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Darwin Correspondence Project

To Gardeners’ Chronicle   [before 9 September 1871]1

Fertilisation of Leschenaultia.—

As “F. W. B.” inquires, in your number of August 26, about the seeding of Leschenaultia, I will give my small experience.2 During 1860 and 1862, I was led to make some observations on the fertilisation of L. formosa and biloba, from having read that with these flowers self-fertilisation was an inevitable contingency; and this, from what I had seen during many years, seemed to me highly improbable.3 I found, as “F. W. B.” states, that before the flower expands, the anthers open and the pollen is shed. This occurs in a considerable number of plants, as in most Leguminosæ, Fumariaceæ, &c.;4 but it can be clearly shown that this by no means necessarily leads to self-fertilisation. In Leschenaultia the pollen, when shed, is neatly collected in a cup-shaped indusium, the mouth of which is at first widely open, but soon closes. Thus far I can follow “F. W. B.;” but he will, I think, find, on further examination, that the pollen must, in order that the flower should be fertilised, be subsequently removed from the indusium, and then placed on an exterior stigmatic surface. This no doubt is effected by insects, tempted to visit the flowers by the copious supply of nectar. On the outside of the indusium there is a viscid surface, and when on two occasions I placed some pollen-grains on the surface, I found, after an interval of about 20 hours, that it was deeply penetrated by numerous pollen-tubes.5 I was so much surprised at this position of the stigma, that I asked Dr. Hooker to dissect some flowers, which he did with care, and he confirmed my conclusion with respect to L. formosa. He also examined two other species, and found no trace of a stigma within the indusium.6 I should here add that Mr. Bentham has subsequently described the structure of the parts in this genus, but I cannot at the present moment lay my hand on his paper.7 When the flower is fully expanded the lips of the indusium fit closely, and cannot be very easily opened. If, however, a finely-pointed, small camel-hair brush be held parallel to the pistil, and be gently inserted into the flower, so as to imitate the entry of an insect, the tip of the brush, by pressing against the slightly projecting lower lip of the indusium, opens it; and some of the hairs enter and become smeared with pollen. If the same brush be now successively inserted into several flowers, pollen-grains will be found left on the exterior viscid stigma.8 During the early part of the summer I treated in this manner several flowers, but with no result. Towards the end of July, however, five flowers were thus treated, and the germens of all soon became much enlarged. Two of them, after a time, shanked off, but three remained on till the autumn, and each contained about 25 seeds.9 My plant produced hundreds of flowers during two or three summers, but the germens of none spontaneously swelled, with the exception of two growing close together, which I imagined had been visited by some insect. These two produced some seeds, but fewer in number than in the above case. All the seeds were in external appearance good, but when sown they did not germinate. The flowers were necessarily fertilised with pollen from the same plant, but it would have been incomparably better if pollen from a distinct seedling plant could have been employed. This would have been all the more advisable, as the late Mr. Drummond, of Swan River, in Australia, to whom I wrote, asking him to observe in the proper season what insects visited the Leschenaultias, informed me that the species growing there in a state of Nature very rarely produce seed.10 It appears at first sight a surprising circumstance that in this genus and in some allied genera, the pollen, whilst the flowers are still in bud, should be scooped out of the anthers, in which it might have remained ready for use, and then be immediately enclosed in a specially contrived receptacle, from which it has afterwards to be removed, so as to be placed on the stigma. But he who believes in the principle of gradual evolution, and looks at each structure as the summing up of a long series of adaptations to past and changing conditions—each successive modification being retained as far as that is possible through the force of inheritance—will not feel surprise at the above complex and apparently superfluous arrangement, or the other still more complex arrangements, though they may all serve for one and the same general purpose. Any one desiring to learn how diversified are the means for preventing self-fertilisation, even within the limits of the same family of plants, should study Mr. Bentham’s short but extremely curious paper, just published, on the styles of the Australian Proteaceæ.11 I cannot resist specifying one of the remarkable contrivances described by Mr. Bentham. In Synaphea the upper anther does not subserve its proper function of producing pollen, but has been converted into a short broad strap, firmly fixed to the edge of the stigmatic disc. By this means the stigma is held in such a position that it cannot receive pollen from the fertile anthers of the same flower; or, as Mr. Bentham puts the case, “the stigma thus held by the eunuch (i. e., the barren anther) is safe from all pollution from her brother anthers, and is preserved intact from any pollen that may be inserted * * by insects or other agencies.”12

Charles Darwin.

