To H. G. Bronn 30 June 1
Down. | Bromley Kent.
Dear & Honoured Sir.
I have been very unwell otherwise I should have written sooner.2 I enclose answers. & some few corrections & additions—3 I fear that the latter on account of my bad handwriting will give you trouble; but I hope that you will insert them, as they are of some importance It would have been better to have worked them into the text, but I really have not time.
When you write to Mr. Schweizerbart will you say that I was in such a hurry when I wrote to him that I believe I forgot to thank him for his kind letter of June 7th.4 Please to say that I should very much like to have a Copy of the 2nd. German Edit of the Origin & of the Orchis Book, & I hope he will send me them through Messrs. Williams & Norgate.5
Believe me | Dear Sir | Yours truly obliged | Ch. Darwin.
I assure you that I feel most fully sensible of the great honour & kindness which you have done me in translating my Orchis-book.
What powers of work you must have!—6
p. 22. fig iii. p. 272 fig XXXI. I know that the letters were omitted. The reader must guess.—
p. 171 indian-rubber = “Gummi” = caoutchouc— The strap or hinge of Labellum is elastic when extended, as well as when bent.—
p. 254. Cocked-hat—an old fashioned hat worn formerly by gentlemen & Officers. [DIAG HERE]
p. 285. I was struck in Brazil & in British Museum with the astonishing diversity of prominences, &c &c on small Homopterous insects: I marvel whenever I think of them.—
Anther-case, I only mean, the walls of the anther, which form the case or receptacle for the pollen.—
p. 104. line 4. for renewed, read removed.
p. 292 “group of spiral vessels” is well translated by your “Spiral gefass-Bündel”. There is no distinction between “groups of vessels” & “vessels”. Sometimes there is only a single vessel in a petal. As the vessels collect from the several organs, the groups get bigger & bigger low down towards, or in, the ovarium, & form the six large ovarian groups or bundles. Generally even a single organ as one petal or one anther has a small bundle or group of vessels.—
[p. 34 (3 lines from bottom) Orchids*] 7[*] With respect to Orchis maculata, my son George Darwin, who is an entomologist and careful observer, has clearly made out the manner of its fertilisation.8 He saw many specimens of a fly (Empis livida) inserting their proboscises into the nectary. He brought home six specimens, with pollinia attached to their spherical eyes on a level with the bases of the antennae. The pollinia had undergone the movement of depression, and stood a little above and parallel to the proboscis: hence they were in a position excellently adapted to strike the stigma. Six pollinia were thus attached to one specimen, and three to another. My son also saw another and smaller species (Empis tenuipes) inserting its proboscis into the nectary; but this species did not act so well or so regularly as the other in fertilising the flowers. One specimen of this latter Empis had five pollinia, and a second had three pollinia, attached to the dorsal surface of the convex thorax. It is very probable that Orchis masculata, O. latifolia, and O. morio are fertilised by Diptera. (Postscript, June 1862.)9 [p. 75 (bottom line) removed*] 10[*] My son has observed that Herminium is visited by various minute Hymenoptera, which are so small that I had overlooked them.11 They belong to at least two groups, the largest being only 1/20 of an inch in length. Each time he brought home 1-2-3, together at last twenty-four specimens all with pollinia attached to them. It is an extraordinary fact that in all the specimens, with only one exception, the viscid discs were attached to the same peculiar spot, namely, to the outer side of the front leg, to the projection formed by the articulation of the femur with the coxa. In one instance alone a pollinium was attached to the inside of the femur. In addition I received a very small beetle from the family of Serricornia, which also carried a pollinium at the same spot of the front legs. These facts demonstrate how magnificently precise the form and depth of these flowers must be calculated to force various different kinds of insects to assume the same position so that they touch the helmet-like viscid disk with the projecting point of the femur. Answering my remark that these insects have to crawl in with their backs turned directly or obliquely towards the labellum he confirmed that this usually is the case. He, however, had seen several that had begun to crawl into the flower in a different position; but they came out, changed their position and crawled in again. One individual had been caught in the flower with the femur glued to the disc which had not yet been pulled off. This instance made me realise that the insects enter on both sides between the labellum and the upper petals and the discs (with rare exceptions) are glued to the outer surface of the femur. It is probable that when the insect retreats, the