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Letter 3921

Scott, John to Darwin, C. R.

16 Jan 1863

    Summary Add

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    Experiments to cut Laelia stigma from rostellum and then to fertilise rostellum are baffled by "a latent instinctive power". Somehow the pollen-tubes find their way to the style.

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    Suggests CD study variation in ferns.

Transcription

Edinburgh | Botanic Gardens

Jany. 16th. 1863.

Sir,

The flowers of Lælia were all experimented upon ere I received your last, and there is no prospect of us having more of its section in flower for some time. I consequently cannot try the experiment you have lately suggested. I have, however, cut a portion of the column from two flowers of Lælia fertilised—as I had thought—by application of pollen-mass to rostellum. But I find I have been greatly deceived on an examination of these—by the mode at least in which this has been effected, though the pollen-mass was carefully applied to the latter organ. The difficulty which I anticipated in a former letter to you has presented itself in such a manner that I fear all our attempts at rostellic fertilisation will be baffled. A latent provisional force, an almost conscious sympathy—if I may so express myself—seems to exist between pollen and stigma, and is strikingly evoked, when these are not directly and normally applied to each other. There have we an interesting exhibition of that latent instinctive power, which nature has more or less lavishly bestowed upon vegetal Life, in anticipation of certain contingencies, by which she overcomes these, and enables the plant to perform its functional requirements. In the case under consideration, I find that in one of the columns a process extends itself from each of the stigmas—i.e. each side of stigmatic cavity—to the rostellum, and thus reaches the pollen-mass, into which the pollen-tubes insinuate themselves and are thus conducted into their normal route. In the other no such process is developed, nevertheless, the pollen-tubes are abundant in the conducting tissue of the style. Whether they extend themselves directly to the stigma; or pass along the anterior surface of the modified pistil—or rostellum—to it I cannot discover. I am inclined, to favour the former view, as I cannot find after a careful examination any trace of a pollen-tube passing down the anterior surface of rostellum to stigma; and I observed a single pollen-tube protruding quite freely from a grain upon rostellum towards the stigma. I am sorry that I have not examined some of these flowers earlier, as I might then have been able to speak more definitely. The latter case is not a little perplexing, as with the solitary exception I have mentioned, I can discover no trace now of pollen-tubes above the stigmatic-tissue where they most abruptly end abundantly. I will now attempt to describe a little more particularly one or two sections of them and you will perhaps suggest some means of preventing the recurrence of this deception, when I have an opportunity of experimenting upon Cattleya, and thus not let us again be so teasingly baffled, but force the tubes either to penetrate the rostellum, or remain inert, and thus satisfactorily show its real functional condition. I have an idea of applying a thickish solution of gum to stigma and allowing it to harden, before applying pollen-mass to rostellum.

In the column first referred to then, I find the stigmas and pollen-mass—the latter of course being applied to rostellum—connected by a firm yellowish process extended from the former part. This contains an abundance of elliptical and fusiform cells, similar to those found in the upper portion of the conducting tissue of the styles. It is surprising to see how these cells become elongated at times, when we follow them down the style, and how much they then resemble in certain respects the pollen tubes. Indeed, a hasty observer might almost pass them over for the latter were it not their closed extremities, and pale yellow nuclei. Their function is puzzling! Through this interpolated process then the pollen-tubes insinuate themselves and pass down into the style. Not a single tube is to be discovered in the tissue of rostellum. In the other column fertilisation is effected by a somewhat different process. There, I can discover no connection between pollen-grains and tubes, which nevertheless appear in great abundance in the stigmatic tissue. I made a number of sections of this column, to see if I could discover any trace of them along the anterior surface of rostellum, which I thought they might have passed along and then reached the stigma, but in vain. I therefore, know not whether of the conjectures I have already offered is the more probable, but this much I vouch, that there is an abundance of tubes in the style, and not one to be observed in the tissue of rostellum

I see in turning to your observations on the structure of the rostellum, that you remark on the absence of the coherent spindle-formed utriculi in that organ, and that this may probably account for its infertility. My experiments so far favour this view. I was perfectly ignorant of the opinion you there state, and was much puzzled with the occurrence of these peculiar cells, as none of the few works I have had an opportunity to consult mentioned even the existence of such. You have, however, afforded us a rich store in your treatise upon these plants; but I have not as yet had much time for digesting the structural portion; I have engaged my spare time principally with that explicating their varied means for fertilisation

I will be glad to afford you all the information I have on the subject of variation in plants, which I intended mentioning in my last had time permitted. And now I ask you to favour me with the nature of the information you require   Is it simply lists of plants now presenting variations with us, or must I furnish you the history of each? If the former, I think I could furnish you with a number. That is supposing you to accept such changes as variegation of leaves, changes in colour of flower and the like. Have you done much amongst the Ferns in regard to variation, or do you intend treating upon them at all, as I see no allusion to them in ``Origin''   The facilities they afford for reproducing variations is wonderful and affords a strong argument in favour of the view I have proposed—`on influence of sexual relations in transmitting variations''— May I take the liberty of asking you if any more probable explanation has occurred to you for their peculiar facility, than that I have proposed? For a time they almost exclusively engaged my attention. I will be glad to hear how you intend treating them.

