Angraecum sesquipedale

Thank you to Brian aka aeranthes@Flickr for sharing the photograph today (original image via BPotD Flickr Pool). The illustration is by John Nugent Fitch, and is now a public domain work after having been originally published in the 1882-1897 publication, The Orchid Album (link to additional illustrations).

Angraecum sesquipedale has a bevy of common names, including Star of Bethlehem orchid, comet orchid and Darwin’s orchid. The latter name is a reference, of course, to Charles Darwin, who wrote the following in 1862 in On the various contrivances by which British and foreign orchids are fertilised by insects, and on the good effects of intercrossing.:

“I fear that the reader will be wearied, but I must say a few words on the Angræcum sesquipedale, of which the large six-rayed flowers, like stars formed of snow-white wax, have excited the admiration of travellers in Madagascar. A whip-like green nectary of astonishing length hangs down beneath the labellum. In several flowers sent me by Mr. Bateman I found the nectaries eleven and a half inches long, with only the lower inch and a half filled with very sweet nectar. What can be the use, it may be asked, of a nectary of such disproportional length? We shall, I think, see that the fertilisation of the plant depends on this length and on nectar being contained only within the lower and attenuated extremity. It is, however, surprising that any insect should be able to reach the nectar: our English sphinxes have probosces as long as their bodies: but in Madagascar there must be moths with probosces capable of extension to a length of between ten and eleven inches!”

“The rostellum is broad and foliaceous, and arches rectangularly over the stigma and over the orifice of the nectary: it is deeply cleft, with the cleft enlarged or widened at the end. Hence the rostellum pretty closely resembles that of Calanthe after the disc has been removed. The under surfaces of both margins of the cleft near its end are bordered by narrow strips of viscid membrane, easily removed; so that there are two distinct viscid discs. To the middle of each disc a short membranous pedicel is attached; and each pedicel carries at its other end a pollen-mass. Beneath the rostellum a narrow, ledge-like, viscid stigma is seated.”

“I could not for some time understand how the pollinia of this Orchid were removed, or how it could be fertilised. I passed bristles and needles down the open entrance into the nectary and through the cleft in the rostellum with no result. It then occurred to me that, from the length of the nectary, the flower must be visited by large moths, with a proboscis thick at the base; and that to drain the last drop of nectar even the largest moth would have to force its proboscis as far down as possible. To effect this, whether or not the moth first inserted its proboscis by the open entrance into the nectary (as is most probable, from the shape of the flower) or through the cleft in the rostellum, it would ultimately force its proboscis into this cleft, for this is the straightest course, and by slight pressure the whole foliaceous rostellum can be depressed: the distance from the outside of the flower to the extremity of the nectary can be thus shortened by about a quarter of an inch. Hence I took a cylinder, one-tenth of an inch in diameter, and pushed it down through the cleft in the rostellum: the margins readily separated, and were pushed downwards together with the whole rostellum. When I slowly withdrew the cylinder the rostellum rose from its elasticity, and the margins of the cleft upturned and clasped the cylinder. Thus the viscid strips of membrane on the under sides of the cleft rostellum came into contact with the cylinder, and firmly adhered to it; and the pollen-masses were withdrawn. By this means alone I succeeded in each case in withdrawing the pollinia; and it cannot, I think, be doubted that a large moth must thus act; namely, by driving its proboscis up to the very base, through the cleft of the rostellum, so as to reach the extremity of the nectary; and then withdrawing its proboscis with the pollinia attached to it.”

“I did not succeed in imitating the fertilisation of the flower so well as I did in withdrawing the pollinia, but I effected it twice. As the margins of the cleft rostellum must be upturned before the discs adhere to the cylinder, they become, during its withdrawal, affixed some little way from its actual base. The two discs did not always adhere at exactly corresponding points. Now, when a moth inserts its proboscis, with the pollinia affixed to it near the base, into the mouth of the nectary, the pollen-masses will probably be first inserted beneath the rostellum; and during the final exertion, when the moth pushes its proboscis through the cleft of the rostellum, the pollen-masses will almost necessarily be placed on the narrow, ledge-like stigma projecting beneath the rostellum. By acting thus with the pollinia attached to the cylinder the pollen-masses were twice torn off and left glued to the stigmatic surface.”

