Darwin's Island Part 5

Such changes in physical structure, together with those towards baldness, an upright gait and an agile mind which marked the transition from ape to man, involved a large cost, the death or s.e.xual failure of millions who could not cope with the new way of life. The more recent changes in skin colour, the ability to drink milk or beer, or to digest grains demanded just the same sacrifice. The speed at which advantageous genes have spread since the origin of farming suggests that the price was high indeed. The same process is at work today. We face a new abundance, quite different from anything in the evolutionary past, and have not yet evolved to deal with it. We may do so, but the process will not be cheap. For most of history, humans have had to cope with shortage rather than excess and have evolved mechanisms to guard against excessive weight loss when food is short. Our bodies deal less well with today's glut. Starvation disguised as surfeit means that evolution's inexorable machine has cranked up again, with natural selection by diet as active as it was ten thousand years ago.

Our individual response to excess depends strongly on our DNA. A study of twin boys and girls suggests that around seven-tenths of the variation in body weight within a population is due to genetic variation. Identical twin children also resemble each other in how much they will eat if offered a huge meal. Adult twins paid to gorge themselves, or to starve, for several weeks also tend to gain, or lose, weight - to resist, or to surrender to, the challenges of the new diet to the same extent.

Dozens, perhaps hundreds, of genes have been blamed for the new wave of obesity. Fatness runs in families but so do frying pans - and fat cat-owners tend to have fat cats. Their pets share their diet but not their DNA. Nature and nurture work together and the inheritance of pot bellies is - like that of almost everything else - not simple. The notion that fat people can blame the way they are born and ignore what they choose to eat is wrong. Instead, like alcoholics they are more at risk of a certain kind of diet than are others and must struggle harder against temptation. Many people fat today would have been thin a century ago, whatever the nature of their DNA.

Even so, genes have a real influence on waistline. Most of those that predispose to obesity have a small effect, but a certain variant, when inherited in double dose (as it is by ten million Britons), increases body ma.s.s by three kilograms above average. Even a single copy, as borne by almost half the inhabitants of these islands, adds a kilogram. The DNA variant concerned changes appet.i.te and is active in parts of the brain involved in hunger or satiety. The gene is harder at work in starved individuals than those who have just eaten. It makes no difference to their weight at birth but babies with two copies begin to pile on the kilograms within just two weeks. Other genes that dispose to obesity alter the efficiency with which food is soaked up or the rate at which the body burns its fuel.

The environment itself has effects that stretch over the generations. Just as alcoholic mothers have damaged babies, women who eat fast food have fat children not only because they pa.s.s on their genes, but because they were themselves overweight while pregnant. About a third of all pregnant Americans - and half of pregnant African-Americans - are obese. Their internal economy s.h.i.+fts to deal with the problem, as does that of their unborn child. The evidence is clear, for those who lose weight, perhaps through stomach surgery, between one child and the next, tend to have lighter babies than before. The foetuses of overweight mothers respond to the high levels of insulin (the hormone that controls blood sugar) in their mother's bloodstream and are born attuned to lay down fat. Genes that dispose to diseases of the obese are then put under further pressure.

The biggest threat to the overweight is diabetes; not the rare variety that affects a few infants and can be treated with insulin but a related illness that comes on later, defies treatment and has a strong tie with an expanded waist. The problem arises from a resistance to insulin. Its symptoms include heart disease, kidney failure, blindness, nerve damage and even gangrene. It was once a disease of the elderly, but is seen more and more in children and adolescents. Just one extra notch on the belt adds a lot to the danger and those in the top tenth of trouser size have twelve times the risk of diabetes than do the slim.

Half a billion people will soon suffer from adult-onset diabetes. Unless matters improve, a baby born in the USA today has a one in three chance of the condition when it grows up and the illness already takes up a sixth of the country's entire health budget. Even in Britain, two million people show signs of it. In some places the figures are dreadful, with half the adult population of the island of Nauru, in the Pacific, afflicted. Genes that increase body weight are most to blame, although others do increase the risk in both the fat and the thin.

