Darwin's Island (23 page)

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 sunshine. 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 Japanese 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. Shelley 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 Shelley’s sympathetic subject is
Mimosa pudica
, in reference to its bashful nature, and the Chinese call it ‘shyness grass’. 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 fashion. 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.

Shelley’s lines are both a literary device and an accurate observation. They also say something about the relationship 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
of 1875 deals with how ivy, brambles and the like find and scramble up their vertical helpers.
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.

 

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, passed as a holiday.
Climbing Plants
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 glass 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 compass. 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
, 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 pass 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 mass 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 sunshine 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 mass 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 space (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 thorny 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.