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Darwin Correspondence Project

From George Henslow   [c. 7 December 1876]1

7 Bentinck Terrace, Regents Pk.

My dear Sir,

I have to thank you very much for kindly ordering a copy of your new work to be sent to me, and which I have just received.2 There is so much matter in it, that it will take a long while to digest; but I think from a few parts which I have read, that my conclusions—I gave in “Nature”—are really in harmony with yours.3 I have paid no attention to Conspicuous flowers, so that my term “self-fertilisation” had sole reference to small inconspicuous flowers; and as far as they are concerned your book certainly seems to corroborate me. I was hoping to find you had experimented on such flowers but you confess not to have done so. I mean to try next year to see what I can do.

In your list of plants fertile without insect-aid pp. 365–369 are included a few of those which I have noticed as being adapted for self-impregnation; & of which I have detected the pollen-tubes: Thus. Fumaria offs: In this, the 3 Anther-cells of each group form a neat little 3-sided box into which the “horn”-like stigmas are shut, so that the pollen-tubes pour into it while the grains are still in situ. From this I inferred (& you have proved) that it is self-fertilising.4 I have also detected the pollen-tubes in Trifolium procumbens & minus.5 Nicotiana Tabacum as growing in the Kew Gardens seemed to be proterandrous, but N. rusticum with smaller, greenish flowers was certainly fertilising itself.6 I have also noticed the pollen tubes in several small-fld. sp. of Veronica. With regard to sp: of Solanum (Note p. 387) S. nigrum I take to be decidedly self-fg. for the anther cells begin by pores, but slit all the way down & so the pollen fall easily on the stigma, the flower being pendulous.7

The conclusion I drew was that these self-fertilising inconspicuous flowers, had acquired the habit—or rather regained it, in consequence of the reduction of the corolla or by suppression of nectariferous affairs or both: for I found (as Bennett had also, though I was not aware of his observations at the time) that as the weather grows colder Two Things followed; (1) buds have a tendency to remain closed (2) the corollas are dwarfed.8 The former is seen well, e.g. in Spergula arvensis9 which goes on “budding” all through the winter but never “blossoming” & yet produces an abundance of seed:— I therefore greatly rejoiced when I came across the foot-note to p. 420—where you say:—“I am greatly surprized &c for you include Viola tricolor (p. 358) in your list of “Plants sterile without insect aid” Now, these cases of Delphinium and Viola anticipate what I had determined to experimentize & have already asked friends to help me:—viz. to see if flowers which are usually proterandrous will not become self-fertilising when their corollas are early removed; so far then, these cases quite corroborrate my deductions!10

On p. 385. you speak of the reduction of the size of the corollas of some papilionacae species:— the Cause, judging by analogy, I take to be change of climate; for the reduction of temperature seems at once to affect & reduce the size of the corolla or perianth. A case was mentioned a few weeks ago in the Gardner’s Chronicle where a Lapageria had a branch which had somehow escaped into the air—the blossoms on which only differed “in being smaller”.11 Similarly I have noticed the corollas of conspicuous flowers get reduced in the autumn; but it would seem that the sexual processes are in no way interfered with. For example, I found last September & October several plants of Tradescantia erecta,12 at Kew which earlier in the season produced conspicuous flowers, but in which the corollas were completely arrested, shriveled: the styles were consequently bent down on the side of the ovary & the stigma thus kept in contact with the anthers. Not a bud expanded, though they were very numerous, yet all set seed: & I have preserved about 100 for next year’s sowing   the embryos are perfectly well formed!

You say you can hear of no instance of a species with all its flowers rendered permanently cleistogamous:13 Perhaps the two following may prove to be so: Cerastium glomeratum and Corrigiola littoralis14 Because, while Spergula arvensis keeps its buds shut in cold weather, yet expands them on a warm sunny day, yet I have found Cer: glom: perfectly closed on a broiling hot day last June in an open hayfield. It grew abundantly, but not a single bud was expanded yet seed was shed in profusion. Cor: litt: was quite closed last Septr at Kew: whether it opens on hot days I can’t say.—

The fact that so many Conspicuous flowers are self-fertilising, but benefitted by crossing; and that to be absolutely sterile with its own pollen is a fact decidedly in the minority, also seems to corroborrate my view that self-fertilisation was the primordial condition of flowering plants; but that in proportion as the corollas or perianths became conspicuous & insect adaptation set in, sexual differentiations were set up & self-fertilisation correspondingly “went to the wall”; but as species spread & got into colder regions than their ancestors: the corollas were checked: & self-fertilisation became again the rule for such inconspicuous flowers:

But I must not bore you with any more of my notions:

Again thanking you, | I beg to remain | Yrs very truly | Geo: Henslow

Ch. Darwin Esq F.R.S.

