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Letter 4055

Scott, John to Darwin, C. R.

21 Mar [1863]

    Summary Add

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    Thanks for CD's answers on Passiflora

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    and Asa Gray review.

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    Has observed gradation of sterility in Oncidium species.

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    Has observed rostellar germination and fertilisation in Laelia. The latter was prevented in Bletia by covering the stigma with plaster of Paris.

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    Gongora atropurpurea capsules are swelling.

Transcription

Edinburgh

March 21st

Sir,

I am sorry to find from your last, that you are labouring under such a weak state of health. I sincerely trust that a little relaxation will enable you to resume with renewed vigour, those arduous and varied works from which you will be so ill spared.

I thank you for your kind attention to my queries on Passiflora &c. I will not trouble you now with any remarks on sterility of passion-flowers with own-pollen, further than merely stating, that I am inclined to believe—on grounds which if desirable <I can> communicate—you will find <that> —sterility—capable of a <local> application only. I know <it to> be so at least in species mentioned. Pray do not think me over-critical. I refer to it only in private, and would be the last publicly to question the validity of these long recorded statements; but being engaged in similar observations for you, I communicate them only as the results of my own practical experiments. Plants which with us may have exhibited various grades of sterility, may prove otherwise in another locality.

I have now one or two singularly capricious cases of this phenomenon in species of Oncidium in store for you.

I return by this evenings mail Asa Gray's Review, for which I am greatly obliged. I may state that my observations on the capability of the rostellum for fertilisation are still entirely negative, & I have now performed not a few experiments. In my examination, however, I frequently found short tubes protruding from grains, immersed in viscous matter of rostellum, but in no case did I find these insinuating themselves into the tissues of that organ   Its viscous secretions, sufficed to generate their vital action, but its conductive prerogative seems lost; as in every case the tubes, were deflected along the anterior surface of the clinandrum towards the stigma. In Laelia I have now seen fertilisation thus accomplished, in Bletia, and Cœlogyne this was prevented by the plaister of Paris which covered the stigmas. This organ, therefore, in these genera still retains its stigmatic function, though the stylar parts are unable to accomplish theirs. I am afraid, however, since I have read Asa Gray's observations, that I have misunderstood you, in supposing <that the> tubes would not only penetrate rostellum, but passing down amongst its vessels, likewise reach the ovary<.>

The capsules of Gongora atropurpurea are now swelling fast, and promise to be larger than that sent of Acropera—by the way, not a single seed has yet germinated. I am at present working on Gongora truncata which presents even greater difficulties than the former: the stigmatic chamber being all but shut. This genus, with which I suppose Acropera is now associated, is indeed a most perplexing one from your point of view. I will not, however encroach further at present, so with sincere wishes for a speedy recovery from your ill health,

I remain | Sir | Yours very respectfully | John Scott

    Footnotes Add

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    f1 4055.f1
    The year is established by the relationship between this letter and the letter to John Scott, 6 March 1863.
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    f2 4055.f2
    See letter to John Scott, 6 March 1863 and n. 1.
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    f3 4055.f3
    See the enclosure to the letter to John Scott, 6 March 1863.
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    f4 4055.f4
    See also letter from John Scott, 3 March 1863 and n. 4. Scott published his observations on sterility in self-pollinated Passiflora in Scott 1864d; they are summarised in Variation 2: 137--8.
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    f5 4055.f5
    Scott's observations on this subject were published in Scott 1864b; CD cited them in Cross and self fertilisation, pp. 330--1.
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    f6 4055.f6
    CD had lent Scott his copy of A. Gray 1862a (see letter to John Scott, 6 March 1863).
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    f7 4055.f7
    In A. Gray 1862a, p. 426, Asa Gray reported that pollen falling onto certain viscid parts of the rostellum in the orchid Gymnadenia tridentata, readily sent pollen tubes into it, and that those parts of the rostellum appeared `to act as stigmas'. In his letter to Scott of 3 December [1862] (Correspondence vol. 10), CD suggested that he might try a similar experiment with Cattleya `or … some allied orchis'. See also Correspondence vol. 10, letter from John Scott, 17 December [1862], and this volume, letters from John Scott, 6 January 1863 and 16 January 1863.
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    f8 4055.f8
    In his letter to Scott of 21 January [1863], CD suggested that Scott apply plaster of Paris to the stigmas of orchid flowers in order to prevent pollen tubes from penetrating them and effecting fertilisation. Scott wished for a means to `force the tubes either to penetrate the rostellum, or remain inert, and thus satisfactorily show its real functional condition' (see letter from John Scott, 16 January 1863).
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    f9 4055.f9
    On Gongora, see letters from John Scott, 6 January 1863 and 3 March 1863. In his letter to CD of 18 February [1863], Scott sent CD a seed capsule of Acropera loddigesii that he had produced by artificial pollination, stating that he had retained some of the seeds to sow.
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    f10 4055.f10
    See letter from John Scott, 6 January 1863 and nn. 3 and 4. In `Fertilization of orchids', p. 153 (Collected papers 2: 150), CD noted that Gongora presented the same difficulty in pollination as Acropera, the entrance into the stigmatic chamber being `so narrow that the pollen-masses cannot be inserted without the greatest difficulty'.
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