In some species the young stems are rigid and grow upright without help for several metres - but once they touch a tree, they pounce. No longer do they need to invest in solid - and expensive - wood. Instead they become thin and flexible and begin to clamber. Certain lianas grow a flexible stem to find the open air, but once they reach a sunny spot they generate huge trunks that swing downwards from the heights and find another plant to use as a support. That noxious North American the poison oak grows as a solid two-metre shrub when it stands alone, but extends ten times higher if it can find an upright. Many other kinds take advantage of a helper when they get a chance, but stand on their own feet (or roots) if they do not. In a tropical forest, young trees of species not often thought of as climbers grow slim and tall as they lean against their neighbours. If that choice is not available, they stand alone and take up an independent life.
In many climbers, some branches have small leaves and move in a wide circle in search of a new gap through which a shoot can insinuate itself. Those that sneak through and find the sun then grow larger leaves that soak up energy. As the stems spiral away from the ground, they develop wide vessels through which to suck up water and food. The liquid has to travel through a passage many metres long, which makes drinking expensive and forces the plant to reduce water loss with waxy leaves and impermeable stems.
A tree pays a high price for its parasites, for they suck water and minerals from the soil and shade their host from the sun. West African trees in the presence of lianas grow at no more than a fifth of the rate of their fellows. A few climbers can kill. The strangler fig, once it has reached the canopy, sends roots down from its eyrie. As they grow, the aerial roots wrap themselves around the supporter’s trunk, fuse together and squeeze it to death. The lethal tenant is left vertical and proud with its own roots in unencumbered soil. In other trees the benefactor crashes to the ground under the weight of its visitor, but by then the fellow-traveller has moved on in the canopy to bask in sunlight at a second tree’s expense.
Some plants twine clockwise and some anti-clockwise - as in the famous case of the right-handed honeysuckle and left-handed bindweed. A mutation called ‘lefty’ in a small mustard plant persuades the normally straight stem to spiral to the left, while another causes a bias in the opposite direction. Each changes the shape of a crucial protein in the cell skeleton. The molecule looks like a string of asymmetric dumb-bells, with each element lying together head to tail to form a helical and hollow cylinder. The mutations enlarge one or other end of the protein and deform the cylinder, which changes the pattern of cell division and causes its owner to twist. In an echo of the Flanders and Swann song, plants with a single copy of each mutation do indeed grow straight up (although they do not fall flat on their face). For reasons unknown, a bean that normally circles to the right increases its yield if forced to twist in the opposite direction.
Climbing plants are of interest to gardeners, to brewers and to wine-drinkers but for Darwin they were an introduction to a whole new range of botanical talents.
Movement in Plants
, his second volume on the topic, shows that leaves, root-tips, shoots and other parts of all species, climbers or not, are in constant motion. They respond to circumstances in much the same way as do animals. Plants might be slower, but they get there in the end.
The hop’s ability to climb is matched by the skills of every seedling as it emerges into a hostile world to fight for light, for water or for food.
Movement
contains a graphic description of what might appear to be the purposive actions made by a newborn plant in its first days. In the struggle to turn into the right position, to push its root into the soil and its shoot into the air, a seed as it germinated reminded Darwin of a man thrown on his hands and knees by a load of hay falling on him. ‘He would first endeavour to get his arched back upright, wriggling at the same time in all directions to free himself a little from the surround pressure . . . still wriggling, would then raise his arched back as high as he could. As soon as the man felt himself at all free, he would raise the upper part of his body, whilst still on his knees and still wriggling.’
To escape to safety the shoot and the root must each respond to light, to gravity, to touch or to other stimuli. We ourselves live in a universe of senses - of sight, sound, smell, taste, touch and, the forgotten sense, position. Seedlings have no noses, tongues, fingers or ears, but they too perceive the outside world. Animals use electricity and chemicals to pass messages through the body - and so do plants. They have no muscles - but they grow to where they need to be, or move with the help of molecular machinery quite like that which drives our own limbs. As Darwin put it, ‘it is impossible not to be struck with the resemblance between the . . . movements of plants and many of the actions performed unconsciously by the lower animals . . . the most striking resemblance is the localisation of their sensitiveness, and the transmission of an influence from the excited part to another which consequently moves’.
