Age and beauty: kin to both irises and onions, orchids have a long history and a large repertoire of enticing tricks.

Publication: Natural History

Publication Date: 01-JUN-04

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COPYRIGHT 2004 Natural History Magazine, Inc.

Ask a glamorous older woman her age and the secret to her beauty, and you're likely to get a Mona Lisa smile and a deft change of subject. Until recently, botanists have met with similar impenetrability when asking these questions about orchids, the glamour queens of the plant kingdom.

The orchid family--Orchidaceae--has a greater wealth of species than any other plant family on Earth: naturally occurring species number around 30,000, and artificially created hybrids in the tens of thousands. Most of them are epiphytes, growing with their roots not in soil but instead harmlessly clasping tree branches high in the forest canopy. A few are parasites; lacking chlorophyll, they extract the necessary nutrients from the organism on which they have made their home. One Australian genus spends its entire life underground. Orchids come in every color except black, and though few have any fragrance, the ones that do run the gamut from the scent of chocolate to that of carrion.

The astonishing diversity of these plants is matched only by the complexity and unconventionality, of their lifestyles. Orchids are so unlike other flowering plants, in fact, that they seem to live in a kind of splendid isolation from the great hierarchy of other organisms. Darwin wrote a book on them--On the various contrivances whereby British and foreign orchids are fertilised by insects, and on the good effects of intercrossing. The hook served as a kind of sequel to his Origin of Species, and was intended to clarify certain points crucial to the theory of natural selection. But only quite recently--and only because of the advent of powerful molecular techniques such as genetic sequencing--have plant biologists been able to reconstruct the history of the family to which these alluring flowers belong.

Darwin argued that natural selection cannot take place unless organisms cross with other individuals. The reason he gave is that the survival of individuals best adapted to prevailing ecological conditions--often called "survival of the fittest"--depends on the existence of a broad spectrum of characteristics to meet whatever those conditions throw at the individuals of a species. Sexual reproduction, with its radical reshuffling of genes in each new generation, gives rise to that variety.

Most plants--particularly the angiosperms, or flowering plants--possess both male and female parts, and so they can, in principle, fertilize themselves. The fact that they do not--indeed, that they have evolved a wide range of strategies for preventing self-fertilization--seems to support Darwin's reasoning. In his book on orchids he documents the elaborate frills and furbelows, gimmicks and traps, that lure and...

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