Dataset: Signaling. Colors (7/8)

Submitted by adminl on Thu, 01/13/2011 - 06:00

Data set 8/9 for the signaling dataset.

Via 0The San Francisco Chronicle. Speaking metaphorically, mammals opted out of color vision when we were scurrying around in the dark, dodging dinosaurs; your dog doesn't see rainbows as well as you do because our common ancestor didn't. But our closer primate ancestors had an advantage if they could recognize ripe fruit in a rain forest, and plants advertise their fruits' mature sweetness and their seeds' readiness for distribution by suffusing them with color. Color vision had evolved previously in invertebrates and vertebrates both, and some mammal lineages tapped back into that latent capacity. In fact, as we animals co-evolved with plants, well-advertised fruits gained better seed distribution via mammals and birds, and poster-bright blooms further strengthened the partnership by helping pollen get distributed to conceive those seeds in the first place. Flowers are a plant's pollination advertising campaign, and every color sends a message to a particular kind of pollinator. Biologists recognize different "pollination syndromes," combinations of flower color, shape, scent and blooming time, fine-tuned to the senses of bees, butterflies, birds or bats. Birds have excellent color vision. 1Bird-pollinated flowers (California fuchsias, for example) are often red, not so much because birds prefer red as because bees don't. Bees aren't red-blind, but they have more trouble picking out red flowers from green foliage than they do blue ones, so it's more efficient for them to specialize in blue flowers. Bees can also see into the ultraviolet. Some flowers that look yellow or white to us appear purple or blue-green to them. Bee flowers often have nectar guides - petal marks pointing to anthers and pistils, like arrows on a runway - that are invisible to humans, but clear as day to a bee. Although hummingbirds and some flower-visiting bats have UV vision, "their" flowers lack such signals. Bird flowers are also less likely to be scented, since most birds lack a well-developed sense of smell. Butterflies vary; some prefer yellow and blue, others blue to purple. Bats and hawk moths go for white flowers. Red-flowering scarlet gilias produce paler flowers when migrant hummingbirds depart and moths replace them as pollinators. North American columbines, sorting themselves into new species, have evolved new colors to attract new kinds of pollination partners. Color signals can be switched off as well as on. In plants as diverse as trillium, lantana and Texas bluebonnet, flowers change color once they've been pollinated, advising prospective customers that the nectar bar is closed. Sometimes plants use specialized leaves to attract pollinators. The showy "flowers" of poinsettia, heliconia and bougainvillea are not flowers at all but pseudanthia made of bracts - modified leaves. Leaf colors have other functions. The default is green, of course, from the chlorophyll that manufactures a plant's food. Some variations on green are not a good thing: a chlorotic (yellow) leaf usually signals a deficiency of nitrogen, magnesium or iron. But healthy plants can have vividly multicolored leaves: Consider the coleus and the croton. Those colors derive from pigments stored in microscopic vacuoles within the cells of the leaf. Most common are anthocyanins, which range from pink through red and purple to blue and occur in flowers and fruits as well as leaves. The pigment overlies a leaf's chlorophyll and doesn't interfere with photosynthesis. Anthocyanins are believed to screen out harmful levels of UV radiation. In a minority of plants, the active ingredients are betalains, used by botanists as markers of relationships among diverse-looking forms. Nepenthes, those strange carnivorous hanging-pitcher plants, got moved all the way to a different order because their red mottlings are betalains. These pigments, usually purple, red or orange, are also what beets (including Swiss chard), ice plants, purple prickly pear cacti and bougainvillea have in common. Their function is obscure, but they might be antifungals. Some leaves are two-toned - green above, purple below. This may allow forest-floor plants to make the best of low-light conditions, bouncing light waves back into the leaf's photosynthetic cells. Don't forget the structural colors. Silvery leaves get their color from hairs called trichomes, which reflect infrared radiation that might otherwise cook the leaf. They're often associated with hot or arid locations, as with the Hawaiian silverswords. Some bluish leaves have a waxy coating that may waterproof the leaf, filter UV radiation or reflect infrared radiation. A new exhibition at the San Francisco Conservatory of Flowers, "Color: A Winter Carnivale," showcases the colors of flowers, fruit and foliage. It features ambient calliope music, carousel animals (from local carver Oscar Pivaral) and a Wheel of Botanical Fortune. Two years in the making, "Color" represents a collaboration between Curator of Education Lisa Van Cleef and designer Anjel Van Slyke. "I ran around to all the Bay Area nurseries asking, 'What's in bloom?'" says Van Cleef. "It's not just about the flowers - the foliage contains so much color." Van Cleef says the exhibition is designed to appeal to kids as well as adults: "How do we get kids as excited about plants as they are about animals? We need to show them how to look at the ordinary. When they get out of here, they'll see plants differently." Joe Eaton and Ron Sullivan are freelance nature and garden writers in Berkeley. E-mail them at home@sfchronicle.com.