Worked out by a team led by scientists from the Royal Botanic Gardens at Kew, south-west London, it has caused a complete rethink of the relationships between many plant families. It shows, for example, that the closest relative of the lotus, the sacred flower of Buddhism, is not the water lily it so much resembles but the the smog-resistant plane tree of London's squares.
When it is published next month in the Annals of the Missouri Botanical Garden, the classification, which for the first time establishes the relationships of all plant families through their genetic material, will do away with 200 years of previous plant taxonomy dating back to Linnaeus. This has hitherto been based on flowers' and trees' morphology - their appearance and visual characteristics, such as how many leaves or petals they have.
But the ability to examine plants at the molecular level, which has become available on a large scale only in the 1990s, makes clear that many of the relationships botanists previously assumed from morphology are wrong.
There are many surprises in the new classification. The papaya is not related to the passion flower, as was previously thought - its closest relative is the cabbage family; roses are closely related to blackthorns, nettles and figs; and peonies are not related to buttercups but to the saxifrage family.
The classification is the work of nearly 100 scientists led by Mark Chase, head of the molecular systematic section of Kew's Jodrell laboratory, and two colleagues, Kare Bremer of the University of Uppsala in Sweden and Peter Stevens of Harvard. It has taken more than seven years and involved the detailed comparison of three genes for each of 565 representatives of all the families of flowering plants. Most of the work has been done at Kew.
"I think you would have to say it is a breakthrough," Dr Chase said. "There has never been such a focused effort at sorting out a major group of organisms as has been done here with the flowering plants, which are of course critical for life on earth, so its importance is economic as well as scientific."
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