Fount of knowledge
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!
daisy 'colour wheel'
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.
bees are red colour blind
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.
Directional find
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
dandelion's landing strip in ultraviolet light
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.
flower iridescence
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
the garden as observatory
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
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Notes:
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”
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©Copyright 2011 Laura Thomas.
All rights reserved. Content created by Laura Thomas @PatioPatch
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Absolutely fascinating Laura.
I learned a great deal from this post.
I agree, I am happy to see them visit but it is also good to know why and what attracts them.
Beautiful written as always…..tku.
I have collected cerinthe seeds for you. If you email me on
[my name] at iconism.net with your address I will send them to you.
cheryl read my post..The joy of gardening……
thanks for you kind comments, Cheryl and especially the offer of your Cerinthe seeds. Will they be safe in my hands I wonder? Have altered your email to stop the bots junking you. Meanwhile will send my address
Very interesting Laura, I hope the predicted true blue roses don’t confuse them.
Alistair read my post..Aruncus Aethusifolius
A very informative post, Laura. I’ve been fascinated ever since I first learned about UV vision in pollinators (and slightly envious – don’t you wish you could see that in the garden?), and the way flowers exploit it to ensure a bloom is visited. Since we started beekeeping it’s actually altered my gardening methods. I think now about which pollinator types would be most attracted to a particular flower type, whether the blooms carry high or low quantities of nectar or pollen, and most importantly, how much to plant! Planting something just because I like it, simply isn’t good enough anymore!
Curbstone Valley Farm read my post..Jenny’s Chicks: Feathers are for Flying!
What an interesting post Laura. I did know that things in the plant world are not always as they appear. I always try to have single blooms in the garden so as not to confuse the bees etc. Have you come across Chris Beardshaw’s “Secret life of the Garden”? It’s full of fascinating information.
Janet read my post..Sandesquoy Garden
Almost, we could have been there. The ultraviolet honey guides remind me to wonder at nature.
Elephant’s Eye read my post..Berg River Canoe Marathon at Iron Bridge
Laura, you might like the UV flower photography at: http://www.pbase.com/kds315/flowers&page=1 So much of it is like helicopter landing pads…
kininvie read my post..Being picky about Shakespeare
thanks for this link, a plethora of great images. Bjørn Rørslett does make the point that UV flower colours are guessed at as we can’t see UV but the patterns are actual
Laura – Great post! I can understand why pollinators loved my bright red, luminescent oriental poppies … but why are they so attracted to foxgloves? I found it very difficult to take photos of foxgloves without bees getting in the way. Their nectar isn’t as easily accessible as other flowers, maybe it tastes better.
b-a-g read my post..Houttuynia Cordata Chameleon (13 JUL 2011)
What an interesting post, so much info.
That was a fascinating read. I have to say it’s stuff I’ve never really given much thought to, but I found myself totally absorbed in your collection of understandings on these topics. Great photos to go with it, too.
Bernieh read my post..Time To Flaunt Some Flowers
That is a nice article Laura, very well written going with beautiful photos. I am fascinated about the armchair scientist, does that mean just researching thru published articles? In my case when i was still in the lab, i am a hands-on scientist, haha. But that was only for a few years, lengthened only by long years in graduate school. One of my older theses is on pigments so i can fully relate with you. Now i wonder if there will somehow be a mutated insect which is already going to the blues and UVs!
Andrea read my post..Shadows from our Colonizers
I’ve enjoyed this post Laura (I might even have understood a little of it!). I always try to have nectar plants in my veg patch (cerinthe is renowned for this, and beautiful) and was fascinated to see hundreds of honey bees buzzing around a giant pink hebe in the shrub border yesterday.
Caro @ UVP read my post..Saturday Snap: Bee Balm Bergamot
Nature is pretty darn complex and there is so much we still do not know. It fascinates me to learn something new from the scientific perspective. I did check out the UV photography as suggested. I always thought pollinators were paired more with flower structure than color, but so many factors contribute. It is so much more than the simple visual color and it makes one want to research that even more in making gardening choices.
Donna read my post..Blue and the Color of WOW
another factor in attraction Donna and in the experiment above the ‘flower’ discs were all the same shape. Wonder if shape influences iridescence?
- bees show some preference for blue flowers Alistair so the roses might be perfect
always been a fave topic of mine so pleased you found it so too.
- now you have hives, Clare the bees should dictate planting at Curbstone Valley!
- thanks for that Janet. Will add to my birthday wishlist
- agree Diana. Behind the science, there is always more wonderment
- err, becuase they fit them like a glove, b-a-g?
- tried to make the info clear and readable, Bridget
- thanks for the images comment Bernie
- if only I’d had the discipline for science, like you Andrea, so instead I just read the work of others
- hebes are the bees knees Caro as is your Bergamot
Thanks for the informative post. I love the UV pics of flowers – very interesting.
The Sproutling read my post..Plant of the Month: July 2011
Wow, simply fascinating! Thanks so much for all the great information. Sometimes I think understanding a bit of the “why” of things brings an even greater appreciation of nature. Wonderful post!
PlantPostings read my post..GBBD: Lilies rule!
Very interesting post, Laura; the exhibition sounds as though it was particularly fascinating. I love the UV image of the dandelion flower… it’s always so strange to think that our perception of the world is just that, a perception.
hillwards read my post..Bloom Day July 2011
The more I learn about the marvel of mother nature, the more in awe I am. Great post!
Ginny read my post..Wordless Wednesday – it’s all in the light
Interesting and a different take on a garden post.
Barbara read my post..Ilminster, Somerset
How utterly fascinating. I had no idea about the UV dimension. Love the two dandelion photos, truly “illuminating”
Janet/Plantaliscious read my post..GBBD July 2011
Very, very interesting! It is nice to learn how these very special creatures navigate their pollination. Thanks!
The Sage Butterfly read my post..The Last Hurrah – GBBD – July 2011
I am fascinated by the intricate details in the world, by how everything is designed to work together, by how much goes on beyond our limited senses! Great post!
debsgarden read my post..Chinese Pistache: A Gawky Adolescent Grows Up
appreciate all your comments and the interest this topic arouses in us gardeners. So much more going on that meets the eye but then plants can’t afford a hit-and-miss of pollination. Nature is eternally fascinating