Obesity is in part inherited, and is a target of natural selection because it kills many people before their time. Its effects on the evolutionary future are made worse because those who suffer from it face not just premature death but s.e.xual failure. Fat people tend to have fewer children than average. Apart from the romantic problems involved, obese men find it harder to sustain an erection, and obese couples copulate less often, than do the fas.h.i.+onably slim. Even worse, a fat man's sperm count drops by around a quarter, perhaps because his over-insulated t.e.s.t.i.c.l.es are too warm. Female fertility, too, drops with every extra kilo. Excess fat interferes with the menstrual cycle and has other harmful effects. Among women anxious to become pregnant even a slight weight excess increases the time before a favourable outcome by a month, and it takes nine months longer for an obese woman to have a good chance of becoming pregnant than for a person of normal size. In addition, overweight women are more liable to miscarry and their children are at higher risk of birth defects.

All this means that natural selection by diet is once more hard at work, as it was when agriculture began. Darwinians, faced with the problems that have emerged from the new way of life, can hence afford a certain grim optimism about the future. Man evolved to deal with a changed diet in the first food revolution, and will no doubt do so in the second, whatever the cost. In this era of global glut, natural selection may act on future generations until they return to slimness and health in an affluent world, just as the descendants of the first farmers evolved their way out of their dietary problems.

The crude tools of evolution are, needless to say, far less effective in moulding the future than is the simple human ability to learn from our mistakes. Societies facing the waistline problem are better advised to consider the risks, plan ahead and eat less than to await the attentions of biology. Everywhere, people are exhorted to improve their diet and take up exercise although so far the propaganda has not been particularly effective. Even Marie Antoinette was trying to help. The famous 'cakes' offered to her starving nation were not rich and lard-laden delicacies, but baked crusts that might otherwise have been thrown away. A simple error gave rise to a legend of political incompetence and to a sticky end. In these days of excess, her regal counsel seems more sensible than it did at the time of the French Revolution. Whether people will take her advice and modify their lethal habits or whether they will wait for natural selection to do the job, it is, to quote Zhou En-lai on that interesting political event, too early to say.

CHAPTER VI.

THE THINKING PLANT.

Deep in the Amazon jungle a creature snakes into the light. As it climbs cautiously through the branches it senses a brighter spot on a distant tree. After weighing up the risks of abandoning its present post it plunges back into the gloom of the forest floor and creeps across the ground until at last it reaches its target, scrambles upwards and triumphs to bask high in the tropical suns.h.i.+ne. The vine - for such it is - shows every sign of foresight in its behaviour. The notion that a plant might act in what appears to be an intelligent way is alien; less so than before time-lapse films speeded up the circling of shoots or the opening of flowers, but unexpected at least. Can such a simple creature really plan ahead?

Romantics have long been convinced that the vegetable kingdom has a mind of its own. Gardeners talk to their crops in the hope that they will flourish, while tree-huggers, when not in close embrace with a trunk, often play a part in the conservation movement. Real enthusiasts for botanical intelligence believe that cacti grow fewer spines when they listen to soft music and put them out again when they see a cat. The j.a.panese even enter into two-way conversations with their green friends. They have patented an electronic device through which a flower can chat to its owner or, when thirsty, ask for water. In the 1920s, the famous Indian physicist Chandra Bose, a pioneer in the study of electromagnetic waves, worked on electrical activity in plants. His subjects did generate a measurable current when damaged (an observation that led to genuine scientific advances) - but Bose was also certain that music and kind words could set off the response.

Dubious as such claims might be, the mental universe of plants is, if nothing else, useful fuel for metaphor. Sh.e.l.ley writes of a garden in which a mimosa droops in response to a rejected lover's despair: 'Whether the sensitive Plant, or that/ Which within its boughs like a Spirit sat,/ Ere its outward form had known decay,/ Now felt this change, I cannot say.' The Latin name for Sh.e.l.ley's sympathetic subject is Mimosa pudica Mimosa pudica, in reference to its bashful nature, and the Chinese call it 'shyness gra.s.s'. Whatever the plant's mental state, it does respond to the outside world. For most of the time, a mimosa's branched leaf stands proud, but a slight touch, or a gust of wind, causes it to droop in a hang-dog fas.h.i.+on. It can take hours to recover. At night, no doubt exhausted by the emotional turmoil of the day, the leaves close up and their owner goes to sleep.