I thank you for your promise, to remember with a copy of your paper on Linum. I am afraid my late paper on Drosera & Dionæa, will not be of much interest to you, such as it is however, I will send to you when printed, but they are very tardy in printing their `Proceedings' here. As I see your name occasionally in the Gard. Chron. you may have observed a short notice of it in No. 2. 1863 under Bot. Soc. of Edin.

I am, Sir. J. Scott.

    Footnotes Add

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    f1 3921.f1
    See Correspondence vol. 10, letter to John Scott, 19 December [1862], for CD's advice to Scott regarding the examination of pollination in Laelia. See also letter from John Scott, 17 December [1862] (ibid.). In his letter to CD of 6 January 1863, Scott detailed his work with Laelia, in particular his attempt on 13 December 1862 to cross Stanhopea oculata with Laelia anceps. CD's response to Scott's letter of 6 January 1863 has not been found; see CD's annotations to that letter for an indication of his reply.
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    f2 3921.f2
    In his letter to Scott of 3 December [1862] (Correspondence vol. 10), CD asked Scott to try to split the labellum of a Cattleya, or an allied orchid, and place a single pollen-mass on the `large tongue-like Rostellum', which CD thought was a modified stigma. CD wondered whether the rostellum retained `some of its primordial function of being penetrated by pollen-tubes'. With the closely related Laelia anceps soon flowering, Scott chose to work with this species instead (see ibid., letters from John Scott, 6 December [1862] and 17 December [1862]).
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    f3 3921.f3
    In his letter to CD of 17 December [1862] (Correspondence vol. 10), Scott reported that he had pollinated two flowers of Laelia in the manner suggested by CD (see n. 2, above). Scott expressed concern that the pollen-tubes, instead of penetrating the rostellum, would `pass along its surface, down sides of stigmatic cavity, and penetrate stigmas proper'. He asked CD whether a `preventive for such an occurrence' could be engineered; CD replied that if Scott examined the column of a specimen in which seed capsules had set, he might be able to determine whether pollen-tubes had penetrated the rostellum (see ibid., letter to John Scott, 19 December [1862]).
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    f4 3921.f4
    In Cattleya and the related Laelia, the stigmatic tissue lies below the rostellum (see Orchids, p. 161 fig. 29); Scott was investigating how the pollen-tubes travelled from the rostellum, through or past the stigma, to the conducting tissue of the style.
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    f5 3921.f5
    In the final chapter of Orchids, chapter 7, CD's account of the rostellum included his argument that the structure was a modified stigma (pp. 307--23).
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    f6 3921.f6
    CD wrote: `The utriculi are believed to be connected with the penetration of the pollen-tubes; and their absence in the rostellum probably accounts for its infertility' (Orchids, p. 311). He also noted that the layer of cells secreting viscid matter was thicker on the rostellum than on the stigma.
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    f7 3921.f7
    Part of chapter 7 of Orchids (pp. 307--30) was devoted to the examination of orchid structure, particularly the structure of the rostellum and pollen-masses.
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    f8 3921.f8
    CD was writing chapter 11 of Variation on `bud-variation', which he began on 21 December 1862 (see Correspondence vol. 10, Appendix II); in his letter to Scott of 19 December [1862], CD asked him to provide any cases of `what Gardeners sometimes call sports, & which I shall call ``bud-variation'''.
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    f9 3921.f9
    Letter from John Scott, 6 January 1863.
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    f10 3921.f10
    CD mentioned ferns in Variation 1: 383, citing Scott 1862a.
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    f11 3921.f11
    In Scott 1862a, pp. 357--60, Scott sought to explain the facility with which ferns, when propagated by spores, inherited the morphological peculiarities of the part of the plant on which the spores were produced. Scott attributed the more frequent inheritance of such peculiarities in ferns, as compared to seed plants, to the nature of the reproductive structures. He argued that the separation of the reproductive structures in seed plants, and their necessary union, meant that any individual peculiarities would tend to be lost by blending inheritance. In ferns, by contrast, he argued that because the spore was the product of a single lateral organ, individual peculiarities would tend to be inherited unmodified. Scott sent a copy of his paper to CD in December 1862, noting the impact on the theory of natural selection of his views regarding blending inheritance in seed plants (see Correspondence vol. 10, letter from John Scott, 6 December [1862]).
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    f12 3921.f12
    The letter in which this offer was made has not been found (see n. 1, above). However, Scott's name appears on CD's presentation list for the Linum paper (see Correspondence vol. 11, Appendix IV). See also Scott 1863a, p. 547 and n.
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    f13 3921.f13
    Scott's paper, `On the propagation and irritability of Drosera and Dionæa', was read at a meeting of the Botanical Society of Edinburgh on 11 December 1862, and was published as an abstract (Scott 1862b). An abstract of Scott's paper appeared in the Gardeners' Chronicle, 10 January 1863, p. 30.
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    f14 3921.f14
    See n. 3, above. See also letter to John Scott, 21 January [1863].
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    f15 3921.f15
    A. Gray 1862a. See letter to John Scott, 21 January [1863] and n. 11.
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    f16 3921.f16
    See n. 8, above. See also letter to John Scott, 21 January [1863].
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