given a number of interesting observations on Goodenia, Scaevola, Selliera, Brunonia. There seem to be mechanisms for cross- and also for self-fertilisation.), the stigmas open out and are pollinated from younger flowers. The mechanism of fertilisation has been described by F. Muller (590/4. In a letter to Hildebrand published in the "Bot. Zeitung," 1868, page 113.), and more completely by Delpino (loc. cit.).

Mr. Bentham wrote a paper (590/5. "Linn. Soc. Journal," 1869, page 203.) on the style and stigma in the Goodenovieae, where he speaks of Mr. Darwin's belief that fertilisation takes place outside the indusium. This statement, which we imagine Mr. Bentham must have had from an unpublished source, was incomprehensible to him as long as he confined his work to such genera as Goodenia, Scaevola, Velleia, Coelogyne, in which the mechanism is much as above described; but on examining Leschenaultia the meaning became clear. Bentham writes of this genus: — "The indusium is usually described as broadly two-lipped, without any distinct stigma. The fact appears to be that the upper less prominent lip is stigmatic all over, inside and out, with a transverse band of short glandular hairs at its base outside, while the lower more prominent lip is smooth and glabrous, or with a tuft of rigid hairs. Perhaps this lower lip and the upper band of hairs are all that correspond to the indusium of other genera; and the so-called upper lip, outside of which impregnation may well take place, as observed by Mr. Darwin, must be regarded as the true stigma."

Darwin's interest in the Goodeniaceae was due to the mechanism being apparently fitted for self-fertilisation. In 1871 a writer signing himself F.W.B. made a communication to the "Gardeners' Chronicle" (590/6. 1871, page 1103.), in which he expresses himself as "agreeably surprised" to find Leschenaultia adapted for self-fertilisation, or at least for self-pollinisation. This led Darwin to publish a short note in the same journal, in which he describes the penetration of pollen-tubes into the viscid surface on the outside of the indusium. (590/7. 1871, page 1166. He had previously written in the "Journal of Horticulture and Cottage Gardener," May 28th, 1861, page 151: — "Leschenaultia formosa has apparently the most effective contrivance to prevent the stigma of one flower ever receiving a grain of pollen from another flower; for the pollen is shed in the early bud, and is there shut up round the stigma within a cup or indusium. But some observations led me to suspect that nevertheless insect agency here comes into play; for I found by holding a camel-hair pencil parallel to the pistil, and moving it as if it were a bee going to suck the nectar, the straggling hairs of the brush opened the lip of the indusium, entered it, stirred up the pollen, and brought out some grains. I did this to five flowers, and marked them. These five flowers all set pods; whereas only two other pods set on the whole plant, though covered with innumerable flowers...I wrote to Mr. James Drummond, at Swan River in Australia...and he soon wrote to me that he had seen a bee cleverly opening the indusium and extracting pollen.") He also describes how a brush, pushed into the flower in imitation of an insect, presses "against the slightly projecting lower lip of the indusium, opens it, and some of the hairs enter and become smeared with pollen." The yield of pollen is therefore differently arranged in Leschenaultia; for in the more typical genera it depends on the growth of the style inside the indusium. Delpino, however (see Hildebrand's version, loc. cit.), describes a similar opening of the cup produced by pressure on the hairs in some genera of the order.)

Down, June 7th {1860}.

Best and most beloved of men, I supplicate and entreat you to observe one point for me. Remember that the Goodeniaceae have weighed like an incubus for years on my soul. It relates to Scaevola microcarpa. I find that in bud the indusium collects all the pollen splendidly, but, differently from Leschenaultia, cannot be afterwards easily opened. Further, I find that at an early stage, when the flower first opens, a boat-shaped stigma lies at the bottom of the indusium, and further that this stigma, after the flower has some time expanded, grows very rapidly, when the plant is kept hot, and pushes out of the indusium a mass of pollen; and at same time two horns project at the corners of the indusium. Now the appearance of these horns makes me suppose that these are the stigmatic surfaces. Will you look to this? for if they be by the relative position of the parts (with indusium and stigma bent at right angles to style) {I am led to think} that an insect entering a flower could not fail to have {its} whole back (at the period when, as I have seen, a whole mass of pollen is pushed out) covered with pollen, which would almost certainly get rubbed on the two horns. Indeed, I doubt whether, without this aid, pollen would get on to the horns. What interests me in the case is the analogy in result with the Lobelia, but by very different means. In Lobelia the stigma, before it is mature, pushes by its circular brush of hairs the pollen out of the conjoined anthers; here the indusium collects pollen, and then the growth of the stigma pushes it out. In the course of about 1 1/2 hour, I found an indusium with hairs on the outer edge perfectly clogged with pollen, and horns protruded, which before the 1 1/2 hour had not one grain of pollen outside the indusium, and no trace of protruding horns. So you will see how I wish to know whether the horns are the true stigmatic surfaces. I would try the case experimentally by putting pollen on the horns, but my greenhouse is so cold, and my plant so small, and in such a little pot, that I suppose it would not seed...