Sh.e.l.ley's lines are both a literary device and an accurate observation. They also say something about the relations.h.i.+p of mind with brain. If a mimosa can act in what seems a rational way even in the absence of any hint of cerebral matter, what does the endless debate on that topic mean? Philosophers, like poets, should perhaps pay more attention to botany.

Charles Darwin, as a competent scientist, had no real interest in such metaphysical ideas (he did, admittedly, claim that plants sometimes recoil in 'disgust'). He was nevertheless curious about their ability to react to the conditions in which they are placed. He wrote two books on the subject. The Movements and Habits of Climbing Plants The Movements and Habits of Climbing Plants of 1875 deals with how ivy, brambles and the like find and scramble up their vertical helpers. of 1875 deals with how ivy, brambles and the like find and scramble up their vertical helpers. The Power of Movement in Plants, The Power of Movement in Plants, published five years later, asked wider and more radical questions about how all plants respond to the outside world. It had, he wrote, 'always pleased him to exalt members of the botanical world in the scale of organised beings', and in those volumes he succeeded. Together, the two books discuss three hundred species. Darwin placed the plant kingdom on a higher scientific plane than ever before, for the experiments in his greenhouse laid the foundations of modern experimental botany. published five years later, asked wider and more radical questions about how all plants respond to the outside world. It had, he wrote, 'always pleased him to exalt members of the botanical world in the scale of organised beings', and in those volumes he succeeded. Together, the two books discuss three hundred species. Darwin placed the plant kingdom on a higher scientific plane than ever before, for the experiments in his greenhouse laid the foundations of modern experimental botany.

His home county was in those days famous for hops. So fond was the British working man of his beer that Kentish fields were filled with poles and wires up which the bitter vines were trained. Each September tens of thousands of labourers and their families came from London to pick the crop and to have what, in Victoria's glorious days, pa.s.sed as a holiday. Climbing Plants Climbing Plants asks a simple question. How does a hop find a support and climb up it? asks a simple question. How does a hop find a support and climb up it?

To succeed, its shoots as they peep above the soil must seek out an upright of the right size even if they emerge some distance away. Then they must twine around it to clamber upwards. The talents of the hop were the introduction to a new world of botanical behaviour.

Most of the work was done with the help of Darwin's son Francis. It was, as ever, interrupted by ill health: 'The only approach to work which I can do is to look at tendrils & climbers, this does not distress my weakened Brain.' Charles noted, first, that a pot plant in his sick-room circled round as it grew. He and Francis began to cultivate a variety of species beneath clear gla.s.s plates upon which the position of the tip could be marked with ink. They saw that the shoot of a young hop travels round all points of the compa.s.s. On a hot day a complete revolution took about two hours. Should the questing tip touch a pole, the hopeful climber then changed its behaviour, snaked around it and found its way to the top. What looked like forethought depended on just three simple talents: the ability to circle, a sense of touch and the capacity to tell up from down.

Father and son went on to study other plants that climb not just with their shoots, but with structures such as tendrils, hooks or adhesive roots. Whatever the details, almost all the climbers gyrated until they found a support and, once found, clambered away from the ground. The Darwins soon discovered that all shoots, even in species that do not climb, in fact circle to a greater or lesser degree. In the same way, all plants can modify their growth to avoid an obstacle, and all can sense gravity. A hop's unusual powers depend - as do many patterns of animal behaviour - on natural selection's ability to modify talents that already exist.

The second book, Movement in Plants Movement in Plants, went further. It describes experiments on the sensitivity of roots, shoots and more to light, gravity, heat, moisture, chemicals, touch and damage. The research was far ahead of its time. Although they did not invent the name, father and son discovered the first hormone - not in animals (an event which had to wait for almost thirty years before scientists at University College London found a chemical messenger in the blood of dogs) but in plants. So impressed was Charles Darwin by the powers of shoots and radicles (the first structures to emerge from the seed at the time of germination) that his book ends with a dramatic claim: 'It is hardly an exaggeration to say that the tip of the radicle thus endowed, and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements.'

