Tuesday, July 20, 2010

Marsh Helleborines - that might have delighted Darwin

A couple of weeks ago we had a short break on the North Norfolk coast, where the saw the most spectacular display of orchids I've ever encountered. The location was Wells Dell, a dune slack at the Wells-next-the-Sea end of the Holkham National Nature Reserve. There were several hundred southern marsh and spotted orchids but for me the stars were scores of marsh helleborines Epipactis palustris.

The flower shape in this species is particularly exotic - almost as elaborate as the tropical orchids in florists' shops.

Charles Darwin was intrigued by the pollination of marsh helleborines, and noted that the front half of the lip of the flower (known botanically as the labellum – the white platform that you can see in this picture) was joined to its rear half (the pink-striped, scoop-shaped bit) by a kind of elastic hinge. He hypothesised that the elasticity of this link tended to catapault bees upward as they left the flower - like a diver's springboard in a swimming pool - so that they contacted the stamens (pollinia) above and carried them away as they left. His eldest son William and some of his network of correspondents helped him investigate the flower’s structure and he reported their findings in the Annals and Magazine of Natural History, as follows:



Notes on the Fertilization of Orchids.
By CHARLES DARWIN, M.A., F.R.S., &c.
To the Editors of the Annals and Magazine of Natural History.

Fertilization of Epipactis palustris. My son, Mr. W. E. Darwin, has carefully observed for me this plant in the Isle of Wight. Hive-bees seem to be the chief agents in fertilization; for he saw about a score of flowers visited by these insects, many of which had pollen-masses attached to their foreheads, just above the mandibles. I had supposed that insects crawled into the flowers; but hive-bees are too large todo this; they always clung, whilst sucking the nectar, to the distal and hinged half of the labellum, which was thus pressed downwards. Owing to this part being elastic and tending to spring up, the bees, as they left the flowers, seemed to fly rather upwards; and this would favour, in the manner explained by me, the complete withdrawal of the pollen-masses, quite as well as an insect crawling out of the flower in an upward direction. Perhaps, however, this upward movement may not be so necessary as I had supposed; for, judging from the point at which the pollen-masses were attached to the bees, the back part of the head would press against, and thus lift up, the blunt, solid, upper end of the anther, thus freeing the pollen-masses.


Various other insects besides hive-bees visit this Epipactis. My son saw several large flies (Sarcophaga carnosa) haunting the flowers; but they did not enter in so neat and regular a manner as the hive-bees; nevertheless two had pollen-masses attached to their foreheads. Several smaller flies (Cœlopa frigida) were also seen entering and leaving the flowers, with pollen-masses adhering rather irregularly to the dorsal surface of the thorax. Three or four distinct kinds of Hymenoptera (one of small size being Crabro brevis) likewise visited the flowers; and three of these Hymenoptera had pollen-masses attached to their backs. Other still more minute Diptera, Coleoptera, and ants were seen sucking the nectar; but these insects appeared to be too small to transport the pollen-masses. It is remarkable that some of the foregoing insects should visit these flowers; for Mr. F. Walker informs me that the Sarcophaga frequents decaying animal matter, and the Cœlopa haunts seaweed, occasionally settling on flowers; the Crabro also, as I hear from Mr. F. Smith, collects small beetles (Halticæ) for provisioning its nest. It is equally remarkable, seeing how many kinds of insects visit this Epipactis, that, although my son watched for some hours on three occasions hundreds of plants, not a single humble-bee alighted on a flower, though many were flying about. In a footnote I have given the results of experiments made by Mr. More, by cutting off the distal and hinged half of the labellum, in order to ascertain how far this part is important.1 He has now repeated the experiment on nine additional flowers: of these, three did not produce seed-capsules; but this may have been accidental. Of six capsules which were produced, two contained about as many seeds as the capsules of unmutilated flowers on the same plant; but four capsules contained much fewer seeds. The seeds themselves were well-formed. These experiments, as far as they go, support the view that the distal part of the labellum plays an important part in leading insects to enter and leave the flower in a proper manner for fertilization.


Darwin, C. R. 1869. Notes on the fertilization of orchids. Annals and Magazine of Natural History (Ser. 4) 4 (September): 141-159.

I think the wonderful display of orchids at Wells Dell might have delighted Darwin. He formulated his ideas on this orchid's pollination - which are contentious - from the observations and experiments of others, but I can imagine him, down on his hands and knees, peering through a magnifying glass and watching insect visitors, finding out for himself.......

10 comments:

  1. Phil it's good to be back you are as entertaining and informative as ever.

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  2. Beautiful photos of the Marsh Helleborines and a very informative text. I'm intrigued with the beauty of orchids and I love searching for them,I suppose You could call me a bit of an orchid anarak.Through my interest in orchids a lifelong walking friend has also got the bug.

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  3. Fascinating post Phil. Enjoyed that.

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  4. That first photograph is an absolute work of art! Great post.

    cheers,
    Wilma

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  5. As you say the flowers do resemble some orchids in their construction.

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  6. Hi David, I know what you mean about orchids.... it's always a thrill to find one, even if it's only one of the commoner species. I caught the bug when I found a bee orchid on the Sussex South Downs when I was a kid..

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  7. Thanks Wilma, orchids like this are natural works of art....

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  8. Hi John, they are amazingly intricate..

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