The little length of stigmatic horns at the moment when pollen is forced out of the indusium, compared to what they ultimately attain, makes me fancy that they are not then mature or ready, and if so, as in Lobelia, each flower must be fertilised by pollen from another and earlier flower.

How curious that the indusium should first so cleverly collect pollen and then afterwards push it out! Yet how closely analogous to Campanula brushing pollen out of the anther and retaining it on hairs till the stigma is ready. I am going to try whether Campanula sets seed without insect agency.

LETTER 591. TO J.D. HOOKER.

(591/1. The following letters are given here rather than in chronological order, as bearing on the Leschenaultia problem. The latter part of Letter 591 refers to the cleistogamic flowers of Viola.)

Down, May 1st {1862}.

If you can screw out time, do look at the stigma of the blue Leschenaultia biloba. I have just examined a large bud with the indusium not yet closed, and it seems to me certain that there is no stigma within. The case would be very important for me, and I do not like to trust solely to myself. I have been impregnating flowers, but it is rather difficult...

I have just looked again at Viola canina. The case is odder: only 2 stamens which embrace the stigma have pollen; the 3 other stamens have no anther-cells and no pollen. These 2 fertile anthers are of different shape from the 3 sterile others, and the scale representing the lower lip is larger and differently shaped from the 4 other scales representing 4 other petals.

In V. odorata (single flower) all five stamens produce pollen. But I daresay all this is known.

LETTER 592. TO J.D. HOOKER. November 3rd {1862}.

Do you remember the scarlet Leschenaultia formosa with the sticky margin outside the indusium? Well, this is the stigma — at least, I find the pollen-tubes here penetrate and nowhere else. What a joke it would be if the stigma is always exterior, and this by far the greatest difficulty in my crossing notions should turn out a case eminently requiring insect aid, and consequently almost inevitably ensuring crossing. By the way, have you any other Goodeniaceae which you could lend me, besides Leschenaultia and Scaevola, of which I have seen enough?

I had a long letter the other day from Crocker of Chichester; he has the real spirit of an experimentalist, but has not done much this summer.

LETTER 593. TO F. MULLER. Down, April 9th and 15th {1866}.

I am very much obliged by your letter of February 13th, abounding with so many highly interesting facts. Your account of the Rubiaceous plant is one of the most extraordinary that I have ever read, and I am glad you are going to publish it. I have long wished some one to observe the fertilisation of Scaevola, and you must permit me to tell you what I have observed. First, for the allied genus of Leschenaultia: utterly disbelieving that it fertilises itself, I introduced a camel-hair brush into the flower in the same way as a bee would enter, and I found that the flowers were thus fertilised, which never otherwise happens; I then searched for the stigma, and found it outside the indusium with the pollen-tubes penetrating it; and I convinced Dr. Hooker that botanists were quite wrong in supposing that the stigma lay inside the indusium. In Scaevola microcarpa the structure is very different, for the immature stigma lies at the base within the indusium, and as the stigma grows it pushes the pollen out of the indusium, and it then clings to the hairs which fringe the tips of the indusium; and when an insect enters the flower, the pollen (as I have seen) is swept from these long hairs on to the insect's back. The stigma continues to grow, but is not apparently ready for impregnation until it is developed into two long protruding horns, at which period all the pollen has been pushed out of the indusium. But my observations are here at fault, for I did not observe the penetration of the pollen-tubes. The case is almost parallel with that of Lobelia. Now, I hope you will get two plants of Scaevola, and protect one from insects, leaving the other uncovered, and observe the results, both in the number of capsules produced, and in the average number of seeds in each. It would be well to fertilise half a dozen flowers under the net, to prove that the cover is not injurious to fertility.