Any creature, animal or vegetable, needs, as it copes with the outside world, to find out what is going on, to pa.s.s the information to the appropriate place and to respond to the challenges presented by Nature. Men and women do the job with eyes, ears, nerves, brains and muscles. Plants have none of those, but cope remarkably well - and in some ways they put our own abilities to shame.

Why climb? Lord Chesterfield got it right. In one of his notorious letters of advice to his son he wrote that 'A young man, be his merit what it will, can never raise himself; but must, like the ivy round the oak, twine himself round some man of great power and interest.' A plant with such an ambition uses its support to reach a lofty place to which it could otherwise never aspire. The helper might come to regret its generosity, but the advantages from the social climber's point of view are clear.

Such behaviour opens up a new universe of opportunity. The plants that first evolved flowers able to attract pollinators, and those that first developed fruits to persuade animals to move seeds, each discovered a whole new set of habitats and a variety of ways of life. As a result their descendants burst into a variety of form. The ability to climb is less dramatic than are fruits and flowers, but those who take it up have also evolved into a vast diversity of kinds. A hundred and thirty different families in the botanical world have climbers. Within each group, those agile creatures are represented by many more species than are their earthbound kin.

Birds, bats, flying squirrels, snakes and fish all take to the air but in different ways, with modified arms, hands, bodies or fins. In the same way, plants have called upon different organs to help them climb. Some, like hops or peas, use tendrils, based on stems or leaves. Others, such as clematis, have altered leaves in other ways, or evolved specialised roots or hooks that allow them to scramble. Roses have hooks. The ivy uses roots to clamber fifty metres and more up cliffs, houses or trees while Virginia creepers go to the opposite extreme and use shoots. In a certain group of ferns the fronds grow around the support to make their way towards the light.

The habit is ancient indeed. Three hundred million years ago, the Earth had vast coal swamps filled with fern-like trees fifteen metres high. The forest had plenty of vines and climbers, which used structures like those of modern plants to struggle into the sun. It became a tangled and impenetrable ma.s.s until at last the whole lot was wiped out as the climate changed.

Tropical forests are still the capital of the scramblers. There, every plant must fight to reach the suns.h.i.+ne against thousands of others. Many jungles are filled with lianas, woody vines that loop down from the trees. In most places they represent less than a tenth of the total ma.s.s of live material - but their tactics are so effective that their leaves fill half the canopy. Almost half of all woody species in the Amazon basin are climbers, with fifty or more different kinds in every hectare. They are fond of gaps, places left open when a moribund giant crashes to the ground or when farmers clear a s.p.a.ce (which is bad news for the farmers themselves as they compete with the creepers to grow a crop). When forests - tropical or temperate - are broken up by loggers, the lianas and their relatives thrive even as the trees upon which they depend are destroyed.

Climbers climb, in the main, to get into the light. Another good reason to take up the habit is to escape, like a baboon pursued by a lion, from ground-based enemies. Leaves near the surface get chewed more by slugs, snails and the like than do those up in the air. In the arid deserts of northern Chile, convolvuli often grow near cacti or th.o.r.n.y shrubs. After an attack by hungry mice, or by scientists with scissors, they at once increase the rate at which they twine and put out more tendrils in the hope that they will reach a shrub and clamber into the safety of its spiny branches.

Darwin noticed that most twiners needed a rather slender pole if they were to make progress - British climbers, indeed, never curl around trees. The upright must also be rough enough to give them a chance to hold on. The climber does not cling with its whole length, but sets up a series of contact points as it moves onwards. Rather like a bloodhound, it sniffs the air now and again as it tracks its route. As it moves, the tip is raised, circles round and comes back to the stem a few centimetres further on. The details vary, with some tendrils set like a coiled spring to twist within seconds around a support as soon as they touch it. Engineers have worked out that for a smooth pillar the climber cannot manage to ascend a support more than about three times its own thickness - a twig, or a vertical wire. The rough bark of a tree makes the job rather easier. Part of the spiral motion as a hop moves on comes from an increase in the rate of growth on one side of the plant compared with the other. In addition, cell walls on that side become looser, bulge up and force the shoot to wind round and round. In time, a tendril can coil in upon itself and grow hard and woody, to lock its support in a fierce embrace.

