More Letters of Charles Darwin — Volume 2

LETTER 618. J.D. HOOKER TO CHARLES DARWIN.

(618/1. The following is part of Letter 144, Volume I. It refers to reviews of "Fertilisation of Orchids" in the "Gardeners' Chronicle," 1862, pages 789, 863, 910, and in the "Natural History Review," October, 1862, page 371.)

November 7th, 1862.

Dear old Darwin,

I assure you it was not my fault! I worried Lindley over and over again to notice your orchid book in the "Chronicle" by the very broadest hints man could give. (618/2. See "Life and Letters," III., page 273.) At last he said, "really I cannot, you must do it for me," and so I did — volontiers. Lindley felt that he ought to have done it himself, and my main effort was to write it "a la Lindley," and in this alone I have succeeded — that people all think it is exactly Lindley's style!!! which diverts me vastly. The fact is, between ourselves, I fear that poor L. is breaking up — he said that he could not fix his mind on your book. He works himself beyond his mental or physical powers.

And now, my dear Darwin, I may as well make a clean breast of it, and tell you that I wrote the "Nat. Hist. Review" notice too — to me a very difficult task, and one I fancied I failed in, comparatively. Of this you are no judge, and can be none; you told me to tell Oliver it pleased you, and so I am content and happy.

LETTER 619. TO W.E. DARWIN. Down, 4th {about 1862-3?}

I have been looking at the fertilisation of wheat, and I think possibly you might find something curious. I observed in almost every one of the pollen-grains, which had become empty and adhered to (I suppose the viscid) branching hairs of the stigma, that the pollen-tube was always (?) emitted on opposite side of grain to that in contact with the branch of the stigma. This seems very odd. The branches of the stigma are very thin, formed apparently of three rows of cells of hardly greater diameter than pollen-tube. I am astonished that the tubes should be able to penetrate the walls. The specimens examined (not carefully by me) had pollen only during few hours on stigma; and the mere SUSPICION has crossed me that the pollen-tubes crawl down these branches to the base and then penetrate the stigmatic tissue. (619/1. See Strasburger's "Neue Untersuchungen uber den Befruchtungsvorgang bei den Phanerogamen," 1884. In Alopecurus pratensis he describes the pollen as adhering to the end of a projection from the stigma where it germinates; the tube crawls along or spirally round this projection until it reaches the angle where the stigmatic branch is given off; here it makes an entrance and travels in the middle lamella between two cells.) The paleae open for a short period for stigma to be dusted, and then close again, and such travelling down would take place under protection. High powers and good adjustment are necessary. Ears expel anthers when kept in water in room; but the paleae apparently do not open and expose stigma; but the stigma could easily be artificially impregnated.

If I were you I would keep memoranda of points worth attending to.

2. X.II. MELASTOMACEAE, 1862-1881.

(620/1. The following series of letters (620-630) refers to the Melastomaceae and certain other flowers of analogous form. In 1862 Darwin attempted to explain the existence of two very different sets of stamens in these plants as a case of dimorphism, somewhat analogous to the state of things in Primula. In this view he was probably wrong, but this does not diminish the interest of the crossing experiments described in the letters. The persistence of his interest in this part of the subject is shown in the following passage from his Preface to the English translation of H. Muller's "Befruchtung der Blumen"; the passage is dated February, 1882, but was not published until the following year.

"There exist also some few plants the flowers of which include two sets of stamens, differing in the shape of the anthers and in the colour of the pollen; and at present no one knows whether this difference has any functional significance, and this is a point which ought to be determined."

