Flowers’ power of seduction

August 23rd, 2011

Guest post by the American Society of Landscape Architects. This post is by Shannon Leahy, ASLA 2011 Summer Intern.

According to this recent TED talk given by Jonathan Drori, life really is all about the birds and the bees, with a lot of help from flowers. Flowering plants, of which there are at least a quarter of a million known species, have evolved to develop fascinating traits in order to reproduce. Flowers are difficult to produce and the endeavor requires a lot of energy and resources. So why do they bother? Drori says that, like so many other things in the world, it all boils down to sexual reproduction. Plants can reproduce by themselves through self-pollination, but sexual reproduction is the means by which they spread their genes to mix with other plants in order to keep up with evolution and adapt to environmental niches.

Sexual reproduction requires the transference of pollen, a powder-like substance that contains male gametes, to other plants. Amazingly, there are as many kinds of pollen as there are flowering plants. Many types of plants, like the grasses and trees that produce hay fever, reproduce through wind-pollination. This process is inefficient because it requires plants to release massive amounts of pollen to get the job done. A more effective method is through a symbiotic process by which flowering plants transfer pollen to insects and birds in exchange for nectar. These pollinators in turn carry the pollen directly to other plants.

Symbiosis has led to beautiful adaptations like the Hummingbird Hawk-moth (Macroglossum stellatarum), which looks remarkably like a hummingbird while feeding on flowers. It has also produced unique physical traits in flowering plants that aid them in attracting insects and birds to “do their bidding.” These tools of seduction include “landing strips,” markings on petals that guide an insect to a flower, and the anthers on a lily, which are cleverly designed to flip up and knock the insect on its back, covering it in pollen.

Among the 20,000 known species of the orchid, there are a variety of unique traits. One type of orchid appears to have jaw-like petals that cover an insect in pollen as it crawls through them to get to the flower’s precious nectar. Another, the Madagascar Star Orchid, or “Darwin’s Orchid” (Angraecum sesquipedale), has a long nectar tube only accessible by an insect with an equally long proboscis. Darwin himself studied the flower and notably predicted that an insect capable of accessing this tube must have coevolved with the species. Watch the video and you will see the one with just the right proboscis for the job.

Since nectar is such a valuable commodity, some species of orchids have developed traits to help them trick pollinators into thinking they have it when they actually do not. They are able to lure insects in because they look like species that do have nectar. Other species of orchids use different methods of deception by exhibiting characteristics that are particularly attractive to a certain type of pollinator, like a pair of black dots that to a male insect look like two female insects resting on a flower, or a glossy metallic surface reminiscent of a beetle’s shell. Still others in part or whole resemble the form of an insect, like an orchid with a petal reminiscent of a bee. Bees flying by catch sight of the petal and, thinking it’s another bee, try to fight it, knocking it about and picking up the flower’s pollen in the process. In an especially devious turn, this same orchid does not offer nectar but strongly resembles another species that does.

Scent is another source of attraction. Ylang-ylang (Cananga odorata), a rather unremarkable looking plant, has an incredibly enticing smell used in many perfumes that is irresistible to insects. A very different species, the Dead horse arum (Helicodiceros muscivorus), is a lily that has evolved to look like carrion. Though it smells horrible and has no food to offer, blow-flies love it and lay their eggs inside it.

Other species of flowering plants use heat to attract insects. The Arum maculatum is a type of lily that can heat up to 15 degrees above ambient temperature. Flies are drawn to the heat and become trapped inside the flower. Once inside they drink the nectar and become covered in pollen. The flower’s bristles eventually wilt and release the flies. Philodendron selloum flowers for two days during which it maintains a constant temperature of 115 degrees with an internal thermo-regulation mechanism. It fuels this process by metabolizing fat, like mammals, rather than starch. Some beetles mate at exactly that temperature and will get inside the flower to do so, becoming covered in pollen in the process.

Color also plays an important role. In tropical regions, bird and butterflies pollinate flowers that are red because, like humans, they see the red, blue, green spectrum and are attracted to the color. Insects, however, see the green, blue, ultraviolet spectrum. Drori conducted a fascinating experiment in order to see what attracts insects to certain flowers. He photographed a Helianthemum, a small yellow flower, and compared several images of it. One image shows the flower in natural light, the way humans see it, while another shows it with the red removed, the color insects cannot see. Then Drori used an ultraviolet filter and a long exposure to produce an image depicting the flower in stark contrast to the background, looking similar to a bull’s eye. One could imagine it would be difficult for a bee to miss it. Not all yellow flowers have this property and, for comparison’s sake, Drori also shows an image of a flower that blends into the background. He notes that the flower could be used as the basis of a sunscreen, which works similarly by absorbing ultraviolet light.

This experiment ends on an intriguing and poetic note. Apparently, the main use of ultraviolet filters is to allow astronomers to take pictures of the clouds surrounding the planet Venus. Venus is also the Roman goddess of love and fertility. Drori notes that while flowers have exerted a great deal of effort to get insects to do their bidding, they have also implicated the rest of us in the process. Flowers have “managed to persuade us to plant great fields full of them and give them to each other at times of birth and death and particularly at marriage, which, when you think of it, is the moment that sort of encapsulates the sort of transfer of genetic material from one organism to another.”

This guest post is by Shannon Leahy, ASLA 2011 Summer Intern.