With respect to your case of Aristolochia, I think further observation would convince you that it is not fertilised only by larvae, for in a nearly parallel case of an Arum and a Aristolochia, I found that insects flew from flower to flower. I would suggest to you to observe any cases of flowers which catch insects by their probosces, as occurs with some of the Apocyneae (593/1. Probably Asclepiadeae. See H. Muller, "Fertilisation of Flowers," page 396.); I have never been able to conceive for what purpose (if any) this is effected; at the same time, if I tempt you to neglect your zoological work for these miscellaneous observations I shall be guilty of a great crime.

To return for a moment to the indusium: how curious it is that the pollen should be thus collected in a special receptacle, afterwards to be swept out by insects' agency!

I am surprised at what you tell me about the fewness of the flowers of your native orchids which produce seed-capsules. What a contrast with our temperate European species, with the exception of some species of Ophrys! — I now know of three or four cases of self-fertilising orchids, but all these are provided with means for an occasional cross.

I am sorry to say Dr. Cruger is dead from a fever.

I received yesterday your paper in the "Botanische Zeitung" on the wood of climbing plants. (593/2. Fritz Muller, "Ueber das Holz einiger um Desterro wachsenden Kletterpflanzen." "Botanische Zeitung," 1866, pages 57, 65.) I have read as yet only your very interesting and curious remarks on the subject as bearing on the change of species; you have pleased me by the very high compliments which you pay to my paper. I have been at work since March 1st on a new English edition (593/3. The 4th Edition.) of my "Origin," of which when published I will send you a copy. I have much regretted the time it has cost me, as it has stopped my other work. On the other hand, it will be useful for a new third German edition, which is now wanted. I have corrected it largely, and added some discussions, but not nearly so much as I wished to do, for, being able to work only two hours daily, I feared I should never get it finished. I have taken some facts and views from your work "Fur Darwin"; but not one quarter of what I should like to have quoted.

LETTER 594. TO A.G. MORE. Down, June 24th, 1860.

I hope that you will forgive the liberty which I take in writing to you and requesting a favour. Mr. H.C. Watson has given me your address, and has told me that he thought that you would be willing to oblige me. Will you please to read the enclosed, and then you will understand what I wish observed with respect to the bee-orchis. (594/1. Ophrys apifera.) What I especially wish, from information which I have received since publishing the enclosed, is that the state of the pollen-masses should be noted in flowers just beginning to wither, in a district where the bee-orchis is extremely common. I have been assured that in parts of Isle of Wight, viz., Freshwater Gate, numbers occur almost crowded together: whether anything of this kind occurs in your vicinity I know not; but, if in your power, I should be infinitely obliged for any information. As I am writing, I will venture to mention another wish which I have: namely, to examine fresh flowers and buds of the Aceras, Spiranthes, marsh Epipactis, and any other rare orchis. The point which I wish to examine is really very curious, but it would take too long space to explain. Could you oblige me by taking the great trouble to send me in an old tin canister any of these orchids, permitting me, of course, to repay postage? It would be a great kindness, but perhaps I am unreasonable to make such a request. If you will inform me whether you have leisure so far to oblige me, I would tell you my movements, for on account of my own health and that of my daughter, I shall be on the move for the next two or three weeks.

I am sure I have much cause to apologise for the liberty which I have taken...

LETTER 595. TO A.G. MORE. Down, August 3rd, 1860.

I thank you most sincerely for sending me the Epipactis {palustris}. You can hardly imagine what an interesting morning's work you have given me, as the rostellum exhibited a quite new modification of structure. It has been extremely kind of you to take so very much trouble for me. Have you looked at the pollen-masses of the bee-Ophrys? I do not know whether the Epipactis grows near to your house: if it does, and any object takes you to the place (pray do not for a moment think me so very unreasonable as to ask you to go on purpose), would you be so kind {as} to watch the flowers for a quarter of an hour, and mark whether any insects (and what?) visit these flowers.

I should suppose they would crawl in by depressing the terminal portion of the labellum; and that when within the flower this terminal portion would resume its former position; and lastly, that the insect in crawling out would not depress the labellum, but would crawl out at back of flower. (595/1. The observations of Mr. William Darwin on Epipactis palustris given in the "Fertilisation of Orchids," Edition II., 1877, page 99, bear on this point. The chief fertilisers are hive-bees, which are too big to crawl into the flower. They cling to the labellum, and by depressing it open up the entrance to the flower. Owing to the elasticity of the labellum and its consequent tendency to spring up when released, the bees, "as they left the flower, seemed to fly rather upwards." This agrees with Darwin's conception of the mechanism of the flower as given in the first edition of the Orchid book, 1862, page 100, although at that time he imagined that the fertilising insect crawled into the flower. The extreme flexibility and elasticity of the labellum was first observed by Mr. More (see first edition, page 99). The description of the flower given in the above letter to Mr. More is not quite clear; the reader is referred to the "Fertilisation of Orchids," loc. cit.) An insect crawling out of a recently opened flower would, I believe, have parts of the pollen-masses adhering to the back or shoulder. I have seen this in Listera. How I should like to watch the Epipactis.