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 pa.s.sage 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 unenc.u.mbered 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 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 circ.u.mstances 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 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.' 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 pa.s.s 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 cla.s.sifier 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 const.i.tutional 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 s.h.i.+fts 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 gra.s.s 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 pa.s.sed ('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 sc.r.a.p 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 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 pa.s.s 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, pa.s.s 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 fas.h.i.+on 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 pa.s.sage of time and more. Some emerge in unexpected places. Human urine applied to a decapitated shoot alters its growth because a plant's auxin pa.s.ses 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 s.e.xual 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.

The leaves that hid the Viet-Cong from aircraft formed a dense screen as the trees that bore them struggled for life - and for light. All plants need sunlight and will fight for that precious resource, often to the death. Every forest is the result of a silent battle between the leaves far above, as each jostles to get a view of the sun. Together, they block its rays. Some bathe in its beams, but others fail, pale and die. To survive they need to pick up the solar radiation, measure its intensity and move, or grow, in response.

The Darwins found that shoots can pick up light and pa.s.s on information but they had no idea of how they noticed the solar presence or of their exquisite sensitivity to wavelength, intensity and direction. They do the job in several ways, with a variety of special molecules, some of which have equivalents in the animal kingdom.

One group of receptors known as phototropins picks up blue light and plays a large part in the growth of shoots towards a source of illumination. Another group, the phytochromes, is sensitive to the longer waves, the red and infra-red. Phytochromes have a protein skeleton matched with a second chemical structure based on linked rings of carbons bent into a molecular knot. The molecule is poised like a set mousetrap and when light strikes it it flips from one shape to another. In the dark or shade the change is reversed. The balance between the alternative forms tells the plant how much light is in the sky. The light-sensitive part looks rather like the chlorophyll found in leaves, and - less predictably - resembles the breakdown products of our own red blood pigment (which is why jaundiced babies can be helped with a dose of intense light).

In a dense forest the proportion of infra-red that hits the ground is no more than a twentieth of that experienced by a plant exposed to full sun, because most of the energetic radiation is soaked up by the leaves above. Because the pigment measures not just light intensity (which changes as the day wears on), but also the ratio of red to infra-red, it can distinguish a shortage of light in the evening or on a cloudy day (when the proportion of the two wavelengths does not change) from an attack of gloom that arises because other leaves have shaded out the solar disc and have stolen the most valuable part of its spectrum. Once the infra-red alarm has sounded, those in the shade must respond to the emergency before the source of energy is blocked altogether.

A plant in this predicament s.h.i.+fts its whole pattern of life. It grows faster and the stalk stretches higher. Each leaf moves to present a flatter surface to what light is available. If the shortage continues, the leaves become thinner and more transparent and their parent becomes less branched as it reaches for the sky. If all this fails, and the light still stays red, the unpalatable truth becomes clear. The victim flowers as soon as it can to give at least some hope that its genes will be pa.s.sed on before it dies in darkness.

The phytochromes are smart, but other pigments are even smarter. A third set of sensors, the cryptochromes, respond not to red, but to blue light. In a further parallel to our own eyes, with their three distinct receptors, a green-sensitive pigment has also been found. Cryptochromes have a structure rather like that of the enzymes that cut and splice DNA. They measure the intensity, rather than the wavelength, of light. Their main job is to sense how long each day is - an important factor when deciding whether to make flowers or fruit as the seasons move on. They are also hard at work in the newborn seedling as it wriggles its way towards adulthood, for they s.h.i.+ft the whole biochemical economy of a young shoot from a life based on the gloomy world of the soil to a career bathed in suns.h.i.+ne.

Many of the sensor molecules have relatives in our own eyes. They too help to work out the length of each day and to a.s.sess wavelength (or colour, as we call it). Some of the magic proteins are the very stuff as dreams are made on for not only do they cause the sensitive plant to droop but they control human sleep rhythms. A long trip in a jet plane leads to unpleasant side-effects - and the cryptochromes help to put them right, which is why a global traveller finds it harder to adjust to local time in gloomy London than in sunny Sydney. Mice in which the blue-light gene has been damaged by mutation sleep more and have less active brains as they snooze - and, for reasons unknown, also show s.h.i.+fts in their response to anti-cancer drugs. The levels of such chemicals in our own eyes are also tied to the annual swings of mood familiar to those with seasonal affective disorder, the Black Dog of winter (although no fit has yet been found between genetic variation in the human genes and liability to that unpleasant illness). The sensitive plants droop in gloomy weather with the help of cryptochromes and so, it seems, do we.