Without eyes, ears or nerves, how can a plant know which way is up, what has touched it or whether the sky is blue or grey? Now, we have begun to find out.
Father and son identified two general kinds of activity - those in which just a response, and not its direction, is related to the external trigger and those that involve a move towards, or away from, an outside stimulus. Among the former, they noted that many plants open and close their flowers in sunlight and shade, or ‘go to sleep’ as they fold their leaves at night, perhaps to reduce the amount of heat lost by radiation. Some, like the mimosa, also responded to a sudden prod with a collapse of the leaves in an attempt to frighten off a hungry insect, or to expose an enemy to the thorns with which its branches are decorated. All those with sensitive leaves slept at night but plenty of the sleepers were quite indifferent to a poke.
For the mimosa and its fellows such actions come from a sudden loss of internal pressure in each frond, which spreads to the leaves next to that actually touched. Certain cells held in a bulge at the base of the leaf-stalk are crucial, for if they are rubbed or tickled they act as hinges and the leaf folds at once. They are more sensitive than are our own fingers. The hinges also control the response to darkness to light. Each has a long hair that acts as a lever and is embedded in a sensory cell. On a stimulus, the magnified movement at the base of each sets off a response in the hinge as charged molecules are pumped across its membrane. At once, water is lost, parts of the internal skeleton of the cell collapse and the leaf folds up. In time, the plant forces water back into the crucial structure and sets it ready to respond to the next challenge. The pattern of two opposed forces at work to close or to open the leaf is rather like our own arrangements, in which one muscle causes a limb to extend and another makes it flex.
Many flowers can tell the time and the ancients sometimes set the hour of prayer with a quick glance at the garden. The great classifier of life Linnaeus even designed a bed filled with blossoms that opened at different hours to make a crude botanical timepiece. The talents of many such species - such as the sunflowers that track the sun through the day - turn on no more than a direct response to outside stimuli. Mimosas have a more subtle sense of the hours, for when placed in constant darkness the rhythm of sleep and wakefulness persists. They have an internal biological clock, independent of light and dark. The plants undergo what Darwin referred to as ‘innate or constitutional changes, independent of any external agency’.
An internal timer, based on the build-up and breakdown of chemicals, maintains the daily rhythm. The clock is not precise and will wander away from strict time if kept in constant light or dark. Different species have internal timers with a cycle that varies from around twenty hours to about thirty. Dawn resets the mechanism, which hence keeps up with the shifts in hours of daylight as the seasons progress. The inner and the outer world interact, for in mimosas the leaves do not fold up at night unless they have been illuminated during the day.
Such movements have what might look like purpose, but they lack direction. Other botanical talents give the impression, almost, of having a definite goal in mind. A plant’s life is ruled by the sun, by water, by food and by predators. To survive, it must avoid its enemies and find its friends and, like an animal, hunt for food, water, shelter and - most of all - sunlight.
The Darwins discovered that young shoots will grow towards even a dim light. That simple observation led them to their most significant result: the discovery of an internal chemical message - a hormone - that altered growth. It was the first of thousands of such chemicals now known.
Their experiment was simple but ingenious. A shoot of grass bent over towards the light. It did so, they found, only if the beam hit its topmost point from one side. If the very tip was covered, or the light was directed to a spot just below it, the shoot remained unmoved. In addition, when the plant was buried in sand with only the tip left in daylight and the rest in pitch blackness, the buried shoot grew towards the source of illumination although the rays never touched it. Short bursts of light had about the same effect as did a single longer glow and even very low levels did the job. The tip of the shoot, they realised, was sensitive to the sun’s rays and somehow the information on where it came from was passed (‘influence is transported’) to the stem below and persuaded it to alter its activity.
