RECORD: Darwin, C. R. 1860. Fertilisation of British orchids by insect agency. Gardeners' Chronicle and Agricultural Gazette no. 23 (9 June): 528.
REVISION HISTORY: Scanned, OCRed, corrected and edited by John van Wyhe 2002-8, textual corrections by Sue Asscher 1.2007. RN3
FERTILISATION OF BRITISH ORCHIDS BY INSECT AGENCY.
I SHOULD be extremely much obliged to any person living where the Bee or Fly Orchis is tolerably common, if he will have the kindness to make a few simple observations on their manner of fertilisation.1 To render the subject clear to those who know nothing of botany, I must briefly describe what takes place in our common British Orchids. The pollen-grains form two pear-shaped masses; each borne on a foot-stalk, with a sticky gland at the end. The pollen masses are hidden in little pouches open in front. When an insect visits a flower, it almost necessarily, owing to the position of the parts, uncovers and touches the sticky glands. These firmly adhere to the head or body of the insect, and thus the pollen-masses are drawn out of their pouches, are dragged over the humid stigmatic surface, and the plant is fertilised. So beautifully are the relative degrees of adhesiveness of the gland, and of the grains of pollen to each other and to the stigmatic surface mutually adapted, that an insect with an adherent pollen-mass will drag it over the stigmas of several flowers, and leave granules of pollen on each. The contrivances by which the sticky glands are prevented from drying, and so kept always viscid and ready for action, is even still more curious; they lie suspended (at least in the two species which I have examined) in a little hemispherical cup, full of liquid, and formed of such delicate membrance, that the side projecting over the gangway into the nectary is ruptured transversely and depressed by the slightest touch; and then the glands, sticky and fresh out of their bath, immediately and almost inevitably come into contact with and adhere to the body which has just ruptured the cup. It is certain that with most of our common Orchids insects are absolutely necessary for their fertilisation; for without their agency, the pollen-masses are never removed and wither within their pouches. I have proved this in the case of Orchis morio and mascula by covering up plants under a bell-glass, leaving other adjoining plants uncovered; in the latter I found every morning, as the flowers became fully expanded, some of the pollen masses removed, whereas in the plants under the glass all the pollen-masses remained enclosed in their pouches.2
Robert Brown, however, has remarked that the fact of all the capsules in a dense spike of certain Orchids producing seed seems hardly reconcileable with their fertilisation having been accidentally effected by insects.3 But I could give many facts showing how effectually insects do their work; two cases will here suffice; in a plant of Orchis maculata with 44 flowers open, the 12 upper ones, which were not quite mature, had not one pollen-mass removed, whereas every one of the 32 lower flowers had one or both pollen-masses removed; in a plant of Gymnadenia conopsea with 54 open flowers, 52 had their pollen-masses removed. I have repeatedly observed in various Orchids grains of pollen, and in one case three whole pollen-masses on the stigmatic surface of a flower, which still retained its own two pollen-masses; and as often, or even oftener, I have found flowers with the pollen-masses removed, but with no pollen on their stigmas. These facts clearly show that each flower is often, or even generally, fertilised by the pollen brought by insects from another flower or plant. I may add that after observing our Orchids during many years, I have never seen a bee or any other diurnal insect (excepting once a butterfly) visit them; therefore I have no doubt that moths are the priests who perform the marriage ceremony. The structure, indeed, of some Orchids leads to this same conclusion; for no insect without a very long and extremely fine proboscis could possibly reach the nectar at the bottom of the extremely long and narrow nectary of the Butterfly-Orchis; and entomologists have occasionally captured moths with pollen-masses adhering to them. If any entomologist reads this, and can remember positively having caught a moth thus furnished, I hope he will give its name, and describe exactly to which part of the moth's body the sticky gland adhered.
We may now turn to the genus Ophrys; in the Fly Orchis (Ophrys muscifera), the pollen-masses, furnished with sticky glands, do not naturally fall out of their pouches, nor can they be shaken out; so that insect-agency is necessary, as with the species of the other genera, for their fertilisation. But insects here do their work far less effectually than with common Orchids; during several years, previously to 1858, I kept a record of the state of the pollen-masses in well-opened flowers of those plants which I examined, and out of 102 flowers I found either one or both pollen-masses removed in only 13 flowers. But in 1858 I found 17 plants growing near each other and bearing 57 flowers and of these 30 flowers had one or both pollen-masses removed; and as all the remaining 27 flowers were the upper and younger flowers, they probably would subsequently have had most of their pollen-masses removed, and thus have been fertilised. I should much like to hear how the case stands with the Fly Orchis in other districts; for it seems a strange fact that a plant should grow pretty well, as it does in this part of Kent, and yet during several years seldom be fertilised.
We now come to the Bee Orchis (Ophrys apifera), which presents a very different case; the pollen masses are furnished with sticky glands, but differently from in all the foregoing Orchids, they naturally fall out of their pouches; and from being of the proper length, though still retained at the gland-end, they fall on the stigmatic surface, and the plant is thus self-fertilised. During several years I have examined many flowers, and never in a single instance found even one of the pollen-masses carried away by insects, or ever saw the flower's own pollen-masses fail to fall on the stigma. Robert Brown consequently believed that the visits of insects would be injurious to the fertilisation of this Orchis; and rather fancifully imagined that the flower resembled a bee in order to deter their visits. We must admit that the natural falling out of the pollen-masses of this Orchis is a special contrivance for its self-fertilisation; and as far as my experience goes, a perfectly successful contrivance, for I have always found this plant self-fertilised; nevertheless a long course of observation has made me greatly doubt whether the flowers of any kind of plant are for a perpetuity of generations fertilised by their own pollen. And what are we to say with respect to the sticky glands of the Bee Orchis, the use and efficiency of which glands in all other British Orchids are so manifest? Are we to conclude that this one species is provided with these organs for no use? I cannot think so; but would rather infer that, during some years or in some other districts, insects do visit the Bee Orchis and occasionally transport pollen from one flower to another, and thus give it the advantage of an occasional cross. We have seen that the Fly Orchis is not in this part of the country by any means sufficiently often visited by insects, though the visits of insects are indispensable to its fertilisation. So with the Bee Orchis, though its self-fertilisation is specially provided for, it may not exist here under the most favourable conditions of life; and in other districts or during particular seasons it may be visited by insects, and in this case, as its pollen masses are furnished with sticky glands, it would almost certainly receive the benefit of an occasional cross impregnation. It is this curious apparent contradiction in the structure of the Bee Orchis—one part, namely the sticky glands, being adapted for fertilisation by insect agency—another part, namely the natural falling out of the pollen-masses, being adapted for self-fertilisation without insect agency—which makes me anxious to hear what happens to the pollen-masses of the Bee Orchis in other districts or parts of England. I should be extremely much obliged to any one who will take the trouble to observe this point and to communicate the result to the Gardeners' Chronicle or to me. Charles Darwin, Down, Bromley, Kent.
2 See Natural selection, pp. 65-6.
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Citation: John van Wyhe, editor. 2002-. The Complete Work of Charles Darwin Online. (http://darwin-online.org.uk/)
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