It is not obvious why he spoke of the problem as if no light had been thrown on it, since in 1881 Fritz Muller had privately (see Letter 629) offered an explanation which Darwin was strongly inclined to accept. (620/2. H. Muller published ("Nature," August 4th, 1881) a letter from his brother Fritz giving the theory in question for Heeria. Todd ("American Naturalist," April 1882), described a similar state of things in Solanum rostratum and in Cassia: and H.O. Forbes ("Nature," August 1882, page 386) has done the same for Melastoma. In Rhexia virginica Mr. W.H. Leggett ("Bulletin Torrey Bot. Club, New York," VIII., 1881, page 102) describes the curious structure of the anther, which consists of two inflated portions and a tubular part connecting the two. By pressing with a blunt instrument on one of the ends, the pollen is forced out in a jet through a fine pore in the other inflated end. Mr. Leggett has seen bees treading on the anthers, but could not get near enough to see the pollen expelled. In the same journal, Volume IX., page 11, Mr. Bailey describes how in Heterocentron roseum, "upon pressing the bellows-like anther with a blunt pencil, the pollen was ejected to a full inch in distance." On Lagerstroemia as comparable with the Melastomads see Letter 689.) Fritz Muller's theory with regard to the Melastomads and a number of analogous cases in other genera are discussed in H. Muller's article in "Kosmos" (620/3. "Kosmos," XIII., 1883, page 241.), where the literature is given. F. Muller's theory is that in Heeria the yellow anthers serve merely as a means of attracting pollen-collecting bees, while the longer stamens with purple or crimson anthers supply pollen for fertilising purposes. If Muller is right the pollen from the yellow anthers would not normally reach the stigma. The increased vigour observed in the seedlings from the yellow anthers would seem to resemble the good effect of a cross between different individuals of the same species as worked out in "Cross and Self Fertilisation," for it is difficult to believe that the pollen of the purple anthers has become, by adaptation, less effective than that of the yellow anthers. In the letters here given there is some contradiction between the statements as to the position of the two sets of stamens in relation to the sepals. According to Eichler ("Bluthendiagramme, II., page 482) the longer stamens may be either epipetalous or episepalous in this family.

The work on the Melastomads is of such intrinsic importance that we have thought it right to give the correspondence in considerable detail; we have done so in spite of the fact that Darwin arrived at no definite conclusion, and in spite of an element of confusion and unsatisfactoriness in the series of letters. This applies also to Letter 629, written after Darwin had learned Fritz Muller's theory, which is obscured by some errors or slips of the pen.)

LETTER 620. TO G. BENTHAM. Down, February 3rd {1862?}

As you so kindly helped me before on dimorphism, will you forgive me begging for a little further information, if in your power to give it? The case is that of the Melastomads with eight stamens, on which I have been experimenting. I am perplexed by opposed statements: Lindley says the stamens which face the petals are sterile; Wallich says in Oxyspora paniculata that the stamens which face the sepals are destitute of pollen; I find plenty of apparently good pollen in both sets of stamens in Heterocentron {Heeria}, Monochoetum, and Centradenia. Can you throw any light on this? But there is another point on which I am more anxious for information. Please look at the enclosed miserable diagram. I find that the pollen of the yellow petal-facing stamens produce more than twice as much seed as the pollen of the purple sepal-facing stamens. This is exactly opposed to Lindley's statement — viz., that the petal-facing stamens are sterile. But I cannot at present believe that the case has any relation to abortion; it is hardly possible to believe that the longer and very curious stamens, which face the sepals in this Heterocentron, are tending to be rudimentary, though their pollen applied to their own flowers produces so much less seed. It is conformable with what we see in Primula that the {purple} sepal-facing anthers, which in the plant seen by me stood quite close on each side of the stigma, should have been rendered less fitted to fertilise the stigma than the stamens on the opposite side of the flower. Hence the suspicion has crossed me that if many plants of the Heterocentron roseum were examined, half would be found with the pistil nearly upright, instead of being rectangularly bent down, as shown in the diagram (620/4. According to Willis, "Flowering Plants and Ferns," 1897, Volume II., page 252, the style in Monochoetum, "at first bent downwards, moves slowly up till horizontal."); or, if the position of pistil is fixed, that in half the plants the petal-facing stamens would bend down, and in the other half of the plants the sepal-facing stamens would bend down as in the diagram. I suspect the former case, as in Centradenia I find the pistil nearly straight. Can you tell me? (620/5. No reply by Mr. Bentham to this or the following queries has been found.) Can the name Heterocentron have any reference to such diversity? Would it be asking too great a favour to ask you to look at dried specimens of Heterocentron roseum (which would be best), or of Monochoetum, or any eight-stamened Melastomad, of which you have specimens from several localities (as this would ensure specimens having been taken from distinct plants), and observe whether the pistil bends differently or stamens differently in different plants? You will at once see that, if such were the fact, it would be a new form of dimorphism, and would open up a large field of inquiry with respect to the potency of the pollen in all plants which have two sets of stamens — viz., longer and shorter. Can you forgive me for troubling you at such unreasonable length? But it is such waste of time to experiment without some guiding light. I do not know whether you have attended particularly to Melastoma; if you have not, perhaps Hooker or Oliver may have done so. I should be very grateful for any information, as it will guide future experiments.

