Wednesday, April 07, 2004

Never Trust an Orchid Have a look at this interesting article from The New York Times. Orchids have long been a subject of fascination for evolutionary biologists. Darwin himself wrote a lengthy book describing the various adaptations they evolved for the purpose of attracting bees. One puzzle in this regard is the prevalence of orchids that do not contain any nectar. Bees are not the brightest creatures in the world, but surely they would not waste their time on flowers that give them no nutritional value. Darwin speculated that these flowers actually did produce nectar, but that the science of his day had not detected it.

It seems that Darwin was wrong on that one. New research, described in the article, show that flowers benefit from not providing nectar to bees. The point is that if the bee is receiving nectar from a particular flower on a larger plant, the bee will subsequently visit other flowers on the same plant. This results in inbreeding, which is not good from the plant's perspective. By not delivering nectar, these plants are encouraging the bees to fly elsewhere, thereby spreading their pollen to more receptive plants.

Bees are attracted to orchids by their smell, and they are capable of remembering that certain smells lead to plants with nectar while other smells do not. Why, then, do they continue to visit flowers that smell like they have nectar but actually do not? The answer, it seems, is that the individual flowers of a deceptive plant do not all have the same smell. This keeps the bee sufficiently confused, apparently.

Here's an excerpt from the article:

Dr. Schiestl notes, however, that while an individual orchid benefits when an insect is unsatisfied enough to fly off, somehow another plant of the same species has to be able to lure that insect in, or else the pollen it carries will never lead to fertilization and its ultimate evolutionary goal: a seed.

"Bees can learn and memorize the odor of an individual flower to decide what it will visit next," he said. "So if it encounters a deceptive flower, it will avoid flowers that smell similarly." So how have green-winged orchids, which any right-thinking pollinator would avoid after the first pointless visit, managed to persist?

The answer appears to be further deceptions still.

Dr. Schiestl said that in a new study, he and colleagues found that a closely related orchid that produced nectar had flowers that all smelled enticingly like one another. The smells of individual flowers of the deceptive Anacamptis morio species, however, differ widely, providing a moving target to the hapless bees trying to avoid being tricked yet again and ensuring confusion, along with a healthy crop of green-winged orchids next year.


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