If you can it any time send me Spiranthes or Aceras or O. ustulata, you would complete your work of kindness.

P.S. — If you should visit the Epipactis again, would you gather a few of the lower flowers which have been opened for some time and have begun to wither a little, and observe whether pollen is well cleared out of anther-case. I have been struck with surprise that in nearly all the lower flowers sent by you, though much of the pollen has been removed, yet a good deal of pollen is left wasted within the anthers. I observed something of this kind in Cephalanthera grandiflora. But I fear that you will think me an intolerable bore.

LETTER 596. TO A.G. MORE. Down, August 5th, 1860.

I am infinitely obliged for your most clearly stated observations on the bee-orchis. It is now perfectly clear that something removes the pollen-masses far more with you than in this neighbourhood. But I am utterly puzzled about the foot-stalk being so often cut through. I should suspect snails. I yesterday found thirty-nine flowers, and of them only one pollen-mass in three flowers had been removed, and as these were extremely much-withered flowers I am not quite sure of the truth of this. The wind again is a new element of doubt. Your observations will aid me extremely in coming to some conclusion. (596/1. Mr. More's observations on the percentage of flowers in which the pollinia were absent are quoted in "Fertilisation of Orchids," Edition I., page 68.) I hope in a day or two to receive some day-moths, on the probosces of which I am assured the pollen-masses of the bee-orchis still adhere (596/2. He was doomed to disappointment. On July 17th, 1861, he wrote to Mr. More: — "I found the other day a lot of bee-Ophrys with the glands of the pollinia all in their pouches. All facts point clearly to eternal self-fertilisation in this species; yet I cannot swallow the bitter pill. Have you looked at any this year?")...

I wrote yesterday to thank you for the Epipactis. For the chance of your liking to look at what I have found: take a recently opened flower, drag gently up the stigmatic surface almost any object (the side of a hooked needle), and you will find the cap of the hemispherical rostellum comes off with a touch, and being viscid on under-surface, clings to needle, and as pollen-masses are already attached to the back of rostellum, the needle drags out much pollen. But to do this, the curiously projecting and fleshy summits of anther-cases must at some time be pushed back slightly. Now when an insect's head gets into the flower, when the flap of the labellum has closed by its elasticity, the insect would naturally creep out by the back-side of the flower. And mark when the insect flies to another flower with the pollen-masses adhering to it, if the flap of labellum did not easily open and allow free ingress to the insect, it would surely rub off the pollen on the upper petals, and so not leave it on stigma. It is to know whether I have rightly interpreted the structure of this whole flower that I am so curious to see how insects act. Small insects, I daresay, would crawl in and out and do nothing. I hope that I shall not have wearied you with these details.

If you would like to see a pretty and curious little sight, look to Orchis pyramidalis, and you will see that the sticky glands are congenitally united into a saddle-shaped organ. Remove this under microscope by pincers applied to foot-stalk of pollen-mass, and look quickly at the spontaneous movement of the saddle-shaped organs and see how beautifully adapted to seize proboscis of moth.

LETTER 597. TO J.D. HOOKER December 4th {1860}.

Many thanks about Apocynum and Meyen.