P.S. — Do you happen to know, when there are only four stamens, whether it is the petal or sepal-facers which are preserved? and whether in the four-stamened forms the pistil is rectangularly bent or is straight?

LETTER 621. TO ASA GRAY. Down, February 16th {1862?}.

I have been trying a few experiments on Melastomads; and they seem to indicate that the pollen of the two curious sets of anthers (i.e. the petal-facers and the sepal-facers) have very different powers; and it does not seem that the difference is connected with any tendency to abortion in the one set. Now I think I can understand the structure of the flower and means of fertilisation, if there be two forms, — one with the pistil bent rectangularly out of the flower, and the other with it nearly straight.

Our hot-house and green-house plants have probably all descended by cuttings from a single plant of each species; so I can make out nothing from them. I applied in vain to Bentham and Hooker; but Oliver picked out some sentences from Naudin, which seem to indicate differences in the position of the pistil.

I see that Rhexia grows in Massachusetts; and I suppose has two different sets of stamens. Now, if in your power, would you observe the position of the pistil in different plants, in lately opened flowers of the same age? (I specify this because in Monochaetum I find great changes of position in the pistils and stamens, as flower gets old). Supposing that my prophecy should turn out right, please observe whether in both forms the passage into the flower is not {on} the upper side of the pistil, owing to the basal part of the pistil lying close to the ring of filaments on the under side of the flower. Also I should like to know the colour of the two sets of anthers. This would take you only a few minutes, and is the only way I see that I can find out whether these plants are dimorphic in this peculiar way — i.e., only in the position of the pistil (621/1. In Exacum and in Saintpaulia the flowers are dimorphic in this sense: the style projects to either the right or the left side of the corolla, from which it follows that a right-handed flower would fertilise a left-handed one, and vice versa. See Willis, "Flowering Plants and Ferns," 1897, Volume I., page 73.) and in its relation to the two kinds of pollen. I am anxious about this, because if it should prove so, it will show that all plants with longer and shorter or otherwise different anthers will have to be examined for dimorphism.

LETTER 622. TO ASA GRAY. March 15th {1862}.

...I wrote some little time ago about Rhexia; since then I have been carefully watching and experimenting on another genus, Monochaetum; and I find that the pistil is first bent rectangularly (as in the sketch sent), and then in a few days becomes straight: the stamens also move. If there be not two forms of Rhexia, will you compare the position of the part in young and old flowers? I have a suspicion (perhaps it will be proved wrong when the seed-capsules are ripe) that one set of anthers are adapted to the pistil in early state, and the other set for it in its later state. If bees visit the Rhexia, for Heaven's sake watch exactly how the anther and stigma strike them, both in old and young flowers, and give me a sketch.

Again I say, do not hate me.

LETTER 623. TO J.D. HOOKER. Leith Hill Place, Dorking, Thursday, 15th {May 1862}.

You stated at the Linnean Society that different sets of seedling Cinchona (623/1. Cinchona is apparently heterostyled: see "Forms of Flowers," Edition II., page 134.) grew at very different rate, and from my Primula case you attributed it probably to two sorts of pollen. I confess I thought you rash, but I now believe you were quite right. I find the yellow and crimson anthers of the same flower in the Melastomatous Heterocentron roseum have different powers; the yellow producing on the same plant thrice as many seeds as the crimson anthers. I got my neighbour's most skilful gardener to sow both kinds of seeds, and yesterday he came to me and said it is a most extraordinary thing that though both lots have been treated exactly alike, one lot all remain dwarfs and the other lot are all rising high up. The dwarfs were produced by the pollen of the crimson anthers. In Monochaetum ensiferum the facts are more complex and still more strange; as the age and position of the pistils comes into play, in relation to the two kinds of pollen. These facts seem to me so curious that I do not scruple to ask you to see whether you can lend me any Melastomad just before flowering, with a not very small flower, and which will endure for a short time a greenhouse or sitting-room; when fertilised and watered I could send it to Mr. Turnbull's to a cool stove to mature seed. I fully believe the case is worth investigation.

P.S. — You will not have time at present to read my orchid book; I never before felt half so doubtful about anything which I published. When you read it, do not fear "punishing" me if I deserve it.

Adios. I am come here to rest, which I much want.

