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Letter 3221

Darwin, C. R. to Hooker, J. D.

[28 July – 10 Aug 1861]
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    Summary Add

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    Puzzled by function of orchids' rostellum.

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    Orchids' pollen concentrated in two pollinia; hence one flower can fertilise only two others. This may explain precision of orchid pollination mechanisms.

Transcription

2. Hesketh Crescent | Torquay

Read this if ever you have time & inclination—

My dear Hooker

Many thanks for names & all information & for your enquiring about Epithecia. Veitch sent me a grand lot this morning. What wonderful structures! I have now seen enough, & you must not send me more, for though I enjoy looking at them much, & it has been very useful to me seeing so many different forms, it is idleness. For my object each species requires studying for days.— I wish you had time to take up the group. I would give a good deal to know what the rostellum is, of which I have traced so many curious modifications. I suppose it cannot be one of the stigmas: there seems a great tendency for two lateral stigmas to appear?

My paper, though touching on only subordinate points will run, I fear, to 100 M.S. folio pages!!! The beauty of the adaptations of parts seems to me unparalleled. I shd. think or guess waxy pollen was most differentiated. In Cypripedium which seems least modified & a much exterminated group, the grains are single. In all others as far as I have seen they are in packets of 4; & these packets cohere into many wedge-formed masses in Orchis; into 8, 4 & finally two. It seems curious that a flower shd. exist which could at most fertilise only 2 other flowers, seeing how abundant pollen generally is: this fact I look at as explaining the perfection of the contrivances by which the pollen, so important from its fewness, is carried from flower to flower. By the way Cephalanthera has single pollen-grains; but this seems to be a case of degradation for rostellum is utterly aborted. Oddly the columns of pollen are here kept in place by very early penetration of pollen-tubes into the edge of the stigma: nevertheless it receives more pollen by insect agency.—

Epithecia has done me one good little turn: I often speculated how the caudicle of Orchis had been formed. I had noticed slight clouds in its substance half way down: I have now dissected them out, & I find they are pollen-grains fairly embedded & useless. If you suppose the pollen-grain to abort in the lower half of the pollinia of Epipactis; but the parallel elastic threads to remain & cohere; you have the caudicle of Orchis & can understand the few embedded & functionless pollen-grains.—

I must not look at any more exotic orchids; hearty thanks for your offer. But if you would make one single observation for me on Cypripedium, I should be glad. Asa Gray writes to me that the outside of pollen-masses is sticky in this genus: I find that the whole mass consists of pollen-grain immersed in a sticky brownish thick fluid. You could tell by a mere lens & pen-knife.— If it is, as I found it, pollen could not get on stigma without insect-aid.— Cypripedium confounds me much: I conjecture that drops of nectar are secreted by surface of Labellum beneath the anthers & in front of stigma, & that the shield over the anthers & form of Labellum is to compel insects to insert their probosces all round both organs. It would be troublesome for you to look to this; as it is always bothersome to catch the nectar secreting; & the cup of Labellum gets filled with water by gardeners watering.—

I have examined Listera, ovata, cordata & nidus avis; the pollen is uniform: I suspect you must have seen some observation founded on mistake from the penetration & hardening of sticky fluid from rostellum, which does penetrate the pollen a little.—

It is mere virtue which makes me not wish to examine more orchids; for I like it far better than writing about varieties of cocks & Hens & Ducks.— Nevertheless I have just been looking at Lindleys list in Veg. K. & I cannot resist one or two of his great Division of Arethuseæ, which includes Vanilla. And as I know so well the Ophreæ, I shd like (God forgive me) any one of the Satyriadæ, Disidæ, & Corycidæ.—

I fear my long lucubration will have wearied you; but it has amused me to write, so forgive me.— Ever your affect | C. Darwin

I am very sorry that I have failed to see Catasetum & Mormodes; I think I could have madout all & means of ejection of pollinia. Let me know affinities of Epithecia. I marvel often as I think over the diversity & perfection of the contrivances.—

There is a Dutch(!) Translation of ``Origin'' come out.—

    Footnotes Add

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    f1 3221.f1
    Dated by the relationship to the letter to J. D. Hooker, 27 July [1861] and by the fact that CD had received a specimen of Epithecia from Hooker by 11 August 1861 (see n. 2, below, and letter to J. D. Hooker, [11 August 1861]).
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    f2 3221.f2
    Epithecia is another name for Epidendrum. CD had asked Hooker to send him orchids from the tribe Epidendreae in the letter to J. D. Hooker, 27 July [1861]. In the same letter CD asked Hooker to confirm the names of some of the orchids he had sent.
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    f3 3221.f3
    James Veitch Jr, of Veitch & Sons Royal Exotic Nurseries, Chelsea, is acknowledged in Orchids for having provided CD with specimens of exotic orchids (Orchids, p. 158 n.)
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    f4 3221.f4
    At the end of the Introduction to Orchids, CD included a section giving definitions of terms, in which he described how the upper stigma is modified into `an extraordinary organ, called the Rostellum, which in many Orchids presents no resemblance to a true stigma. The rostellum either includes or is formed of viscid matter' (Orchids, p. 6).
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    f5 3221.f5
    Owing to the length of his manuscript, CD published his material on orchids in book form (Orchids) rather than in a scientific journal.
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    f6 3221.f6
    CD described the orchid genus Cypripedium in Orchids, pp. 270--6, noting their single pollen grains on p. 271.
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    f7 3221.f7
    In Orchids, pp. 155--6, CD discussed how the transition from orchid flowers with well-developed rostellums to those of Cephalanthera, with their aborted rostellums, might have arisen `with each gradation useful to the plant'.
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    f8 3221.f8
    CD reiterated this view of the development of the caudicle in Orchids, pp. 327--8.
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    f9 3221.f9
    Asa Gray's letter has not been found, but see the letter to Asa Gray, 21 July [1861] and n. 4.
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    f10 3221.f10
    CD discussed the necessity for insect aid in the pollination of Cypripedium in Orchids, pp. 273--6.
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    f11 3221.f11
    See Orchids, pp. 279--80.
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    f12 3221.f12
    Hooker had studied the rostellumn of Listera ovata and published a paper on the subject (Hooker 1854b). The point to which CD refers, however, is not discussed in this work. In Orchids, p. 139, CD stated that Hooker had described the structure of this species `minutely, and of course correctly', though he had not attended to the role of insects in its fertilisation, whereas Christian Konrad Sprengel had `misunderstood both the structure and the action of the rostellum'.
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    f13 3221.f13
    CD had been writing up his materials on variation in fowl and ducks for inclusion in Variation. He completed the manuscript in May (see `Journal'; Appendix II).
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    f14 3221.f14
    See letter to J. D. Hooker, 27 July [1861], n. 8.
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    f15 3221.f15
    Winkler trans. 1860.
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