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"This creates a white sheen which makes the petals seem glossy." This kind of thin pigmented film is unique in the world of plants. Dr van der Kooi, of Lausanne University in

Switzerland, said: "Butterflies use similar structures to produce colour - as do some birds - but buttercups are the only known flowers to do so." His team is the first to measure

light spectra and conclude the cell layer acts as a thin film. There are hundreds of varieties of buttercups. They are poisonous to eat for humans and cattle. People sometimes hold a fresh

buttercup flower under the chin. If a yellow reflection from the flower's shiny petals can be seen the person is said to "like butter".  This custom is still taught to young

children and shows how buttercup petals reflect light.  Dr van der Kooi said alongside the air chamber the petals have a starch layer which again scatters light and sends some of it back

through the pigmented epidermis. He said: "Thus, the light reflected by the starch travels through the pigments twice, giving it a very rich yellow colour." He used a physical

model of the petals which he had previously developed to explain how these unique anatomical features are responsible for both the intensity of colour and the sheen of the petals. Dr van der

Kooi whose findings are published in the Journal of the Royal Society Interface said the advantages are twofold. He said: "The first is under the right conditions the gloss reflecting

from the epidermis can produce a bright flash visible to pollinating insects from a great distance." The sun needs to be high in a clear sky for this to happen. The second is the

buttercup is 'heliotropic' which means its flowers follow the sun - especially on cold days. When it's cold the petals form a paraboloid-shaped cup - like a satellite dish -

thus reflecting the sunlight towards the central area of the flower where the reproductive structures are located. Dr van der Kooi said: "We are still doing measurements but our

hypothesis is this light helps to increase the temperature of the reproductive organs which enhances seed and pollen maturation." Previous measurements by others have shown the centre

of the flower can be several degrees warmer than the ambient temperature. He said: "But this may also be due to the shielding effect of the petals from the wind." This work has

thus solved much of the mystery surrounding the buttercup. Dr van der Kooi said: "We can now explain the effects of the complex anatomy of the petals." 'Build Me Up

Buttercup' was the name of a popular song by The Foundations during the 1960s. In 1916 an elderly man went around a field painting all the buttercups blue.