Roots, in contrast, prefer darkness and make a real effort to avoid daylight. Once again, blue light does the job, with its own special receptor molecule in the tip. The root has another talent which helps it delve into the soil, for roots can sense the force of gravity. For climbers, too, the Earth's attraction is important, although they prefer to move in the opposite direction. Darwin found that the crucial sense organ for gravity resided in the tip of the root and the shoot and that to cut off that tip much confused the growing plant. Now it has been tracked down - and, once again, it has some uncanny similarities with the human system that does the same job.

Men and women maintain their equilibrium with a set of liquid-filled tubes in the inner ear, arranged in three dimensions, left and right, forward and back or up and down. They contain a liquid that washes back and forth as we stand, sit or move about. Tiny grains of calcium carbonate rest on special cells on the inner surface of each tube and s.h.i.+ft as gravity or acceleration directs them. The movements of the fine hairs on each cell are translated into electrical messages to the brain to give us a sense of where we stand.

Plants do the same with special cells in roots and shoots. Each contains small grains of starch which, like the minute particles within our ears, s.h.i.+ft as their owner moves. Mutants unable to make starch lose both their sense of gravity and the ability to circle. Darwin speculated that the questing movements he found in hops and the like depend on the Earth's attraction, but he was not altogether right, for, in an experiment that would have flabbergasted him, the tips of plants held in weightless conditions on the s.p.a.ce Station continue to make their measured rounds.

A closer look at both people and plants shows further parallels in the gravity sensor. In the ear, a molecular rack and pinion uses a pair of proteins that play a part in muscle to pick up the movement of the small grains as they are washed back and forth. In the plant a pair of almost identical molecules does the same job.

Poets, mystics and romantics often imagine the vibrations of a sixth, seventh or eighth sense (although Sh.e.l.ley had more sense than to do so). One popular candidate is magnetism - a topic tarnished from its earliest days when the German mountebank Franz Anton Mesmer claimed that 'animal magnetism' - the supposed ability of some people to open blocked bodily channels in the afflicted - could cure blindness and more. The idea was used by Mozart in Cos fan tutte Cos fan tutte but blown out of the water by a French governmental commission headed by Benjamin Franklin. There are still plenty of magnetic therapists, who sell hundreds of millions of dollars' worth of magic bracelets, insoles and blankets in the United States each year. but blown out of the water by a French governmental commission headed by Benjamin Franklin. There are still plenty of magnetic therapists, who sell hundreds of millions of dollars' worth of magic bracelets, insoles and blankets in the United States each year.

Biology has gained a renewed interest in our interactions with the Earth's magnetic field. The subject still attracts odd claims: some say that blindfold students can find their way home thanks to a supposed internal compa.s.s, while aerial shots hint that cattle tend to line themselves up to face north or south. A strong magnetic field does spark off brain activity, but what relevance that has to daily life is not clear.

Many creatures do have a strong and unexpected ability to sense direction, with the help of a magnetic compa.s.s. Migratory birds, for example, have iron-rich cells in their brains, and use the Earth's field to find their way back and forth across the globe as the seasons change. To do so, they use the products of a gene remarkably similar to that of a plant's blue-light sensor. Its molecule is an essential part of an internal timer employed by the migrants as they measure the angle made by the sun as it sweeps across the sky and use the information to orient themselves north or south. In addition, it helps the birds to navigate by the Earth's lines of magnetic force.