The latter I want about some strange movements in cells of Drosera, which Meyen alone seems to have observed. (597/1. No observations of Meyen are mentioned in "Insectivorous Plants.") It is very curious, but Trecul disbelieves that Drosera really clasps flies! I should very much wish to talk over Drosera with you. I did chloroform it, and the leaves which were already expanded did not recover thirty seconds of exposure for three days. I used the expression weight for the bit of hair which caused movement and weighed 1/78000 of a grain; but I do not believe it is weight, and what it is, I cannot after many experiments conjecture. (597/2. The doubt here expressed as to whether the result is due to actual weight is interesting in connection with Pfeffer's remarkable discovery that a smooth object in contact with the gland produces no effect if the plant is protected from all vibration; on an ordinary table the slight shaking which reaches the plant is sufficient to make the body resting on the gland tremble, and thus produce a series of varying pressures — under these circumstances the gland is irritated, and the tentacle moves. See Pfeffer, "Untersuchungen aus d. bot. Institut zu Tubingen," Volume I., 1885, page 483; also "Insectivorous Plants," Edition II., page 22.) The movement in this case does not depend on the chemical nature of substance. Latterly I have tried experiments on single glands, and a microscopical atom of raw meat causes such rapid movement that I could see it move like hand of clock. In this case it is the nature of the object. It is wonderful the rapidity of the absorption: in ten seconds weak solution of carbonate of ammonia changes not the colour, but the state of contents within the glands. In two minutes thirty seconds juice of meat has been absorbed by gland and passed from cell to cell all down the pedicel (or hair) of the gland, and caused the sap to pass from the cells on the upper side of the pedicel to the lower side, and this causes the curvature of the pedicel. I shall work away next summer when Drosera opens again, for I am much interested in subject. After the glandular hairs have curved, the oddest changes take place — viz., a segregation of the homogeneous pink fluid and necessary slow movements in the thicker matter. By Jove, I sometimes think Drosera is a disguised animal! You know that I always so like telling you what I do, that you must forgive me scribbling on my beloved Drosera. Farewell. I am so very glad that you are going to reform your ways; I am sure that you would have injured your health seriously. There is poor Dana has done actually nothing — cannot even write a letter — for a year, and it is hoped that in another YEAR he may quite recover.

After this homily, good night, my dear friend. Good heavens, I ought not to scold you, but thank you, for writing so long and interesting a letter.

LETTER 598. TO E. CRESY. Down, December 12th {1860?}.

After writing out the greater part of my paper on Drosera, I thought of so many points to try, and I wished to re-test the basis of one large set of experiments, namely, to feel still more sure than I am, that a drop of plain water never produces any effect, that I have resolved to publish nothing this year. For I found in the record of my daily experiments one suspicious case. I must wait till next summer. It will be difficult to try any solid substances containing nitrogen, such as ivory; for two quite distinct causes excite the movement, namely, mechanical irritation and presence of nitrogen. When a solid substance is placed on leaf it becomes clasped, but is released sooner than when a nitrogenous solid is clasped; yet it is difficult (except with raw meat and flies) to be sure of the result, owing to differences in vigour of different plants. The last experiments which I tried before my plants became too languid are very curious, and were tried by putting microscopical atoms on the gland itself of single hairs; and it is perfectly evident that an atom of human hair, 1/76000 of a grain (as ascertained by weighing a length of hair) in weight, causes conspicuous movement. I do not believe (for atoms of cotton thread acted) it is the chemical nature; and some reasons make me doubt whether it is actual weight; it is not the shadow; and I am at present, after many experiments, confounded to know what the cause is. That these atoms did really act and alter the state of the contents of all the cells in the glandular hair, which moved, was perfectly clear. But I hope next summer to make out a good deal more...

LETTER 599. TO J.D. HOOKER. Down, May 14th {1861}.

I have been putting off writing from day to day, as I did not wish to trouble you, till my wish for a little news will not let me rest...

I have no news to tell you, for I have had no interesting letters for some time, and have not seen a soul. I have been going through the "Cottage Gardener" of last year, on account chiefly of Beaton's articles (599/1. Beaton was a regular contributor to the "Cottage Gardener," and wrote various articles on cross breeding, etc., in 1861. One of these was in reply to a letter published in the "Cottage Gardener," May 14th, 1861, page 112, in which Darwin asked for information as to the Compositae and the hollyhock being crossed by insect visitors. In the number for June 8th, 1861, page 211, Darwin wrote on the variability of the central flower of the carrot and the peloria of the central flower in Pelargonium. An extract from a letter by Darwin on Leschenaultia, "Cottage Gardener," May 28th, 1861, page 151, is given in Letter 590, note.); he strikes me as a clever but d — d cock-sure man (as Lord Melbourne said), and I have some doubts whether to be much trusted. I suspect he has never recorded his experiment at the time with care. He has made me indignant by the way he speaks of Gartner, evidently knowing nothing of his work. I mean to try and pump him in the "Cottage Gardener," and shall perhaps defend Gartner. He alludes to me occasionally, and I cannot tell with what spirit. He speaks of "this Mr. Darwin" in one place as if I were a very noxious animal.

Let me have a line about poor Henslow pretty soon.