Whenever you have occasion to write, pray tell me whether you have Rhododendron Boothii from Bhootan, with a smallish yellow flower, and pistil bent the wrong way; if so, I would ask Oliver to look for nectary, for it is an abominable error of Nature that must be corrected. I could hardly believe my eyes when I saw the pistil.

LETTER 624. TO ASA GRAY. January 19th {1863}.

I have been at those confounded Melastomads again; throwing good money (i.e. time) after bad. Do you remember telling me you could see no nectar in your Rhexia? well, I can find none in Monochaetum, and Bates tells me that the flowers are in the most marked manner neglected by bees and lepidoptera in Amazonia. Now the curious projections or horns to the stamens of Monochaetum are full of fluid, and the suspicion occurs to me that diptera or small hymenoptera may puncture these horns like they puncture (proved since my orchid book was published) the dry nectaries of true Orchis. I forget whether Rhexia is common; but I very much wish you would next summer watch on a warm day a group of flowers, and see whether they are visited by small insects, and what they do.

LETTER 625. TO I.A. HENRY. Down, January 20th {1863}.

...You must kindly permit me to mention any point on which I want information. If you are so inclined, I am curious to know from systematic experiments whether Mr. D. Beaton's statement that the pollen of two shortest anthers of scarlet Pelargonium produce dwarf plants (625/1. See "Animals and Plants," Edition II., Volume II., page 150, for a brief account of Darwin's experiments on this genus. Also loc. cit., page 338 (note), for a suggested experiment.), in comparison with plants produced from the same mother-plant by the pollen of longer stamens from the same flower. It would aid me much in some laborious experiments on Melastomads. I confess I feel a little doubtful; at least, I feel pretty nearly sure that I know the meaning of short stamens in most plants. This summer (for another object) I crossed Queen of Scarlet Pelargonium with pollen of long and short stamens of multiflora alba, and it so turns out that plants from short stamens are the tallest; but I believe this to have been mere chance. My few crosses in Pelargonium were made to get seed from the central peloric or regular flower (I have got one from peloric flower by pollen of peloric), and this leads me to suggest that it would be very interesting to test fertility of peloric flowers in three ways, — own peloric pollen on peloric stigma, common pollen on peloric stigma, peloric pollen on common stigma of same species. My object is to discover whether with change of structure of flower there is any change in fertility of pollen or of female organs. This might also be tested by trying peloric and common pollen on stigma of a distinct species, and conversely. I believe there is a peloric and common variety of Tropaeolum, and a peloric or upright and common variation of some species of Gloxinia, and the medial peloric flowers of Pelargonium, and probably others unknown to me.

LETTER 626. TO I.A. HENRY. Hartfield, May 2nd {1863}.

In scarlet dwarf Pelargonium, you will find occasionally an additional and abnormal stamen on opposite and lower side of flower. Now the pollen of this one occasional short stamen, I think, very likely would produce dwarf plants. If you experiment on Pelargonium I would suggest your looking out for this single stamen.

I observed fluctuations in length of pistil in Phloxes, but thought it was mere variability.

If you could raise a bed of seedling Phloxes of any species except P. Drummondii, it would be highly desirable to see if two forms are presented, and I should be very grateful for information and flowers for inspection. I cannot remember, but I know that I had some reason to look after Phloxes. (626/1. See "Forms of Flowers," Edition II., page 119, where the conjecture is hazarded that Phlox subulata shows traces of a former heterostyled condition.)

I do not know whether you have used microscopes much yet. It adds immensely to interest of all such work as ours, and is indeed indispensable for much work. Experience, however, has fully convinced me that the use of the compound without the simple microscope is absolutely injurious to progress of N{atural} History (excepting, of course, with Infusoria). I have, as yet, found no exception to the rule, that when a man has told me he works with the compound alone his work is valueless.

LETTER 627. TO ASA GRAY. March 20th {1863}.

I wrote to him {Dr. H. Cruger, of Trinidad} to ask him to observe what the insects did in the flowers of Melastomaceae: he says not proper season yet, but that on one species a small bee seemed busy about the horn-like appendages to the anthers. It will be too good luck if my study of the flowers in the greenhouse has led me to right interpretation of these appendages.

LETTER 628. TO J.D. HOOKER. Down, November 28th {1871}.

If you had come here on Sunday I should have asked you whether you could give me seed or seedlings of any Melastomad which would flower soon to experiment on! I wrote also to J. Scott to ask if he could give me seed.

