From John Scott1 11 November 1862
Edinburgh | Botanic Gardens 〈Nov. 11th, 1862.〉 To Charles Darwin Esq. Sir,
I take the liberty of addressing you for the purpose of directing your attention to an error in one of your ingenious explanations of the structural adaptations of the Orchidaceæ in your late work. This occurs in the genus Acropera, two species of which 〈you〉 assume to be unisexual, and so far as known represented by male individuals only.2 Theoretically you have no doubt assigned good grounds for this view, nevertheless, experimental observations that I am now making, have already convinced me of its fallacy. And I thus hurriedly—and as you may think prematurely—direct your attention to it—before I have seen the final result of my own experiments, that you might have the longer time for re-considering the structure of this genus for another edition of your interesting book—if, indeed, it be not already called for.3 I am, furthermore, induced to communicate the results of my yet imperfect experiments in the believe that the actuating principle of your late work is the elicitation of truth, and that you will gladly avail yourself of this even at the sacrifi〈ce〉 〈of much〉 ingenious theoretical argumentation.
Since I have had an opportunity of perusing your 〈work on〉 Orchid Fertilisation, my attention h〈as〉 been 〈particul〉arly 〈direc〉ted 〈to〉 the curiously constructed floral organs 〈of〉 Acropera. I unf〈or〉tu〈nate〉ly have as yet had only a few flowers for experiment〈al〉 en〈quiry,〉 〈otherwise〉 my remarks might have been clearer and more satisfact〈ory〉. 〈Such as they are,〉 however, I respectfully, lay before you, with a full assur〈ance of their ver〉acity, and I sincerely trust that as such you will receive them.
Your observations, appear to have been chiefly directed to the 〈A. lu〉teola, mine to the A. Loddiegesii, which, however, as you remark is 〈in〉 a 〈very simi〉lar structural condition with the former;4 having the same 〈narrow〉 stigmatic chamber, abnormally developed placenta, &c. In regard 〈to the〉 former point—contraction of stigmatic chamber—I may remark that it does not appear to be absolutely necessary that the pollen-masses penetrate this chamber for effecting fecundation. Thus a raceme was produced upon a plant of A. Loddigesii in the Botanic Gardens here lately; upon this I left only six flowers, these I attempted to fertilise, but with two only of the six, have I been successful. I succeeded in forcing a single pollen-mass into the stigmatic chamber of one of the latter, but I failed to do this on the other; however, by inserting a portion of a pedicel with a pollinium attached, I caused the latter to adhere, with a gentle press—to the mouth of the stigmatic chamber. Both of these, as I have already remarked, are, nevertheless, fertilised, one of them I have cut off for examination—and its condition I will presently describe, the other is still upon the plant and promises fair to attain maturity. In regard to the other four flowers, I may remark that though similarly fertilised—part having pollinia inserted, others merely attached—they all withered and dropped off without the least swelling of the ovary. Can it be then that this is really a Mono-synoicous species?—part of the flowers male others truly hermaphrodite.5
In making longitudinal sections of the fertilised ovary 〈before〉 mentioned, I found the basal portion entirely destitute of 〈ovules〉 their place being substituted by transparent cellular ram〈ification〉 of the placentæ. As I traced the placentæ upwards the ovules bec〈ame〉 〈gr〉ad〈ually〉 〈more abundant〉 towards its apex. A transverse section 〈n〉e〈ar〉 the apex of 〈th〉e ovary, however, still exhibited a more than ordinary 〈plac〉ental development—i.e. congenerically considered—each end giving 〈off two〉 branches, which meet each other in the centre of the ovary, the ovules being irregularly and sparingly disposed upon their surfaces.
