Although a dilettante gardener, I’m even more an armchair scientist but annually take a dip into the fount of scientific knowledge at the Royal Society’s summer exhibition. 1 This august institution, situated in the grand edifice of Carlton House Terrace, showcases recent science and technology research and gives students, young and old, the opportunity to question the researchers about their work.
Since I’m a naturalist at heart, this year’s main topic of interest was ‘Why Nature Dresses to Impress’ coupled with a delve in the ‘Physics of structural colour’. 2 Here follows a résumé of what I learnt and seemingly understood, along with some background knowledge. Veracity is therefore not guaranteed!
The Vision of Colours: Our garden flowers invariably offer up all the spectrum of the rainbow that we learnt to sing about as children. Whilst we may wax lyrical on their hues, at the structural level of vision we are merely registering different lengths of light waves from shorter blues, medium greens, to the longer reds. Further on from the blue spectrum is ultraviolet light, undetectable to our eyes but not to birds and pollinating insects, which means they have a very different view of the garden to the one we perceive.
Pigments and Absorbtion: Blooms manifest as a particular hue because they selectively absorb some light waves and reflect others, according to their inherent pigments. Thus a red Hollyhock appears so because it has absorbed blue and green, leaving only the longer red wavelength visible, whilst a purple Peony absorbs only green, reflecting both blue and red.
Similarly there are 3 pigments in our eye’s colour vision structures (cones) which are receptive to short, medium and long wavelengths. These photoreceptors activate at different levels and ratios of excitation according to what colour is received. If one or more of the cones is dysfunctional we are effectively blind to certain hues.
We know that a bee’s vision cannot register the longer wavelengths which begs the question as to how and why they visit red flowers. The answer seems to lie outside the sphere of pigmentation.
UV Patterns and Fluorescence: Flower foragers are not attracted by colour per se but to the food sources of nectar carbohydrates and/or pollen proteins. Given that blooms invariably secrete these larders in convoluted shapes and complex interiors, they need to provide coded directions for pollinators to follow. Scents and white flowers (reflecting all colour wavelengths) are strong clues for fly-by-nights but daytrippers are more reliant on visual cues.
Coloured stripes and petal striations emanating from the centre are one obvious marker as can be seen in my enhanced image of a hoverfly locating the sweet, sticky pistil of a Petunia. With the added facility of UV vision however, pollinators are able to discern a plant’s food sources even more clearly and rather differently. The fascinating flower photography of Bjørn Rørslett illustrates not only the landing strip patterns that plants construct to guide flyers in but also the fluorescence of nectaries which positively glow in ultraviolet light. 3
Diffraction and Iridescence: Aside from pigmentation, electron microscope studies of garden plants have shown that they also construct colour through iridescence. Structural colours result from the process of diffracting light. The plant cuticles of flower and fruits are layered in a diffraction grating akin to how CD/DVD grooves are structured and likewise change colour according to the angle of light direction. What this would mean for pollinators is that the colour of flowers would alter according to their angle of approach, thus creating a dynamic rather than a static target.
Whether floral iridescence does influence foraging behaviour is a question for behavioural ecologists. Witney et al investigated the mechanism of iridescence in tulip flowers and then conducted a series of lab experiments with bumblebees (Bombus terrestris) and a variety of optical ‘flower’ discs. Their results suggest that bees are more prone to visit luminescent flowers, irrespective of underlying pigment, UV signals, or polarization effects. 4 Critics however have argued that since diffraction gratings not only iridesce but also contribute to hue or polarization of petal colouration, there is no proven certainty that luminescence is the isolated factor in the bees’ foraging behaviour. 5
I am quite content to merely observe and enjoy the spectacle of pollinators visiting the flowers in my garden but having some understanding of the science of attraction is an added fascination.
Postscript: You might also be interested in webcasts of public events and prize lectures at Royal Society tv
1. RoyalSociety – Summer Science Exhibition 2011
2. Dept of Plant Sciences: University of Cambridge
3. Bjørn Rørslett Flowers in Ultra-Violet
4. Witney et al (2009) Floral Iridescence, Produced by Diffractive Optics, Acts As a Cue for Animal Pollinators
5.Morehouse & Rutowski (2009) ‘Comment on “Floral Iridescence, Produced by Diffractive Optics, Acts As a Cue for Animal Pollinators”
©Copyright 2011 Laura Thomas.
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