Footnotes

The date is established by the publication of this letter in Gardeners’ Chronicle on 9 September 1871.
A letter from Frederick William Burbidge about the fertilisation of Leschenaultia formosa was published in Gardeners’ Chronicle on 26 August 1871, p. 1103. The letter is lightly annotated in CD’s copy of the Gardeners’ Chronicle in the Cory Library, Cambridge Botanic Garden.
See Correspondence vol. 8, letter to J. D. Hooker, 18 [April 1860], and Correspondence vol. 10, letter to J. D. Hooker, 1 May [1862] and n. 2. CD probably read Auguste Saint-Hilaire’s Leçons de botanique, where Saint-Hilaire argued that cross-fertilisation would be impossible for Goodenia, a genus closely related to Leschenaultia (Saint-Hilaire 1841, pp. 63–4, 72). CD’s annotated copy is in the Darwin Library–CUL (see Marginalia 1: 734–8). See Correspondence vol. 8, letter to J. D. Hooker, [22 January 1860] and n. 7.
The Leguminosae are now the Fabaceae (legumes, peas, beans, and pulses); the Fumariaeae are the family of fumitories, fumeworts, and bleeding hearts.
CD described his experiments on Leschenaultia formosa in his Experimental notebook (DAR 157a: 50–2). See also Correspondence vol. 8, letter to J. D. Hooker, 14 May [1860].
CD probably refers to the letter to J. D. Hooker, 26 April [1860] (Correspondence vol. 8), in which he asked Hooker to look into the relative positions of the indusium, stigma, and styles of Leschenaultia formosa, and the letter from J. D. Hooker, [28 April 1860] (ibid.), in which Hooker described his results. One of the other species dissected by Hooker was L. biloba, but the third has not been identified; see Correspondence vol. 10, letter from J. D. Hooker, [16 May 1862]. It is described as ‘Red Leschenaultia not formosa’ in ibid., letter from J. D. Hooker, [17 May 1862]; this cannot be L. biloba, which is blue, but could be L. laricina or L. macrantha.
George Bentham published a paper about Australian Goodenovieae in the Journal of the Proceedings of the Linnean Society in 1869 (Bentham 1867). The former family Goodenovieae is now subsumed within the family Goodeniaceae. Leschenaultia is a genus in the family Goodeniaceae.
CD recorded performing this experiment on 14 May 1860 in his Experimental notebook (DAR 157a: 50), as well as in his letter to J. D. Hooker, 14 May [1860] (Correspondence vol. 8).
CD experimented on five flowers on 25 July 1860. He recorded having five pods, containing a total of twenty-six seeds, on 3 December 1860 (Experimental notebook (DAR 157a: 52)).
‘Notes on the styles of Australian Proteaceæ’ (Bentham 1871).
Synaphea is a genus of flowering plants in the family Proteaceae. The genus is endemic to western Australia. The quotation is taken from Bentham 1871, p. 64.

Bibliography

Bentham, George. 1867. Note on the stigmatic apparatus of Goodenovieæ. [Read 19 December 1867.] Journal of the Linnean Society (Botany) 10 (1869): 203–6.

Bentham, George. 1871. Notes on the styles of Australian Proteaceæ. [Read 6 April 1871.] Journal of the Linnean Society (Botany) 13 (1873): 58–64.

Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 26 vols to date. Cambridge: Cambridge University Press. 1985–.

Marginalia: Charles Darwin’s marginalia. Edited by Mario A. Di Gregorio with the assistance of Nicholas W. Gill. Vol. 1. New York and London: Garland Publishing. 1990.

Summary

In response to a query [from "F. W. B."], CD describes his experience with seeding Leschenaultia, which demonstrates that insect agency is required.

Letter details

Letter no.
DCP-LETT-7927
From
Charles Robert Darwin
To
Gardeners’ Chronicle
Sent from
unstated
Source of text
Gardeners’ Chronicle, 9 September 1871, p. 1166

Please cite as

Darwin Correspondence Project, “Letter no. 7927,” accessed on 15 October 2019, https://www.darwinproject.ac.uk/letter/DCP-LETT-7927.xml

Also published in The Correspondence of Charles Darwin, vol. 19

letter