They can pick up the field only in daylight, and the blue sensor is what allows them to do so. It works at the atomic level. Electrons come in pairs that spin around the nucleus in opposite directions. As a result they cancel out each other's ability to act as magnets. When energy - from light, heat or chemical reactions - enters the system, some particles are knocked off balance and are left with just a single spinning electron. Such 'free radicals' need to marry another partner as soon as they can. To find a match they must change their direction of rotation. A magnetic field at right angles to the spin makes that task harder. As a result, a bird can use unpaired electrons as a compa.s.s needle that allows them to sense the Earth's magnetism. The cryptochromes generate lots of free radicals when they pick up the energy of blue light and transmit it into the cell and with their help the happy migrant gains an improved sense of direction.

Magnetism also has an effect on plant growth. The starch grains in the tip of a shoot can be moved with a magnet and the field then helps tell a plant where it stands (an attempt to test the results of that experiment without gravity perished in the Challenger Challenger disaster). A magnetic field also slows the growth of shoots - but only in blue light. Mutants that lack the relevant sensor are quite indifferent to its presence. Perhaps all this hints at a forgotten shared sixth sense, in both animals and plants. Darwin often spoke of 'fool's experiments', ideas that were most unlikely to work but were worth a try. He speculated on the role of magnetism in animal navigation but he would be amazed to find that it applied to the movements in the other kingdom of life. The experiment was too foolish even for him. disaster). A magnetic field also slows the growth of shoots - but only in blue light. Mutants that lack the relevant sensor are quite indifferent to its presence. Perhaps all this hints at a forgotten shared sixth sense, in both animals and plants. Darwin often spoke of 'fool's experiments', ideas that were most unlikely to work but were worth a try. He speculated on the role of magnetism in animal navigation but he would be amazed to find that it applied to the movements in the other kingdom of life. The experiment was too foolish even for him.

In another instance of what his son called his 'wish to test the most improbable ideas', he persuaded Francis to play the ba.s.soon to a mimosa to test whether it would respond. Sweet music, like kind words, had no effect; and a later experiment in which pop music was blasted at them for hours also left them unmoved. Even so, plants do hold conversations. They use not sound to do the job, but scent. A series of chemical messengers have evolved to help, and together they provide a sense of smell to add to those of sight, gravity and the rest.

Many pump out a simple gas called ethylene, which causes them to grow faster and also helps fruit to ripen. In dense vegetation, the gas persuades leaves to struggle harder towards the light, for it reveals the presence of compet.i.tors. The ability to smell can be used for more sinister ends. Dodder, also known as devil's guts, witch's shoelaces, h.e.l.lbine and the like, is a parasite related to the morning glory. It is a pest of carrots, potatoes, clover and garden flowers. Its leaves are tiny and contain almost no chlorophyll. The seeds can survive for ten years. Once they germinate, the shoot pokes above the soil and searches for a potential victim. It circles round until it succeeds. Then it inserts fine needles whose cells fuse to those of its quarry. They suck out vital fluids and the plant loses its own root, to live as a parasite. It has a strong preference for juicy species like tomatoes over tougher kinds such as wheat.

The dodder, like a bloodhound, sniffs out its prey. When allowed to germinate in a closed container with two tunnels, one that leads to a chamber with healthy tomatoes and the other to a similar s.p.a.ce with wheat, it directs its growth towards its preferred host - the soft and juicy tomato - and is repelled by its alternative.

A plant can also use its sense of smell for protection. Some species talk to themselves, for a damaged leaf causes others nearby to get ready for attack - but not when the leaf is sealed in a plastic bag, as proof that an airborne signal is involved. Leaves chewed by insects pour out signals that are picked up by their neighbours, who prepare their own defences. Other species listen in to foreign messages. Tobacco seedlings grown close to a sagebrush bush switch on their own anti-insect chemicals when the bush is pruned. A few can even call for help. A beetle that chews a lima bean takes a real risk, for its victim sends out an aerial message that persuades leaves nearby to make a sugary secretion that attracts ants and wasps. The visitors then attack the beetle.