Years later, in 1913, came direct proof of a chemical messenger. The amputated tip of a stem was placed in daylight on a piece of soft sponge. The sponge soaked up the crucial substance as the scrap of tissue pumped it out and, when it was laid on to a cut stem whose own tip had been removed, the shoot at once altered its pattern of growth. The botanical envoy was named ‘auxin’ (after the Greek
auxein
, ‘to grow’). It was the first hormone.
For plants and animals alike, to learn about the world outside is not enough. To respond to the messages of opportunity or danger that pour in, information must be transmitted from the point of arrival to a body part that can respond to them. News about the outside world travels through an animal’s body through many routes. Nerves pass it on from cell to cell (and all cells, nerves or not, talk to their neighbours), while distant tissues communicate with the help of chemicals released into the bloodstream. Plants have no nerves, but they, too, pass instructions between cells through special channels that traverse their thick walls and allow the living parts to touch. Darwin’s hormone travels downwards from the shoot tip in that fashion and the channels in addition transfer proteins, nucleic acids, hormones and even viruses. Plants also have open vessels - but unlike our own bloodstream, liquid does not circulate but moves from roots towards shoots or leaves, whence it is lost by evaporation. As a result, any flow of information is one-way. The hormones that travel through the vessels include proteins and molecules that control cell division and cell death as well as others that control the decision to flower or to store food. Other signals tell the dark world beneath the soil when spring has come while yet more keep an eye on the balance between food and growth or send warnings about the arrival of an enemy.
Dozens of plant hormones are known. The chemicals resound through their tissues in response to light, heat, damage, the passage of time and more. Some emerge in unexpected places. Human urine applied to a decapitated shoot alters its growth because a plant’s auxin passes unchanged through the body of those who eat it (and the substance was in fact itself first purified from that invaluable fluid). Now the messengers are studied not just with chemistry but with mutant plants whose altered growth is due to an aberrant response to hormones.
Most plant hormones are simpler and smaller than our own. Some have a chemical structure based on closed carbon rings, but many are small proteins. A few even look rather like the steroids that control human sexual attributes. Like mammalian chemical messengers, they are often arranged in pairs, with some that promote an action and others that oppose it. Each has a receptor on the target tissue to which it binds, and each acts - as do those of animals - to stimulate or repress the action of particular genes. Some cause individual cells to expand or to contract, while others change the rate of cell division - should, for example, cells divide faster on the dark rather than the light side of a shoot, then the whole structure bends towards the source of illumination.
Such molecules determine when their masters will ripen, lose their leaves, move towards or away from light and gravity, fight infection, and more. Those concentrated in the tip of a shoot act to suppress the activity of sections of the plant that lie below them. Auxin diffuses downwards and prevents the growth of buds that might compete with the tip itself for light. Cut off the tip and those segments burst into life - which is why gardeners prune their fruit trees to get a dense bush.
The locks into which the auxin keys will fit have also been discovered. Many of the inherited changes in shape treasured by gardeners result from errors in the hormone genes or their receptors. The auxins persuade the shoot to grow up or the root down, the flower to bloom and the fruit to swell under the influence of auxin from its seeds. The sinister movements of the sundew as it rolls its leaves over a trapped insect are due to another member of the same chemical family. In some ways the auxins resemble the substances involved in nerve transmission more than they do animal hormones. Indeed, some of our own nerve transmitters are found in plants but quite what they do is not yet clear.
The auxins and their relatives have often been turned to useful ends. Gardeners and farmers use artificial versions to help cuttings to take root. Agent Orange - so named after the colour code on its barrels - was an artificial auxin that caused vegetation to grow itself to death. It was used for a decade at the time of the Vietnam War. Over seventy-five million litres were used in an attempt to destroy the guerrillas’ crops and to open up the forest to expose the enemy. Its military value was never proved and the chemical was abandoned when found to be contaminated by the poison dioxin. Even so, synthetic auxins are still used as herbicides and appear to be safe.