Nature's Palette: The Science of Plant Color
- David Lee
“Why grass is green, or why our blood is red, Are mysteries which none have reach'd into.” John Donne's words were true in the seventeenth century. Today they certainly aren't, as David Lee makes clear in Nature's Palette, an enchanting survey of colour in plants. Among the mysteries is why some plants are electric blue, such as Selaginella, a relative of ferns that thrives in the rainforests of Malaysia. Its leaves are as blue as some butterfly wings, and for the same reason: it is a 'structural' colour, produced by light scattered from a periodic banded microstructure in the outer cell walls. The shaded undergrowth of the forests is home to several other blue-leafed plants, but it's not clear why they should reflect so much light when it's such a scarce commodity. After 30 years of looking, Lee still has no good explanation.
Blue leaves abound in seventeenth-century Dutch flower and landscape paintings. One of the most striking examples is Adam Pynacker's Landscape with Sportsmen and Game, in which a garish clump of blue foliage on the gloomy woodland floor appears every bit as surreal as Selaginella. This is not a botanical oddity, however, nor is it what the artist intended. Over time the yellow pigment in the green mixture has faded. But there is a botanical explanation, as painters commonly used yellow pigments from plant extracts, such as saffron or weld, that are damaged by light. Plants require merely a seasonal burst of exuberant advertising, so their pigments don't last — as generations of artists and dyers have found to their cost.
Dutch flower paintings also depict tulips streaked with feathery white. These 'broken tulips' appear unpredictably among otherwise identical bulbs, and are short-lived and hard to propagate. As a result, they were so rare in Baroque Holland that it was often cheaper to hire a painter to capture their beauty than to acquire a bulb. Speculators bought thousands of bulbs in the hope of finding a few such variants that would sell for a fortune, creating the infamous 'tulip mania', the archetypal economic bubble driven by speculation. As Lee explains, we know now that the broken tulips were diseased, infected by a virus that interferes with pigment production.
Lee's book is packed with many such gems from botanical and social history. So captivating is his passion for botany that his occasionally bewildering thickets of carotenes and anthocyanins can be forgiven. His paean provides a compelling case that botany is full of intellectual challenges, many shamefully neglected. He shows, for example, that plants are extraordinary biological devices, with leaves that act as arrays of lenses or mobile light antennaes, or that change colour on exposure to light, thanks to the mobility of their chloroplasts. But only one Nobel prize has ever been awarded for fundamental biological research on plants — for Barbara McClintock's discovery of transposons in maize, which had to weather a storm of scepticism before being rewarded.
Like its subject, the book can be overwhelmingly heady. A scary glimpse into the hothouse world of Miami orchid breeding suggests there is a lurid thriller waiting to be written on the subject by Carl Hiaasen. And the cloying scents of the 'gallant south', evoked so unforgettably in the Billie Holiday song Strange Fruit, are all the more palpable for knowing that some flower odours mimic rotting carcasses to attract carrion flies. So obsessive is the flower-lover's demand for novelty that a biotech company, Florigene, set itself up with the sole mission of making blue roses and carnations. A jackpot awaits if they succeed, but I can't help thinking the romance will be well and truly lost on the way.
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Ball, P. Botanists' blues. Nature 449, 982 (2007). https://doi.org/10.1038/449982a