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The orchid, a plant of many adaptations

Orchidaceae is one of the largest and most diverse of all plant families. There are twice as many species of orchids as there are birds. Prized by collectors but also one of the most popular house plants, this varied and diverse flower has cleverly evolved to make its home across the globe.

In pictures: the secret life of orchids

Featured orchid: The mirror orchid, nature's dazzling mimic

Research at Kew reveals how this orchid's brilliant blue flower gets its iridescent shine from 'structural colour'

To the human eye, the most striking thing about the Mediterranean mirror orchid, Ophrys speculum, is the shiny blue ‘mirror’ at the centre of its lower lip. The mirror, with its thick fringe of brown hairs, has intrigued naturalists for centuries. The purpose of this flower, like those of other species in this genus, is to deceive. The orchid attracts pollinating insects – in this case most often solitary wasps – by mimicking the female so well that males try to mate with it, inadvertently picking up and depositing sacs of pollen in the process. To a male wasp, the shining mirror (or speculum) probably resembles the lustrous wings of a potential mate.

Paula Rudall of Kew’s Jodrell Laboratory is part of a team at Cambridge and Kew investigating structural colour in plants – the generation of colour by means other than chemical pigments. The team recently showed how the fruits of Pollia condensata – a forest plant from sub-Saharan Africa − generate nature’s most intense blue. Pollia berries achieve their colour without pigments. Instead, layers of coiled fibrils in the cells of the fruit wall selectively interfere with incoming light to reflect only the brightest blue (Proceedings of the National Academy of Sciences, vol 109, p15,712).

So how does the Mediterranean mirror orchid achieve its striking appearance? Does it do it with pigments, or is this another case of ‘structural colour’? The answer, says Rudall, is both (New Phytologist, vol 196, p1,038). ‘The unusually intense colour is achieved partly by blue pigments – primarily a type of cyanidin − located in the speculum’s epidermis, and partly because the shape of the epidermal cells in this region optimally reflects incoming light,’ says Rudall. ‘We compared the petals of this species with the less shiny lips of other Ophrys species. In O. speculum, the surface of the speculum is unusually smooth.’ In many plants, the epidermal cells of petals are conical or domed, and the layer of cuticle covering the epidermis is sometimes sculpted into ridges or pimples. By contrast, the epidermis of the mirror in O. speculum consists of unusually flat cells covered by a featureless cuticle. The result is that some of the light falling on the speculum bounces straight back, as from a mirror. The appearance shifts with the flower’s orientation: at angles where reflection is strong, the mirror effect dilutes the apparent blueness.

However, when the angle is shifted, the intense blue pigment dominates. ‘For an approaching insect, the result must be dazzling,’ says Rudall. ‘We also found that the speculum is highly reflective in the UV spectrum, which is known to be visible to many insects. It is possible that the localised bright colour could stimulate the insect to land in the best position for pseudocopulation and eventual pollen transfer.’

This article was first published in Kew Magazine (Spring 2013)

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