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.

Angraecum sesquipedale, The fragrant Star of Bethlehem Orchid

Angraecum sesquipedale flowering at the Fuqua Orchid Center

Angraecum sesquipedale

By far the most-asked question from nighttime visitors to the Orchid Display House is 'What smells so good in here?'

Winter is the season for night-fragrant Angraecoid orchids, two in particular. The first is Angraecum eburneum superbum, which starts flowering for us in November. The second is one of our most famous orchids, Angraecum sesquipedale, aka Darwin's Orchid, the Star of Bethlehem Orchid or the Comet Orchid, opening this week.

Angraecum sesquipedale is fascinating for lots of reasons, but today's post is just about its dreamy almost narcotic fragrance. The Darwin connection I'll take up in another post.

Angraecum sesquipedale is completely scentless during the day, and our daytime visitors just nod politely and walk away when told that it produces one of the best scents in the orchid family. But wait!

As darkness gathers the orchid releases a beguiling combination of scents--lily, Gardenia and Nicotiana. Interestingly, the scent changes as the flower matures. As Roman Kaiser writes in The Scent of Orchids, this description "applies to the fully developed flower on its 3rd or 4th night after blossoming. During the first two  nights, the scent is rather mixed, with a fairly marked indole note. This latter compound is found in the majority of night-scented flowers."

Night-fragrant flowers often produce scents reminiscent of jasmine, honeysuckle, tuberose, lilies and Gardenia. In the perfume trade, these scents are known as 'white-floral', and have been reproduced in a number of great perfumes.

Night-fragrant orchids that produce these scents are often white and are highly attractive to nocturnal moths and, apparently, people.

The effect on our nighttime visitors is remarkable. Many seem oblivious to the dazzling lights display as they veer off into the darkness in search of the source of the intoxicating scent.