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Darwin Correspondence Project

To Gardeners’ Chronicle   18 October [1857]1

Mr. Swayne in the 5th volume of the Horticultural Transactions incidentally speaks of the advantage of artificially fertilising the early Bean.2 Can you tell me to what sort of Bean he refers? (We presume to the Early Mazagan; but we have no special information.)3 and who has followed this plan, and how has it been effected? My motive for asking is as follows: every one who has looked at the flower of the Kidney Bean must have noticed in how curious a manner the pistil with its tubular keel-pistil curls like a French horn to the left side—the flower being viewed in front. Bees, owing to the greater ease with which they can reach the copious nectar from the left side, invariably stand on the left wing-petal; their weight and the effort of sucking depresses this petal, which, for its attachment to the keel-petal, causes the pistil to protrude. On the pistil beneath the stigma there is a brush of fine hairs, which when the pistil is moved backwards and forwards, sweeps the pollen already shed out of the tubular and curled keel-petal, and gradually pushes it on to the stigma. I have repeatedly tried this by gently moving the wing petals of a lately expanded flower. Hence the movement of the pistil indirectly caused by the bees would appear to aid in the fertilisation of the flower by its own pollen; but besides this, pollen from the other flowers of the Kidney Bean sometimes adheres to the right side of the head and body of the bees, and this can scarcely fail occasionally to be left on the humid stigma, quite close to which, on the left side, the bees invariably insert their proboscis. Believing that the brush on the pistil, its backward and forward curling movement, its protrusion on the left side, and the constant alighting of the bees on the same side, were not accidental coincidences, but were connected with, perhaps necessary to, the fertilisation of the flower, I examined the flowers just before their expansion. The pollen is then already shed; but from its position just beneath the stigma, and from its coherence, I doubt whether it could get on the stigma, without some movement of the wing petals; and I further doubt whether any movement, which the wind might cause, would suffice. I may add that all which I have here described occurs in a lesser degree with Lathyrus grandiflorus. To test the agency of the bees, I put on three occasions a few flowers within bottles and under gauze: half of these I left quite undisturbed; of the other half I daily moved the left wing-petal, exactly as a bee would have done whilst sucking. Not one of the undisturbed flowers set a pod, whereas the greater number (but not all) of those which I moved, and which were treated in no other respect differently, set fine pods with good seeds.4 I am aware that this little experiment ought to have been repeated many times; and I may be greatly mistaken, but my belief at present is, that if every bee in Britain were destroyed, we should not again see a pod on our Kidney Beans. These facts make me curious to know the meaning of Mr. Swayne’s allusion to the good arising from the artificial fertilisation of early Beans. I am also astonished that the varieties of the Kidney Bean can be raised true when grown near each other. I should have expected that they would have been crossed by the bees bringing pollen from other varieties; and I should be infinitely obliged for any information on this head from any of your correspondents. As I have mentioned bees, a little fact which surprised me may be worth giving:—One day I saw for the first time several large humble-bees visiting my rows of the tall scarlet Kidney Bean; they were not sucking at the mouth of the flower, but cutting holes through the calyx, and thus extracting the nectar. I watched this with some attention, for though it is a common thing in many kinds of flowers to see humble-bees sucking through a hole already made, I have not very often seen them in the act of cutting.5 As these humble-bees had to cut a hole in almost every flower, it was clear that this was the first day on which they had visited my Kidney Beans. I had previously watched every day for some weeks, and often several times daily, the hive-bees, and had seen them always sucking at the mouth of the flower. And here comes the curious point: the very next day after the humble-bees had cut the holes, every single hive bee, without exception, instead of alighting on the left wing-petal, flew straight to the calyx and sucked through the cut hole; and so they continued to do for many following days. Now how did the hive-bees find out that the holes had been made? Instinct seems to be here out of the question, as the Kidney Bean is an exotic. The holes could scarcely be seen from any point, and not at all from the mouth of the flower, where the hive-bees hitherto had invariably alighted. I doubt whether they were guided by a stronger odour of the nectar escaping through the cut holes; for I have found in the case of the little blue Lobelia, which is a prime favourite of the hive-bee, that cutting off the lower striped petals deceived them; they seem to think the mutilated flowers are withered, and they pass them over unnoticed. Hence I am strongly inclined to believe that the hive-bees saw the humble-bees at work, and well understanding what they were at, rationally took immediate advantage of the shorter path thus made to the nectar.6 C. Darwin, Down, Bromley, Kent, Oct. 18.

Footnotes

The year is given by the date of publication in Gardeners’ Chronicle and Agricultural Gazette.
Swayne 1824.
This sentence was added (in square brackets) by the editor of Gardeners’ Chronicle.
CD’s notes on this experiment are in DAR 49: 48.
CD had previously observed hive-bees boring holes in flowers and had written on this to the Gardeners’ Chronicle in 1841 (see Correspondence vol. 2, letter to Gardeners’ Chronicle, [16 August 1841]).
CD also discussed this subject in Natural selection, pp. 475–6.

Summary

Describes his experiments with kidney beans to test the agency of bees in their fertilisation. His results suggest they are essential.

Asks what George Swayne could mean by the advantage of artificial fertilisation of early beans [Trans. Hortic. Soc. Lond. 5 (1824): 208–13].

Has observed that hive-bees, which normally suck nectar from the flower of the kidney bean, will use holes cut through the calyx by humble-bees, though the holes cannot be seen from the mouth of the flower. Suggests hive-bees see humble-bees at work and understand what they are doing and "rationally" take advantage of the shorter path to the nectar. [See also 2359.]

Letter details

Letter no.
DCP-LETT-2155
From
Charles Robert Darwin
To
Gardeners’ Chronicle
Sent from
Down
Source of text
Gardeners’ Chronicle and Agricultural Gazette, 24 October 1857, p. 725

Please cite as

Darwin Correspondence Project, “Letter no. 2155,” accessed on 22 August 2019, http://www.darwinproject.ac.uk/DCP-LETT-2155

Also published in The Correspondence of Charles Darwin, vol. 6

letter