Plants seem so passive. Tree branches bow to the wind, losing their leaves. Lettuce just sits there as snails help themselves to a free salad bar. And grass lets everyone walk all over it.
But all that apparent listlessness is deceptive. In truth, plants are incredibly active members of their communities. Plants move on their own all the time—creeping, digging, reaching, blooming. However, most of the action happens too slowly for our eyes to see unaided. In his 1995 documentary series The Private Life of Plants, David Attenborough and his crew created beautiful time-lapse videos to showcase plants' hidden mobility. Climbing jungle vines race one another up tree trunks, stretching towards the sunlight. The thorny bramble whips from side to side, shoving competing plants out of the way to expand its territory.
A spindly orange vine known as dodder is a particularly striking example of dynamic plant life. Dodder is a parasite—it lives off of other plants. Instead of waiting around for a suitable host, the vine hunts one down. Conseulo De Moraes of Penn State University planted a young dodder near a tomato plant and continuously filmed the pair for several days. Her time-lapse video reveals a growing dodder flailing around, tasting the air like a snake, until it finally brushes the tomato's stem and begins to encircle its victim. Eventually it would sink tiny nozzles into the tomato plant to suck out vital juices.
De Moraes discovered something surprising about the dodder: it can smell. The vine sniffs out its hosts, growing toward telltale chemicals released by its neighbors. And it is picky. Dodder prefers juicy tomato plants to slender wheat and healthy plants to sick plants. Tel Aviv University biologist Daniel Chamovitz discusses dodder and many other fascinating plants in his upcoming book, What A Plant Knows, an excerpt from which appears in the May issue of Scientific American.
Dodder is hardly the only plant whose mobility and abilities would surprise most people. The plant kingdom is full of unusual talents that are more common than biologists first realized. The Venus flytrap is only one of several different kinds of carnivorous plants that have developed astonighing ways to catch and digest insects and other small animals. Almost all plants have evolved chemical defenses against herbivores and many plants recognize when their neighbors are under attack, preparing for battle themselves. Alpine buttercups track the sun's arc over the course of a day to keep their blossoms warm and appealing to heat-seeking pollinators. The telegraph plant swivels its leaves to maximize exposure to sunlight, adjusting so quickly that you can see the leaves moves in real time. Some plants may even distinguish between family and strangers, sharing resources only with the former.
Like most organisms, plants sense and respond to their environments. To appreciate just how sensible plants are, we have to look at the world—or even smell the world—from their perspective
The spindly orange vine known as dodder (Cuscuta pentagona) is a parasitic plant. Time-lapse video reveals that a dodder seedling twirls through the air, sniffing volatile chemicals released by neighboring plants in search of a suitable host. When it finds one, dodder entwines its victim and inserts nozzles into the host's stem, siphoning of vital nutrients.
If you look closely at the inner pink lobes of the Venus flytrap's (Dionaea muscipula) trap, you will see several hairs. The trap shuts when an insect touches two or more of these hairs - or the same hair more than once – within a 30-second window. Tripping the hairs triggers an electrical impulse that propagates through the carnivorous plant's modified leaves, opening pores in cells walls so that water rushes in and snaps the trap shut. Scientists can also force the trap to close by applying an electric current to its lobes.
Alpine buttercups (Ranunculus adoneus) are known for their solar tracking abilities – their small yellow flowers follow the sun's daily journey from east to west. Researchers think that the behavior helps keep the flowers warm, which boosts chances of pollination by heat-seeking insects. Scientists have shown that the buttercup responds specifically to the blue wavelengths of sunlight: artificial blue light, but not red, gains the flower's loyalty.
Like Alpine buttercups, a small Asian shrub named the telegraph plant (Codariocalyx motorius) tracks the sun - not with its blooms, but with its leaves. Small leaflets attached to the base of larger leaves constantly swivel to monitor changing levels of sunlight, adjusting the position of the primary leaves as needed. The leaves move so quickly you can see them dance with the naked eye.
If you stroke the sensitive plant (Mimosa pudica), also known as touch-me-not, its fern-like row of leaves reflexively folds in half. Same thing if you blow on the plant or shake it. Whereas the Venus flytrap evolved rapid movement to catch insect prey, the sensitive plant probably shrinks from touch to discourage any insects or herbivores looking for a leafy snack.
Irises bloom in the spring and early summer. They know that the time for flowering has arrived because they can sense that the days are getting longer and the nights are getting shorter. In the lab, scientists have induced an iris to flower in winter by switching on the lights for a few seconds each night, punctuating the darkness. Irises raised in this fashion respond particularly well to red light.
The wild cucumber's (Sicyos angulatus) spidering tendrils, which grab onto fences and other plants for support, are super-sensers. Most people cannot feel the weight of a string weighing less than 0.07 ounces (2 grams). But the tendrils of the wild cucumber respond to the touch of a string weighing only 0.009 ounces (0.25 grams) and immediately start twirling around the tiny thread.
Like most plants, morning glories (Ipomea nil) usually grow up towards the sun. But this strain of morning glory, called Shidare asagao, has lost its balance – it has lost its ability to sense gravity. It doesn’t know where “up” is. Hence the vines of this morning glory fall down, making for a beautiful ornamental plant.
In lab experiments, researchers have shown that a weedy beach plant known as sea rocket (Cakile) recognizes its siblings and restrains its root growth in their presence. The idea is that siblings benefit from sharing nutrients and helping each other pass on genes they have in common. When planted near strangers, however, sea rocket grows as many roots as possible, since there is no benefit to helping out an unrelated plant. Sea rocket probably identifies family based on chemicals that roots secrete into the soil.
Common bread wheat (Triticum aestivum), also known as “winter wheat," only flowers and makes grain following a cold winter. If winter snows do not blanket the sprouts, they never flower. But if prior to planting, farmers keep the seeds in the freezer for a while, then the sprouts will flower even in the absence of snow. In other words, the plants remember their exposure to the cold. How do they encode this memory? The cold induces characteristic changes in the pattern of molecules hitched to DNA, a process known as epigenetics that in turn modifies gene expression.