femur and hip joint brush against the lower side of the disc and pull out the pollinia. I know of hardly any other case where the pollinia stick to the legs of insects, although this occurs in Asclepiadeae as I have seen myself. (Postscript, June 1862.)12 [p. 82 (13 lines from bottom) nectary*] 13[*] My son went at night to a bank where G. conopsea grows and soon caught several moths, like Plusia Chrysites with six pollinia, Plusia gramma with three, Anaitis plagiata with five, and Triphena pronuba with five pollinia attached to their proboscises.14 The two viscid discs in the flower form an arched roof over the nectary and are large, compared with their diameter. That is why they become attached on the sides of the proboscis and, after a vertical movement of depression, occupy a proper position for striking the lateral stigma on the same side. Now if the moths are sucking the nectar they are resting on the labellum and there are no guiding-ridges. That is why it can frequently occur that the proboscises are inserted obliquely. In this case one pollinium alone is usually removed, as I convinced myself by repeated experiments with a bristle. This explains how it happens that so many pollinia are attached to the proboscises of these moths. (Postscript, June 1862.)15 [p. 88 (top line) other*] 16[*] Professor Asa Gray has written to me that he had compared Platanthera hookeri of North America with my description of Chlorantha and found that they correspond in most respects; he, however, also found some curious differences.17 The two viscid discs stand widely separated from each other; consequently a butterfly, unless of gigantic size, would be able to suck the copious nectar without touching either disc; but this risk is avoided in a most remarkable manner. The central line of the stigma is prominent, and the labellum, instead of hanging down, is curved upwards, so that the front of the flower is divided into two halves. Thus the moth is compelled to go either to one or the other side to suck nectar, and its head will be brought into contact with one of the two discs. The drum of the pollinium, when removed, contracts in the same manner as in Pl. chlorantha. (Postscript, June 1862.)18 [p. 152 (1 line from bottom) insects*] 19[*] Prof. Dickie has been so good as to observe the flowers on living plants.20 He informs me that, when the pollen is mature, the crest of the rostellum is directed towards the labellum, and that, as soon as touched, the viscid matter explodes, the pollinia becoming attached to the touching object; after the explosion, the rostellum bends downwards and spreads out, thus protecting the virgin stigmatic surface; subsequently the rostellum rises and exposes the stigma; so that everything here goes on as I have described under L. ovata. These flowers are frequented by minute Diptera and Hymenoptera. (Postscript, June 1862.)21 [p. 156 (bottom line) Listera*] 22[*] I have recently had better opportunity to observe this orchid,23 and found that the rostellum lost its power of explosion in about four days, the viscid matter then turning brown within the loculi of the rostellum. This fact does not correspond with some earlier observations, but the weather in that year was unusually hot. After the four days had elapsed, the pollen had become very incoherent and some had fallen on the two corners of the stigma, which was penetrated by the pollen-tubes. Hence, if insects should fail to remove the pollinia by causing the explosion of the rostellum, and carry them to their destination, this orchid certainly seems capable of occasional self-fertilisation. But the scattering of the incoherent pollen was largely aided by the presence of small insects of the genus Thrips, insects that cannot be excluded by any net, and that are numerous in these flowers and scatter the pollen in all parts of the flowers.24 [p. 276 (3 lines from top) convex*] 25[*] Professor Asa Gray carefully examined several American species of Cypripedium after reading these sheets and has found various major differences of structure and several beautiful adaptations.26 He infers from these observations that in several cases small insects are entering the labellum and he completely agrees with me that these insects are necessary for the fertilisation of all species. With regards to the state of the stigma he confirms that in C. acaule, where the pollen is much more granular than in other species, that, also, the stigma is slightly concave and viscid. (Addition made in June 1862.)27
Encloses answers and corrections [concerning Orchids]. Thanks HGB for translating it.
- Letter no.
- Darwin, C. R.
- Bronn, H. G.
- Sent from
- Source of text
- Bronn trans. 1862; DAR 143: 155; Houghton Library for Rare Books and Manuscripts, Harvard University
- Physical description
- 2pp; Enclosure 1 2pp, A
Please cite as
Darwin Correspondence Project, “Letter no. 3630,” accessed on 8 December 2016, http://www.darwinproject.ac.uk/DCP-LETT-3630