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Letter 4840

Leighton, W. A. to Darwin, C. R.

29 May 1865

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    Describes the floral structure of the lupin and gives his observations on its pollination by bees.

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Luciefelde | Shrewsbury

May 29. 1865.

My dear Sir,

Noticing the operations of a small Humble Bee, about the size of a horse bean, on the flowers of Lupinus polyphyllus, I was induced to examine more particularly the blossoms. I found that if the blossom be examined before the standard is reflexed, that the tube of stamens bears alternately five very large sagittate anthers, and five very small (comparatively) rounded or oblong anthers. (This is noticed by Vaucher)   The sagittate-anthered stamens are about 23rds. the length of the pistil—the pollen of their anther-cells is mature and is even now falling away. On the contrary the small oblong-anthered stamens reach only in length to the base of the sagittate anthers & are about 13rd. the length of the pistil. Moreover their pollen is not ready and their anther-cells are closed.

[DIAG HERE]

Examining the stigma I could not discover that as yet, any pollen was deposited on it. Nor did the microscope disclose any appreciable difference in the size or form of the pollen grains of the two sets of anthers.

Now if we examine the blossom after the standard has become reflexed, we find that the large sagittate-anthered stamens still maintain their relative length with the pistil—that their pollen has fallen & the anther cells & the entire stamens are withering away. But that the shorter stamens have become elongated to the length of the pistil,—four of them having their upper portions entirely covered with pollen—all trace of the anther cell having disappeared—and the 5th. or that opposite to the standard having withered away. The stigma also is now observed to have pollen grains on it.

When the blossoms were thus fully matured, I observed that the humble bee alighting on the alæ, bore them downwards by his weight, and in so doing at the same time caused a slight downward movement in the closed keel, whereby the pollen was extruded through the apex, and that the extruded pollen coming in consequence into contact with the body of the bee as he inserted his proboscis to the base of the stamens, became attached to it, & was carried away by him to other blossoms.

The following questions readily suggest themselves:— What becomes of the immense quantity of pollen of the large sagittate anthers? It can scarcely be useless— is it carried by insects or otherwise to fertilize other individual plants possessing pistils of similar relative length? Is the plant really fertilized by the shorter stamens when elongated to an equal relative length with the pistil, <an>d is this fertilization effected by the agency of the bee?

The few plants in my garden presenting similar structure, I am unable to carry this investigation further. I therefore venture to send it to you, though in doing so I feel that I may be incurring the risk of ``sending coals to Newcastle''

Believe me, my dear Sir | Yours most truly | Willm. Allport Leighton

Charles Darwin Esq.

    Footnotes Add

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    f1 4840.f1
    The reference is to Jean-Pierre-Etienne Vaucher's Histoire physiologique des plantes d'Europe (Vaucher 1841), 2: 213, which CD read in 1862 (see Correspondence vol. 10). There is an annotated copy of Vaucher 1841 in the Darwin Library--CUL; CD noted the observation that lupins exhibit two sorts of anthers in the margin (see Marginalia 1: 813). There are notes on the different sets of anthers in Lupinus nanus, dated 5 August 1862, in DAR 76: B92.
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    f2 4840.f2
    Leighton had been a boyhood friend of CD's and had attended botanical lectures and excursions with CD at Cambridge University; he and CD occasionally corresponded on botanical subjects (see, for example, Correspondence vol. 10, and Leighton's memoir of CD in DAR 112: B97--8). CD did not publish the observations in this letter and no reply to it has been found. However, CD made notes on Leighton's information, remarking both that self-fertilisation was inevitable, and that the pollen was in `enormously greater quantity' than required for self-pollination (see DAR 76: B94). CD also preserved a newspaper cutting from the Reader, 10 February 1866, p. 154, that reported on Leighton's short article on pollination in Lupinus polyphyllus (Leighton 1866; the cutting is DAR 76: B93). CD had observed pollination in Lupinus, a genus in the family Leguminosae, in the garden at Maer Hall, Staffordshire, during the 1840s (see CD's note in DAR 77: 62c; see also DAR 49: 3--15). CD was interested in pollination in Leguminosae, since the occurrence of self-pollination in several species in this family appeared to present a problem for his general view that plants were generally adapted to facilitate crossing between individuals (see Origin, p. 97, Natural selection, pp. 68--71, and Correspondence vols. 4, 6, and 7). CD's later experiments with L. luteus and L. pilosus indicated that although these species seeded freely when self-pollinated, crossing was nevertheless advantageous in that it conferred greater vigour on the seedlings of the second generation (see Cross and self fertilisation, pp. 147--50, 367, 396--7; CD's notes on these experiments are in DAR 78: 192--9).
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