Plants can taste chemicals in solution, as well as smell them in the air. They extract information from the liquids that bathe their roots and flow across their leaves. Roots and shoots sense the presence of enemies and grow away from them. They hunt for food, too, for when a root hits a rich spot, it stops, sprouts and sucks up what is on offer. The substances that they pump into the soil may attract friends such as helpful fungi, but are also hijacked by enemies. Witchweeds are pests of tropical crops such as sugarcane. They grow on the roots of their host and - like the dodder - drain its vitality. They, too, pick up the taste of a dissolved substance used by the host to attract fungi. In the same way corn seedlings whose roots are chewed by grubs pump out a chemical that attracts predatory worms. The American black walnut scares away compet.i.tors with its own secretions and leaves a dead zone beneath its shade - and it is no coincidence that our own tongues are t.i.tillated by the poisons found in pepper, coffee, lettuce and more, which evolved not to satisfy the gourmet but to fight off an enemy.

Darwin himself saw that plants must have a sense of touch, for the climbers themselves, as soon as they contact a vertical object, change their behaviour, give up their wide sweeps and begin to twine. Roots, too, probe the soil and grow their way around a stone too large to move - although in this case they avoid, rather than embrace, the object. He found that the senses of touch and of direction interact, for a root held vertical will grow away from an object that blocks its path - but the same structure kept horizontal will always try to extend downwards, whatever obstacle is placed in the way.

Tree-huggers carry out what seems the entirely witless experiment of embracing a trunk to exchange energies with it; to inject their own vitality into the plant and to obtain some as yet undiscovered botanical spirit in return. In fact, to touch - or to hug - a plant has another unexpected effect, for it inhibits its growth. The pines of Highland forests are small, twisted and bent because they have been caressed - or battered - by the winds. Their equivalents around Down House live in calmer air and soar upwards. Identical seedlings grown in calm and windy places always end up with quite a different appearance. The plants are sensitive indeed for to bend a young tomato plant for half a minute stops its growth for a whole hour. That is why stormy places make for stunted trees.

The most conspicuous response to touch is that of the mimosa, so admired by Sh.e.l.ley. Two centuries after his poem, botanists tried a simple experiment: take a series of chemicals known to act as hormones, dissolve them and water the mimosa to see which bits of DNA respond. For every substance, a previously unknown gene increased in activity by a hundred times. At first it looked as if a crossroads in the hormone labyrinth had been discovered - but sprinkling the plants with pure water had the same effect. Mere contact had done the job. The first of the 'touch genes' had been discovered. A drop of rain - or a gust of wind - causes them to leap into action. More than five hundred separate parts of DNA alter their activity when a leaf is prodded, some by ten times and more. A hundred respond in the opposite way, but why, n.o.body knows.

The touch genes do many things. They cause the cell wall to firm up or loosen in response to stress and growth patterns to alter as a result, which explains the gnarled Highland trees. Some members of the clan respond to night and day, or to the seasons as they pa.s.s (which is why leaves fall off in autumn), and others are involved in disease resistance. They might some day be engineered to give fruits that fall from the tree in a breeze as soon as they are ripe or crops that grow tall in windy places. Half the touch genes also respond when placed in darkness, but quite why they do is still not clear. The sensitive plant is, it appears, more sensitive than anyone imagined.

The inner world of plants has emerged as almost as rich as our own. Darwin was cautious in his parallels between the sensory and intellectual lives of the two kingdoms. The most he would say is that 'It is impossible not to be struck with the resemblance between the foregoing movements of plants and many of the actions performed unconsciously by the lower animals.' Now we know that the similarities go far further than he thought. One persuasive parallel involves the sense of touch. To everyone's surprise, some of the signal proteins used by plants to sense a gentle tap resemble certain molecules that do a similar job for us. They control our heartbeats, switch on hormones that determine growth and alter the blood chemicals that change mood from happy to depressed. In a further twist to the tale, young rats caressed by their mothers respond with an increase in the activity of certain genes related to those that react to touch in plants. A shortage of embraces stunts the animals' physical and emotional growth.

There is something magical in the way that scientific rationalism connects raindrops with heartbeats and battered trees with depressed infants. Sh.e.l.ley himself saw that science told us much that poetry cannot. He filled his Oxford room with electrical gadgetry and saw no contradiction between the worlds of the spirit and of science. He would have been delighted to learn that cooling pa.s.sions are linked to falling leaves and that the Darwinian universal of shared ancestry shelters beneath its ample branches both the mimosa and its poet.

CHAPTER VII.

A PERFECT FOWL.