By Rick Weiss
Washington Post Staff Writer
Monday, September 4, 2006
Amid growing evidence that vines are staging a global botanical coup -- taking over more terrain, clambering up more walls and smothering more trees than ever -- scientists are redoubling their efforts to understand these unusually animated plants.
Their studies don't point to easy cures for overrun gardens or sagging forest canopies. But they are revealing the remarkable array of adaptations that allow these wily plants to infiltrate and dominate new terrain so quickly and with such apparent impunity.
Studies are also addressing such long-standing questions as, "Am I imagining it, or can vines actually sense which direction there is something to climb on?" As it turns out, some can -- one of several surprisingly complex behaviors that leave scientists who study vines amazed, but also a little on edge.
"They're greedy opportunists," said Oliver Phillips, a vine ecologist at the University of Leeds in England, who, as do other experts, speaks about vines with a mix of respect and horror.
By definition, vines are plants that gain altitude by climbing on the backs of others, rather than by standing up on their own. They are, in botanical terms, "mechanical parasites," mooching from others the woody backbones that they lack.
"Each time a tree like a sequoia gets really high, you've got a huge investment in the wood, which requires an enormous amount of photosynthesis," said Peter Raven, director of the Missouri Botanical Garden. "Vines make much less of an investment and get to the same place."
Vines focus on sprinting skyward and, once above the fray, making leaves -- lots of leaves -- to soak up the prodigious amounts of sunlight up there, shading and slowly starving supporting trees.
Their emphasis on leaf production is unparalleled among terrestrial plants (and reminiscent of seaweeds, similarly unburdened by a need for sturdy trunks). Vines might constitute only 5 percent of a forest's botanical biomass, but their leaves can account for 40 percent of a forest's leaf biomass.
How do they do it?
For one thing, vines are metabolically geared to gulp huge quantities of carbon dioxide, the gaseous fuel plants use to turn sunlight into sugar. Recent studies suggest that vines are already taking advantage of rising carbon dioxide levels to help them outpace other plants.
Plumbing is important, too. Uncluttered by weight-bearing wood, vine stems have room for unusually wide-bore water "pipes." The laws of fluid dynamics declare that a pipe whose diameter is merely doubled can carry 16 times the amount of water. That increased capacity gives vines the hydraulic wherewithal to keep their sky-high carpets of foliage totally pumped -- a feat that trees, with their more restrictive piping, cannot match.
But perhaps the most intriguing adaptations wielded by vines are the behavioral ones, including the mesmerizing act of groping for the perfect something to climb on; the almost prehensile snagging or grabbing of that target; and finally -- depending on the kind of vine -- the scaling of that upright, using any one of four basic strategies.
Some vines seem to reach blindly for support, but many are more methodical. Some reverse their light-seeking machinery and reach for the nearest shady spot, which has a good chance of being a tree trunk or other climbable surface. Others respond to airborne hormones released by woody plants. Like moths seeking mates in the evening air, they follow their noses in search of something to hang on to.
Charles Darwin was among the first to categorize the climbing modes of vines. He became fascinated by the plants while bedridden for months with a chronic illness, and he kept scrupulous notes as he watched several specimens pull themselves up by their botanical bootstraps.
Today, Darwin's "The Movements and Habits of Climbing Plants," published in 1865, remains one of the best descriptions of the ratchet-like hooks, sticky rootlets, twining growth and specialized grabbing appendages that constitute vines' quartet of tricks.
Consider, for example, the climbing palms, which flourish in rainforests, are the source of rattan for furniture and can grow taller than California redwoods. They have barb-like hooks that easily slide upward over the bark and branches of a supporting tree but will not slip back. As the vine climbs, it uses those hooks as a mountaineer uses pitons and carabiners, hitching itself up, up and up.
Must . . . hang . . . on!
English and poison ivies, among others, have stems bearing "aerial rootlets" that resemble caterpillar feet. These do not absorb water or nutrients but can pry deep into bark, or the pores of brick or stone, and so hang onto their perches with Ivy League tenacity.
Twiners, including morning glory and oriental bittersweet, have become the object of intense study by experts in plant biomechanics -- who want to know how spiraling gyres keep from slipping down the shafts they embrace -- and also molecular biologists, who are looking into what determines the direction they spiral. Experiments indicate that twining "handedness" is largely genetically programmed, with most vines twisting counterclockwise when viewed from the top down. (And no, they don't reverse direction in the Southern Hemisphere.)
"Even high-powered mathematical physicists are thinking about the stability of helical forms, including twining vines," said Wendy Silk of the University of California at Davis, who has performed seminal studies on how twiners twine.
Silk and her colleagues are investigating vines' mysterious capacity to form a coil whose natural diameter is slightly narrower than the diameter of the upright they are climbing, thus assuring a good grip. Her team has found that gravity's downward pull on a vine's lower reaches actually tightens the vine's grip, just as those woven Mexican finger-gripper trinkets tighten the harder they are pulled.
Using engineered, sensor-laden poles that can detect subtle pressures, Silk has found that twining vines align their curves so that any downward force on the stem gets naturally transformed into a thousand-fold stronger squeezing force.
Finally, there are the grapes and other vines endowed with tendrils or similar finger-like structures that are responsive to touch -- a behavior with the wonderfully mellifluous name of thigmotropism.
These specialized plant cells respond to physical stimulation by producing hormones that spur disproportionate growth of neighboring cells. That results in a curling growth pattern, round and round the original irritating object.
Such talents are perfect for exploiting ecological changes being wrought by humans, including patchy deforestation, which has been giving vines exactly the kind of scalable "edges" they love.
"I'm not sure what's going to keep us from becoming a world full of vines," said Francis Putz, a vine researcher at the University of Florida.
Luckily, we are still primates. If the roads get overgrown, we can always swing to work.