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Darwin Correspondence Project

To Gardeners’ Chronicle   [before 11 August 1866]1

All who have tried have found much difficulty in crossing papilionaceous flowers.2 Several years ago, Dr. Herbert remarked to me that with the exception of Erythrina no hybrids had been made in this great family.3 Gärtner crossed 36 flowers of different varieties of the common Pea, and he did not get a single pod perfectly developed and with the full complement of seed; he crossed 10 flowers of Kidney Beans, and did not get a single pod.4 Some years ago I crossed the varieties of the Sweet Pea, and many more flowers dropped off unimpregnated than were fertilised.5 The difficulty arises from the anthers opening at so early an age that they must be removed long before the flower expands. After the operation the immature stigma is liable to exposure to the air; and it is difficult to judge when to apply the pollen. Moreover there is some reason to suspect that the stigma requires successive applications of pollen. To show the difficulty of fertilising papilionaceous flowers, I may mention that I lately removed all the pollen that I could with a soft brush from six recently expanded flowers of Lupinus pilosus6 protected from the visits of insects, and then applied pollen from a distinct individual of the same species. Although in this case there was no operation at an early age, yet five flowers out of the six dropped off unimpregnated. Had these flowers remained untouched, all, judging from the others, probably would have set, and the only difference would have been that their stigmas would have been surrounded by a mass of pollen as long as the flowers continued in bloom. This case is worth mentioning as showing how erroneous the belief is that fertilisation usually takes place in unopened flowers, in which the pollen is shed at an early age. These trials on the Lupines, and others formerly on Sweet Peas, led me to try the following plan. I rolled up thin paper into a cylinder, rather thinner than a knitting needle. I then tied a thread tight round, and cut off the cylinder beneath the thread, so that a little pipe closed at one end or cap, about the fifth of an inch in length, was left. This was easily filled with pollen from the keel-petal of any desired variety, and could then be placed on the pistil and secured below the stigma by being tied with a thread. I then castrated four flower-buds of the Sweet Pea, and placed on the young stigmas caps filled with pollen from another variety, and four fine pods were soon formed. I also fertilised eight castrated flowers of two species of Lupins with pollen from distinct plants of the same species, but from these I have got only four pods. I may add, that as an experiment I filled one of the little caps with pollen of Lathyrus grandiflorus and placed it on the stigma of a Sweet Pea (Lathyrus odoratus), and to my great surprise, considering how distinct these species are, a fine pod has been formed. I am certain no pollen could have been left in the flower of the Sweet Pea, as the anthers were removed whilst quite immature; and if these hybrid seeds grow, a curious hybrid will be produced. I should not have thought this plan of fertilising papilionaceous flowers worth mentioning had it not been applicable in all cases in which early castration is necessary, and likewise in certain cases mentioned by Gärtner, in which the stigma requires, or is benefited by, successive applications of pollen. In all such cases some trouble would be saved and certainty gained by the use of the little caps filled with the desired kind of pollen.

Charles Darwin

Down, Bromley, Kent

Footnotes

The letter was published in the Gardeners’ Chronicle and Agricultural Gazette on 11 August 1866; see also Collected papers 2: 132–4.
CD began artificially pollinating leguminous flowers in 1857 as a means of investigating the role of insects in pollination, and of testing his belief that no plant self-fertilises perpetually. See his discussion of Leguminosae in Natural selection, pp. 68–71, and Correspondence vols. 6 and 7. See also Correspondence vol. 13, letter from George Henslow, 1 November 1865, nn. 8 and 9.
CD had corresponded with William Herbert on plant hybridisation and reversion (see Correspondence vol. 2) and had visited Herbert in 1845 (Correspondence vol. 3, Appendix II). Erythrina is mentioned in Natural selection, p. 70 n. 4, as one of the few cases of hybrids forming in the family Leguminosae.
CD refers to Karl Friedrich von Gärtner’s crossing experiments with Pisum and Phaseolus (see Gärtner 1849, p. 720).
On CD’s experiments with sweetpeas (Lathyrus odoratus), see Natural selection, pp. 70–1, and Correspondence vol. 9, letter to Journal of Horticulture, [before 14 May 1861] and n. 7.
CD had asked Joseph Dalton Hooker to identify the species in his letter of 21 [July 1866]. For more on CD’s experiments with Lupinus pilosus and his publication of the results, see Correspondence vol. 13, letter from W. A. Leighton, 29 May 1865, n. 2.

Summary

Describes the difficulties of crossing papilionaceous flowers. Believes the lack of success is a consequence of the need for early castration and successive applications of pollen on the stigma. Gives details of a method he has used to cross such flowers successfully.

Letter details

Letter no.
DCP-LETT-5189
From
Charles Robert Darwin
To
Gardeners’ Chronicle
Sent from
Down
Source of text
Gardeners’ Chronicle and Agricultural Gazette (1866): 756

Please cite as

Darwin Correspondence Project, “Letter no. 5189,” accessed on 17 November 2018, http://www.darwinproject.ac.uk/DCP-LETT-5189

Also published in The Correspondence of Charles Darwin, vol. 14

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