“If the Angræcum in its native forests secretes more nectar than did the vigorous plants sent me by Mr. Bateman, so that the nectary becomes filled, small moths might obtain their share, but they would not benefit the plant. The pollinia would not be withdrawn until some huge moth, with a wonderfully long proboscis, tried to drain the last drop. If such great moths were to become extinct in Madagascar, assuredly the Angræcum would become extinct. On the other hand, as the nectar, at least in the lower part of the nectary, is stored safe from depredation by other insects, the extinction of the Angræcum would probably be a serious loss to these moths. We can thus partially understand how the astonishing length of the nectary may have been acquired by successive modifications. As certain moths of Madagascar became larger through natural selection in relation to their general conditions of life, either in the larval or mature state, or as the proboscis alone was lengthened to obtain honey from the Angræcum and other deep tubular flowers, those individual plants of the Angræcum which had the longest nectaries (and the nectary varies much in length in some Orchids), and which, consequently, compelled the moths to insert their probosces up to the very base, would be fertilised. These plants would yield most seed, and the seedlings would generally inherit longer nectaries; and so it would be in successive generations of the plant and moth. Thus it would appear that there has been a race in gaining length between the nectary of the Angræcum and the proboscis of certain moths; but the Angræcum has triumphed, for it flourishes and abounds in the forests of Madagascar, and still troubles each moth to insert its proboscis as far as possible in order to drain the last drop of nectar.”

Darwin’s 1862 prediction of a coevolved moth with a proboscis of 35cm was ridiculed by some as being impossible. Others suggested the long nectaries were proof of supernatural creation (read: Darwin’s Madagascan Hawk Moth Prediction (PDF)). Wallace and Darwin responded by detailing how evolutionary processes could develop both a long-nectaried orchid species and a co-evolved moth. Though Darwin did not live to see it, in 1903 a moth was discovered in Madagascar with the characteristics as predicted by Darwin.

For more reading / photographs:

Happy Birthday, Charles.

Angraecum sesquipedale
Angraecum sesquipedale

18 responses to “Angraecum sesquipedale”

  1. Eric in SF

    One of the most regal and stunning orchid species on the planet.
    I need to dig up my old old film photo of a 6 foot tall, multi-dozen flowered specimen of this plant I encountered at my very first big orchid show, the Pacific Orchid Exposition, back in 2001.
    Here are some more great links on the moth that pollinates this species:
    http://www.flickr.com/photos/kqedquest/3256354461/
    http://www.pbs.org/wnet/nature/episodes/deep-jungle-new-frontiers/video-darwins-moth/1374/

  2. Old Ari

    interesting, the original has one bloom, where the illustrator has produced a lawyer complete with wig.

  3. Bonnie

    It took a moment, but I see the lawyer! Also a girl in a bonnet, and something in a beard.
    But I mustn’t lose sight of Darwin’s fascinating prediction of the moth! Too cool!

  4. Jean

    A super entry. Great to read a long passage by this most inspiring scientist, and what a story!

  5. Sue in Bremerton

    The words of Charles Darwin so very much impressed me. His attention to the detail of what he was looking for and how he went about it are amazing. He must have had remarkable thought processes. He went in my people to respect list a lot nearer the top instead of close to the bottom of it.
    Thank you so much.

  6. Stuart

    You can see moth actually unfurling its proboscis and drinking the nectar in this youtube video.

  7. Suzanna

    That video was amazing!!! Thank you. What a treat.

  8. Eric in SF

    Thanks, Stuart, that’s the video I was looking for and couldn’t find it!

  9. Ann Kent

    Awesome entry, comments, and further links. Appreciate the opportunity to learn from text, traditional illustration, photo, and video. Thank you everyone. Ann

  10. Viola

    WOW! Extraordinary in every way. Thank you for sharing this amazing story, Daniel, Eric, and Stuart.

  11. Anne

    All of this information makes me so grateful for scientists. I never doubt that there are amazing things like this story out there but to actually see the thought process and the vindication….amazing.

  12. elizabeth a airhart

    oh my we are so lovely and do come up
    and see some time – are we not
    i did print out the write up i can read then
    follow all the links– thank you
    botanical drawings are so important they
    are what we have before eric came along
    with his camera –
    you tube has ever so many films about flowers
    the time lapses are quite good
    thank you all really thank you all

  13. Don Fenton

    My favourite flower! About 40 years or so ago, I entered a plant of this in an agricultural show; on the day, however, the flower had died, finished, lost substance and browned a bit. I put it in anyway, because few people would have had the opportunity to see such a thing, and won a 1st Prize!

  14. Brian O'Brien

    See this link for a pollination video of a far-flung (south Florida) relative (the Ghost Orchid) of Angraecum sesquipedale that exhibits a similar pollination syndrome (sphinx moth, night-fragrant white flower).

  15. elizabeth a airhart

    thank you for the link brian-fine films

  16. mar evangelista

    very dainty flower. . . so attrective

  17. Sheila

    Fascinating to read Darwins own words.
    He was a truly remarkable man.

  18. Deborah Lievens

    I love this stuff!

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