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Wednesday, March 27, 2013

Orchids with Spurs

Aerangis biloba, native to Africa
As much as I love Darwin's Orchid (Angraecum sesquipedale), that handsome fellow with the floral spurs, I must confess that I think its first and second cousins are cuter.

Angraecum arachnites from Madagascar
Tiny Angraecum arachnites and Aerangis biloba have been flowering for the last few weeks in the Orchid Display House. And they are irresistible. Completely absent is the 'I dare you to look away' magnetism of Angraecum sesquipedale. Likewise the overpowering fragrance (present only at night, how obvious). You have to search for these little guys.

The spur on this flower is sharply bent at the lower end
And look; each tiny flower has a long floral spur, too. The spur is a hollow tube with nectar at its furthest end--designed to tempt hungry hawkmoths.

So, what's up with the spur? Lots of flowers, like Impatiens, have nectaries. Why are these so long? The amazingly long spurs of some orchids in the subtribe Angraecinae is the result of coevolution between these orchids and long-tongued hawkmoths.

Jumellea arachnantha from the Comoro Islands
How does this happen? Pollination in Angraecum sesquipedale is more likely to be successful when the flower's spur is a little longer than the moth's tongue. Only when the moth pushes its proboscis deeply into the spur to reach the last bit of nectar does the pollinarium (the pollen masses and sticky disk) become attached to the base of the moth's tongue. Charles Darwin discovered this experimentally in the 1860's. He reasoned that moths with longer tongues are better fed and thus more likely to leave descendants than shorter tongued moths; orchids with longer spurs are likewise more likely to pass on their genes. Over many generations the orchid and pollinator become locked into a trend (sometimes described as a race) selecting for ever-increasing tongue and spur length.*

Darwin's Orchid and its pollinator are an extreme example of coevolutionary specialization--demonstrated by the almost absurd length of spur (up to 32 cm in Angraecum sesquipedale) and tongue (up to 25 cm Xanthopan morgani ssp. praedicta).

Extreme coevolutionary specialization is rare and only happens over a very long period of time in an isolated and stable environment. As Darwin noted presciently, it almost certainly puts both plant and pollinator at risk of extinction if environmental conditions change.

And conditions are changing. Should we be concerned? Are our orchids with spurs vulnerable? Absolutely. All orchids are dependent upon other organisms (pollinators, and mycorrhizal fungi for example) for their survival--our orchids with spurs are simply among the more vulnerable.

Recent studies indicate that this process may be more complicated than Darwin imagined--the coevolutionary "race" between a plant and a single pollinator species may actually involve a sequential shift among a series of pollinators.


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