# From George Henslow   6 November 1865

10 South Crescent | Bedford Sq: W.C.

Nov 6/65

My dear Sir,

I beg to thank you very much for yr kind & instructive letter:1 there are one or two passages in it which,—with your permission I should like to read in connection with a few notes I purposing giving at the next Linnean Meeting.2

I should much like to read Sprengels account of Salvia, & am glad to find my surmise to have been corroborrated by yourself. (Would you kindly give me the reference to Sprengls paper?)3 With regard to Medicago, perhaps the word Irritability is not exactly the true one. Elasticity also does not express the facts which are these. The stamens are at first straight & horizontal included within the alæ & Carina; the former of which have little processes overlying the stamens & pressing down upon them: this, they are able to do by being firmly locked with the carina.4

The instant any object, e.g. a pin, is directed down the channel of the Vexillum, it cannot help pressing upon these “processes” laterally. The stamens then spring up & the carina spreads out &, with the alæ, drop. But the stamens not only rise up but immeadiately become curved towards the Vexillum in the form of a $\frac{1}{2}$ circle; so that there seems to me to be some other force required, besides mere elasticity, especially as this would imply the possibility of making them return to their original horizontal position: which they will not do, but crack transversely if it be attempted. Having however once assumed their ‘curved’ ‘rigid’ & ‘vertical’ position it cannot be altered.5

I have read Cohn’s paper & have been much interested in it, especially as I had been, also, examining some Centaureas,6 & without knowing anything had been previously observed, thought I saw some peculiarity, but I found it was simply elasticity. I did not discover the independent irritability.7 My own conclusion was this;—that as the style rises & stretches the anthers to their full extent by pushing against their closed valvate extremities, until it succeeds in forcing its way through,—so on the other hand, the instant the style is through, the tension of the filaments is relaxed & the anthers are drawn down again. So that two opposing forces are brought into play, the object being to enable the ring of hairs (if present) more effectually to sweep out the pollen, & so elevate on the outside of the two stigmatic ends which now & not till now, separate.

I presumed all this was intimately connected with intercrossing.8

Many thanks for your kind offer of giving me further information; which, I should like to avail myself off at another time.

Yours very truly | Geo. Henslow

P.S. I beg to thank you for the printed matter which I will return9

## Footnotes

Henslow cited CD’s observations of Medicago lupulina and other species in his paper delivered at the Linnean Society on 16 November 1865 (G. Henslow 1865; see letter from George Henslow, 1 November 1865 and n. 5).
See letter from George Henslow, 1 November 1865. In the missing portion of his letter of [2–5 November 1865], CD evidently referred Henslow to the work of Christian Konrad Sprengel, who argued that floral structures were designed to bring about pollination by insects (Sprengel 1793); Sprengel described the anther movements of Salvia pratensis, which enabled pollen to be transferred from different flowers on the backs of bees, in ibid., p. 61. CD scored this passage in his extensively annotated copy of the book, which is in the Darwin Library–CUL (see Marginalia 1: 774–85). CD referred to Sprengel’s description of ‘the admirable mechanical adaptations in [Salvia] for favouring or ensuring cross-fertilisation’ in Cross and self fertilisation, p. 93 n. See also Correspondence vol. 3, letter to J. D. Hooker, [11–12 July 1845] and n. 13, and Correspondence vol. 12, letter to Friedrich Hildebrand, 25 June [1864] and n. 9.
The carina is the structure formed by the two anterior petals of a papilionaceous flower. For the other botanical terms, see the letter from George Henslow, 1 November 1865 and n. 3.
Henslow said that he had been unable to discover whether the curvature of the stamens was caused ‘by distention of the cells on the convex side or a corresponding emptying of those on the concave’ (G. Henslow 1865, pp. 327–8).
Ferdinand Julius Cohn’s paper on the contractile tissue of plants (Cohn 1860) was based principally on observations of the movements of stamens in Centaurea. CD’s heavily annotated copy is in the Darwin Pamphlet Collection–CUL. On CD’s interest in Cohn’s work, see the letter from T. H. Huxley, 1 May 1865, n. 7.
Cohn described how, on contact, the stamens of Centaurea first contracted, then began to extend themselves and acquire a curved condition similar to that which occurs in an irritated muscle; Cohn claimed that the movement of the plant was not due to mechanical elasticity, but to an ‘expansive Kraft’ (expansive force) and ‘selbst fortgeleitete Kraft’ (innate motor power) (Cohn 1860, pp. 4–6, 39–40, 47–8). A long abstract of the paper was published in English in the Annals and Magazine of Natural History 11 (1863): 188–202. CD cited this abstract in Insectivorous plants, pp. 256–7.
Cohn observed that insects first lighted on the flowers, causing the contraction of the stamens and the consequent extrusion of pollen from the anthers; the insects then carried the pollen to other flowers, thereby effecting cross-pollination (Cohn 1860, pp. 44–5, 48).
Henslow may refer to a copy of CD’s letter to the Gardeners’ Chronicle, [before 13 November 1858] (Correspondence vol. 7), on the agency of bees in the fertilisation of papilionaceous flowers. See letter from George Henslow, 1 November 1865 and n. 9. See also letter from George Henslow, 2 December 1865.

## Summary

Pleased CD confirms his observations on Salvia.

Spring action of Medicago stamens described.

## Letter details

Letter no.
DCP-LETT-4931
From
George Henslow
To
Charles Robert Darwin
Sent from
London, South Crescent, 10
Source of text
DAR 166: 151
Physical description
7pp

Darwin Correspondence Project, “Letter no. 4931,” accessed on 17 December 2017, http://www.darwinproject.ac.uk/DCP-LETT-4931

Also published in The Correspondence of Charles Darwin, vol. 13

letter