In regard to the mere question of fertilisation, then I am perfectly satisifed, but there are other points which require further elucidation. Among these I may particularly refer to the contracted stigmatic chamber, and the slight viscidity 〈of it〉s disk. 〈The latter〉, however, may be a consequence of uncongenial conditions 〈as you do not〉 mention particularly its examination by any author 〈in its〉 natural habitat.— If such be the case, the contracted stigmatic chamber will offer no real difficulty, should the viscous exudations be only sufficient to render the mouth adhesive. For as I have already shown the pollen-tubes may be emitted in this condition, and effect fecundation without being in actual contact with the stigmatic surface, as occurs pretty regularly in the fertilisation of the Stapelias, for example. But indeed, your own discovery of the independent germinative capabilities of the pollen-grains of certain Orchidaceæ is sufficiently illustrative of this.6
I may also refer to the peculiar abnormal condition, that many at least of the ovaries present on a comparative examination of their placentæ; and of which I beg to suggest the following explanation, though it is yet founded on limited observations. In examining certain young ovaries of A. Loddiegesii, I found some of them filled with the transparent membranous fringes of more or less distinctly cellular matter, which from your description of the ovaries of luteola, appears to differ simply in the greater development in the former species. Again in others I found small mammillary bodies, which appeared to be true ovules, though I could not perfectly satisfy myself as to the existence of the micropyle, or nucleus. I unfortunately neglected to apply any chemical test. The fact, however, that in certain of the examined ovaries few or none of the latter bodies occurred—the placenta al〈on〉e 〈being〉 developed in an irregular membranous form, taken in conjunct〈ion〉 〈wi〉th the results of my experiments—before alluded to—on 〈their〉 fe〈rtilisation〉, leads 〈m〉e 〈to〉 infer that two sexual conditions are presented by 〈the〉 flowers of this plant In short that many of the ova〈ries〉 are now 〈nor〉mal〈ly〉 abortive, though Nature occasionally makes futile efforts for 〈their〉 per〉fect development, in the production of ovuloid bodies, these 〈then I regard as the〉 male flowers. The others that are still capable of 〈fertilisation, and〉 likewise possessing male organs, are hermaphro〈dite,〉 and must as I think from the results of your comparative examinations present a somewhat different condition. As it can scarcely be supposed that ovules in the condition you describe, could ever be fertilised.
Th〈is is〉 at least the most plausible explanation I can offer for the different results 〈in〉 my experiments 〈on〉 the fertilisation of apparently similar morphologically constructed flowers, others may however occur to you. For there is not, as 〈in〉 the Catasetum any external change visible in the respective unisexual and bisexual flowers.7 And yet it would appear from your researches that the ovules of Acropera are in a more highly atrophied condition than occurs in Catasetum,8 though as you likewise remark M. Neuman has never succeeded in fertilising C. tridentatum.9 If there be not then, an arrangement of the reproductive structures, such as I have indicated, how can the different results in M. Neuman’s experiments and mine be accounted for? However as you have examined many flowers of both A. luteola and Loddigesii, such a difference in the ovulary or placental structures could scarcely have escaped your observation. But be this as it may the—to me at least—demonstrated fact still remains, that certain flowers of A. Loddigesii are capable of fertilisation and that though [there] are good grounds for supposing that important physiological changes are going on in the sexual phenomena of this species, there is no evidence whatever, for supposing that ex〈terna〉l morphological changes have so masqued certain individuals, as to prevent their recognition.
I would now, Sir, in conclusion beg you to excuse me for this infringement upon your valuable time, as I have been induced to write 〈you in the〉 believe that you have had negative results from other exper〈iments〉 before you ventured to propose your theoretical explanation, and consequently that you have been unknowingly led into error. I will 〈contin〉ue as opportunities present themselves to examine the many peculiarities you have pointed out in this as well as others of the Orchid 〈family.〉 And at present I am looking forward 〈with〉 anxiety for the 〈maturation〉 of the ovary of A. Loddigesii which will bear testi〈mony〉 〈to the〉 veracity of the remarks I have ventured to lay before you
I remain | Sir | Your obedient servant | John Scott
To Charles Darwin Esq.
CD is mistaken in considering Acropera unisexual, with only male flowers [Orchids, pp. 203–10]. JS has successfully fertilised two A. loddigesii flowers. One is ripening. Dissection of the other shows the pollen accomplishes fertilisation without contacting any stigmatic surface. Abortive ovules found in flowers that did not become fertilised when pollinated. JS suggests Acropera has both unisexual male and hermaphrodite flowers.
Please cite as
Darwin Correspondence Project, “Letter no. 3800,” accessed on 12 February 2016, http://www.darwinproject.ac.uk/DCP-LETT-3800