When I first thought of the title for this Halloween post, I had fun in mind — white flowers that have ghostly or skeletal effects — and there are those, like the cotton grass above and the trillium and others below. But the more I thought about white flowers, the more questions I had. How did they become white? Is it a loss of pigment or a color of its own? Why are there so many of them? Depending on the region, they can far outnumber flowers in the blue to red to orange range, and outstrip the numerous species of yellow flowers. Studies show that pollinators, given a choice, will gravitate to colors. So what’s the evolutionary advantage of white? Is there one? It turns out that white flowers are full of mystery. Which is, indeed, fun.
The earliest angiosperms, more than 100 million years ago, are thought to have been white, cream or pale green. Since Darwin, people — including me — have been happily saying that the more vivid colors slowly evolved to attract pollinators, whose vision long predated the flowers. And that appears to be true. Or, at least, there’s no strong body of evidence saying it’s not true. But, as it turns out, there’s no strong body of empirical evidence saying it is true. Empirical evidence implies that we can see something happen in real time, and it’s hard to see an evolutionary process in our brief lifespan.
There are studies that show, for example, flowers becoming redder in as little as a single generation as more hummingbirds pollinate them. Further studies show that when given choices, pollinators will choose colors over white flowers, though that may be because the colorful ones stand out more vividly against green foliage. Finding flowers efficiently is crucial to the success of both flower and pollinator, so the easier the flower is to see, the better. Very important, the stronger the relationship a pollinator has with a specific color, the more likely it is to bring matching pollen from one flower to fertilize another in the same species.
So, we know that pollinators have an intimate relationship with flower color. Or, more accurately, with the color’s wavelength, since the purple we see is not what the pollinator sees. But, with the explosion of genetic information in recent years, there’s also a growing appreciation for other factors that are at play, especially in how white flowers have evolved. Flowers in the blue to purple to red range use anthocyanins to create their color, the chemicals that make foods like grapes and raspberries so good for us. If the dominant anthocyanin is delphinidin, the flower is purple, if pelargonidin, red, if cyanidin, magenta to lavender. Other flavonoids, such as anthoxanthins, along with a variety of carotenoids, create yellows and oranges.
In the course of mutations that alter the expression of specific enzyme and protein pathways, the amounts of these color-inducing chemicals can vary, changing the color of the flower. Mutations may also cause the pathways to stop working altogether. The resulting loss of function can return the flower to its primordial white, a state that’s likely to be irreversible since it would take a series of very specific mutations for those particular pathways to work again.
There is a widely accepted division of flower/pollinator relationships: bees prefer flowers in the blue range, while hummingbirds gravitate to red, butterflies to pink, moths and beetles to white. And studies do back up these general preferences. But there’s a lot of variation. If bees weren’t interested in pollinating white flowers, we wouldn’t have almonds, apples, plums or any number of other fruits in the Rosaceae family. Thus, other factors are apparently important, among them scent, availability, abundance, learned behavior, competition, as well as the match of plant shapes to pollinator characteristics. It also may be that the subtle pinks that make white apple blossoms so poignantly beautiful to us are neon signs to bees. More mysteries. As every study says, ‘more research is needed.’
As fascinating as I find all this, I’m somewhat resistant to the idea that the gorgeous hues of reds, purples and lavenders I love so much are a result of ‘the number of hydroxyl groups attached to the B-ring of the molecule,’ or that tender, luminous whites are due to the functional failure of these groups. Reducing something as magical as color to the action or loss of enzyme and protein pathways seems like a comedown. On the other hand, my seeing and treasuring these colors is possible only because my body relies on similar pathways. Which brings another mysterious dimension forward: the fact that flowers and I share biological functions and genes, and, in sharing them, share each other.
Not only that, but without a strong connection to a variety of pollinating animals and insects, and the biology and genetics we have in common with them, neither flowers nor I would be here to begin with. All those pathways need constant nourishment. Like me, the pollinators depend on flowers for nutrition and survival. Flowers depend on these friendly forces, which can include me, for reproduction. We all depend on a huge array of microbes and fungi to create the nutrients we thrive on from the soil at our feet. We depend on the movements of air currents, the hydrology of water, the minerals released from rocks.
Sitting among flowers on a forest path, or the desert floor, or out in a meadow, we’re held in a vast array of interlinking pathways, beating our hearts, feeding our cells; moving water, air, nutrients; creating color, vision, scent. All mysteriously designed to keep every one of us — flower, leaf, dirt, human, bee, bird, beetle — alive and blossoming.
I’d love to have you on the journey! If you add your email address, I’ll send you notices of new adventures.