CD annotations

1.1 I have … can do. 1.8] crossed pencil
4.5 the blossoms … smaller”. 4.6] scored red crayon; ‘4’ blue crayon
4.8 For example, … shriveled: 4.11] scored red crayon
6.3 also seems … condition 6.4] scored red crayon
6.3 self-fertilisation … condition 6.4] underl red crayon
Top of letter: ‘p 4.’ red crayon

Footnotes

The date is established by the references to Cross and self fertilisation; copies sent by John Murray were received around 7 December 1876 (letter from A. W. Bennett, 7 December 1876; CD’s presentation list for Cross and self fertilisation (Appendix III)).
Henslow’s name appears on CD’s presentation list for Cross and self fertilisation (Appendix III).
See Henslow 1876b; see also letters from George Henslow, 20 October 1876 and 24 October 1876, and letter to George Henslow, 23 October [1876]. Henslow published further on his thesis that self-fertilised plants were the fittest to survive in the struggle for life in Henslow 1877.
In Cross and self fertilisation, p. 366, CD noted that he had never seen an insect visit Fumaria officinalis (common fumitory), and that covered-up and unprotected plants produced an equal number of capsules.
Trifolium procumbens and T. procumbens var. minus are synonyms of T. campestre, field clover.
Nicotiana tabacum is the tobacco plant; N. rustica is Aztec tobacco.
In Cross and self fertilisation, p. 387, CD noted that species of plants with small and inconspicuous flowers nevertheless continued to produce some flowers that opened and could intercross, and concluded that crossing must therefore be of some benefit to them. However, he added that he had not experimented on such flowers, and, in a footnote, suggested that it would be useful to try some species of Solanum. Solanum nigrum is deadly nightshade.
Alfred William Bennett published on self-fertilisation in winter-flowering plants in Bennett 1869.
Corn spurry.
In Cross and self fertilisation, p. 420 n., CD described experiments in which the corolla was removed from various flowers, which none the less yielded seeds. CD was not surprised that self-fertilised flowers produced seed in these circumstances, but was surprised that Delphinium consolidum and Viola tricolor did so. In ibid., p. 358, CD listed V. tricolor and D. consolidum among plants that were sterile or significantly less fertile than usual when insects were excluded. See also Henslow 1876b.
Gardeners’ Chronicle, 4 November 1876, p. 588, contained a report of a white Lapageria (the Chilean bellflower) that had sent a shoot between a door jamb and a wall; the shoot flowered outside, although the flowers were smaller than usual.
Tradescantia erecta is widow’s tears or false dayflowers.
See n. 7, above.
Cerastium glomeratum is sticky mouse-ear chickweed or clammy chickweed. Corrigiola litoralis is strapweed.

Bibliography

Bennett, Alfred William. 1869. On the fertilisation of winter-flowering plants. Nature 1: 11–13.

Cross and self fertilisation: The effects of cross and self fertilisation in the vegetable kingdom. By Charles Darwin. London: John Murray. 1876.

Summary

Considers some flowers especially adapted for self-fertilisation, and believes all flowers are self-fertilising under some conditions. Gives examples of plants in which he believes all flowers are cleistogamous. Believes self-fertilisation is the primordial condition of flowering plants.

Letter details

Letter no.
DCP-LETT-8204
From
George Henslow
To
Charles Robert Darwin
Sent from
London, Bentinck Terrace, 7
Source of text
DAR 166: 149
Physical description
6pp †

Please cite as

Darwin Correspondence Project, “Letter no. 8204,” accessed on 3 June 2020, https://www.darwinproject.ac.uk/letter/DCP-LETT-8204.xml

Also published in The Correspondence of Charles Darwin, vol. 24

letter