Thieves stole a rare plant no bigger than a £1 coin from Kew Gardens.
Police are appealing for information after the nymphaea thermarum was swiped from the Princess of Wales Conservatory between 8.30am and 2.55pm on Thursday, January 9.
It is understood the plant would have been dug or pulled up from a shallow water lily pond.
Richard Barley, director of horticulture at Kew Gardens, said: "Our staff are dedicated to the conservation of plants and when incidents of this nature occur it is a blow to morale.
"We take theft of our invaluable scientific collection of plants very seriously and this matter is with the Metropolitan Police."
The plant is the smallest waterlilly in the world and does not exist in the wild.
Its only known natural habitat was in Mashyuza, south-west Rwanda, but disappeared about two years ago.
The lily, which measures 1cm in diameter, was brought back from the brink of extinction at Kew Gardens.
The plant is one of two in existence in the world - the other is in cultivation in Germany.
Anyone with information is asked to contact the Richmond CID on 020 8721 5934.