Several years ago I raised a lot of seedlings of a Melastomad greenhouse bush (Monochaetus or some such name) (628/1. Monochaetum.) from stigmas fertilised separately by the two kinds of pollen, and the seedlings differed remarkably in size, and whilst young, in appearance; and I never knew what to think of the case (so you must not use it), and have always wished to try again, but they are troublesome beasts to fertilise.

On the other hand I could detect no difference in the product from the two coloured anthers of Clarkia. (628/2. Clarkia has eight stamens divided into two groups which differ in the colour of the anthers.) If you want to know further particulars of my experiments on Monochaetum (?) and Clarkia, I will hunt for my notes. You ask about difference in pollen in the same species. All dimorphic and trimorphic plants present such difference in function and in size. Lythrum and the trimorphic Oxalis are the most wonderful cases. The pollen of the closed imperfect cleistogamic flowers differ in the transparency of the integument, and I think in size. The latter point I could ascertain from my notes. The pollen or female organs must differ in almost every individual in some manner; otherwise the pollen of varieties and even distinct individuals of same varieties would not be so prepotent over the individual plant's own pollen. Here follows a case of individual differences in function of pollen or ovules or both. Some few individuals of Reseda odorata and R. lutea cannot be fertilised, or only very rarely, by pollen of the same plant, but can by pollen of any other individual. I chanced to have two plants of R. odorata in this state; so I crossed them and raised five seedlings, all of which were self sterile and all perfectly fertile with pollen of any other individual mignonette. So I made a self sterile race! I do not know whether these are the kinds of facts which you require.

Think whether you can help me to seed or better seedlings (not cuttings) of any Melastomad.

LETTER 629. TO F. MULLER. Down, March 20th, 1881.

I have received the seeds and your most interesting letter of February 7th. The seeds shall be sown, and I shall like to see the plants sleeping; but I doubt whether I shall make any more detailed observations on this subject, as, now that I feel very old, I require the stimulus of some novelty to make me work. This stimulus you have amply given me in your remarkable view of the meaning of the two-coloured stamens in many flowers. I was so much struck with this fact with Lythrum, that I began experimenting on some Melastomaceae, which have two sets of extremely differently coloured anthers. After reading your letter I turned to my notes (made 20 years ago!) to see whether they would support or contradict your suggestion. I cannot tell yet, but I have come across one very remarkable result, that seedlings from the crimson anthers were not 11/20ths of the size of seedlings from the yellow anthers of the same flowers. Fewer good seeds were produced by the crimson pollen. I concluded that the shorter stamens were aborting, and that the pollen was not good. (629/1. "Shorter stamens" seems to be a slip of the pen for "longer," — unless the observations were made on some genus in which the structure is unusual.) The mature pollen is incoherent, and must be {word illegible} against the visiting insect's body. I remembered this, and I find it said in my EARLY notes that bees would never visit the flowers for pollen. This made me afterwards write to the late Dr. Cruger in the West Indies, and he observed for me the flowers, and saw bees pressing the anthers with their mandibles from the base upwards, and this forced a worm-like thread of pollen from the terminal pore, and this pollen the bees collected with their hind legs. So that the Melastomads are not opposed to your views.

I am now working on the habits of worms, and it tires me much to change my subject; so I will lay on one side your letter and my notes, until I have a week's leisure, and will then see whether my facts bear on your views. I will then send a letter to "Nature" or to the Linn. Soc., with the extract of your letter (and this ought to appear in any case), with my own observations, if they appear worth publishing. The subject had gone out of my mind, but I now remember thinking that the imperfect action of the crimson stamens might throw light on hybridism. If this pollen is developed, according to your view, for the sake of attracting insects, it might act imperfectly, as well as if the stamens were becoming rudimentary. (629/2. As far as it is possible to understand the earlier letters it seems that the pollen of the shorter stamens, which are adapted for attracting insects, is the most effective.) I do not know whether I have made myself intelligible.

LETTER 630. TO W. THISELTON-DYER. Down, March 21st {1881}.

I have had a letter from Fritz Muller suggesting a novel and very curious explanation of certain plants producing two sets of anthers of different colour. This has set me on fire to renew the laborious experiments which I made on this subject, now 20 years ago. Now, will you be so kind as to turn in your much worked and much holding head, whether you can think of any plants, especially annuals, producing 2 such sets of anthers. I believe that this is the case with Clarkia elegans, and I have just written to Thompson for seeds. The Lythraceae must be excluded, as these are heterostyled.