Darwin's Island (31 page)

As the sage of Down House collected orchids from the fields and heaths around his comfortable home and examined the specimens sent to him from afar, he became more and more impressed by the ingenuity of the ways in which as they pass on pollen: ‘Hardly any fact has struck me so much as the endless diversities of structure, - the prodigality of resources, - for gaining the very same end, namely, the fertilisation of one flower by the pollen from another plant.’ He glimpsed but a small part of the game played by all plants as they fulfil their sexual destiny.

As Darwin showed, nine years after the orchid book, in
The Descent of Man, and Selection in Relation to Sex
, a male peacock’s flashy rear says nothing about the merits of tails, but a lot about his status as a high-quality mate who can afford a gorgeous adornment. The same is true of plants. More food allows them to make more blooms and to proclaim their excellence to a larger audience. To remove a few flowers may also allow them to grow more fruits, as proof of how expensive it is to be attractive. The brightest and most generous individuals get more pollinators and pass on more of their genes, which promotes yet more brightness and generosity in the next generation and, almost as an incidental, leads to an outburst of diversity as the balance of sexual advantage species in different lineages.

Orchids belong to the great subdivision of the flowering plants that generates just a single leaf as the seed germinates. It contains the grasses (crops such as rice included), bananas, tulips and more. The orchids themselves are among the largest families of all for only the group that contains daisies and sunflowers possesses more species. Around twenty-five thousand different kinds are known - about an eighth of all plants with flowers - and no doubt many more remain to be discovered. Britain has just forty-six native kinds, several of which are rare.

Because orchids are so attractive they are important in the conservation movement (and cynics call them ‘botanical pandas’). They may look fragile but many are tough. Their capital lies in the wet and cool hill-forests of the tropics, and a third of all known species are found in Papua New Guinea. Plenty more live in the Arctic or in temperate woodlands, fields and marshes. They grow on the ground or high in the branches of trees, or on rocky slopes and grasslands. A few live underground and never see the light of day. In some places the plants are short of water and, like cacti, develop thickened stems or tubers to store a reserve. Some have leaves as big as their relatives the bamboos while a few are parasites with almost no foliage at all. Others, such as the vanillas, make vines twenty metres long. Some kinds are tiny, with a flower head that would fit on the head of a pin, while the flower of a certain tree-dweller from New Guinea is fourteen metres around and weighs about a thousand kilograms (a specimen caused amazement at the Great Exhibition in 1851). Plenty of others have multiple displays several metres long. A few have opted out of the endless and expensive conflict and are pollinated by the wind while one Chinese kind has abandoned the whole business of sex and indulges in a strange internal dance in which its male element curves backwards and inserts itself into its own female orifice.

Some of the flowers are simple. They are dark and look rather like the entrance to a burrow, which attracts a bee to come in for a snooze and pollinate as it does so. Many others use far more elaborate tactics. Some are perfect six-pointed stars while others resemble a glass-blower’s nightmare with fine tendrils that hang together in delicate and lurid bunches. Yet others look as if they are moulded from thick pink plastic. The flowers are scarlet, white, purple, orange, red or even blue. One species is pollinated by a wasp. It generates a chemical identical to that emitted by a leaf chewed by grubs - the wasp’s favourite food. The wasp as it visits gets not a meal of tasty flesh, but a load of unwanted pollen. For those over-impressed by the beauties of botany, certain orchids smell like putrid fish to attract carrion-feeding flies.

The biological war between flower and insect, like the whole of evolution, involves an endless set of tactics, but no strategy. It has produced a vast variety of blooms, each of which evolved in a manner that depends on the preferences of their pollinators and on what turns up in the form of mutations. Darwin noted what strong evidence the orchids were against the then common notion that the beauties of nature emerged from some kind of plan: their structures ‘transcend in an incomparable degree the contrivances and adaptations the most fertile imagination of the most imaginative man could suggest’. They were another weapon in the battle against the idea of design, a ‘flank movement on the enemy’.

However remarkable the details, all their flowers are based on the same fundamental plan. It resembles that of the distantly related, but simpler, lily (and Goethe himself, with his interest in botany, described orchids as ‘monstrous lilies’). The parts are arranged in threes, or multiples of that figure. The central lobe is often enlarged into a coloured lip which acts both as a flag to attract insects and as a landing strip that allows the visitors to reach the sweet reward at its base. Often, the flower rotates to turn upside down as it develops. The male organ sits at the end of a long column and the male cells, the pollen, are not powdery as in other plants but instead are held together in large masses, with up to two million minute grains in each. They are covered with a sticky secretion that can attach the whole lot to an insect. The female part lies deeper within, on the same column as the male. Once fertilised, the orchid may produce thousands of tiny seeds in every capsule - no more than a minute proportion of which have any hope of success.

When the pollinator enters, some means is found to attach male sex cells to it. Many orchids have a spring-loaded mechanism that fires a mass of pollen in the right direction. It sticks on with powerful glue. As Darwin found by stimulating the flowers with a pencil, the stalk of the transferred pollen sac quickly dries out and the mass of male cells takes on a more vertical position, just right to fertilise the female part of the next plant visited. In a few kinds, should the mass miss its target, its energy is enough to shoot it for a metre away from the plant (the pollen is ‘shot like an arrow which is not barbed’). The blow is unpleasant enough to cause an insect that has been hit to concentrate, if it can, on the female part of the flowers it visits subsequently, which is a real help to the male who scared it off. In other species, the pollen masses crack open to the sound of a buzz like that of a particular species of bee. Yet other kinds have a see-saw that tips the insect on to the crucial male cells. It pays the plant to do the job as well as it can, for many orchids are limited in their ability to reproduce by a shortage of visitors.

Once the pollinator has been enticed to arrive it expects to be repaid. The first reward of all, in the earliest flowers, was pollen itself, which is expensive as it contains lots of protein. Even so, plenty of orchids still provide a solid meal made up of the stuff, or of bits of tissue that resemble it. Others give nectar, which is simpler and can be provided in very dilute form. Honey bees, for example, must extract the sugary liquid from several million flowers, a few of which may be orchids, to make a kilo of honey.

Some botanical entanglements are intimate indeed. Certain bees are so tied to their partners that their own sex lives have come to depend upon them. They obtain their sexual scents - their pheromones - from an orchid flower and without a visit they are unable to mate. The pheromones may have more than a dozen ingredients. As in the Chanel factory, the bees practise ‘enfleurage’: they mix an odoriferous base taken from the plant with an oily substance of their own that helps the smell to persist. A special grease is smeared on to the flower and the sexual mix transmitted to pockets in the bees’ hind legs. The habit evolved from the insects’ ancient habit of marking their sexual readiness - like dogs around lampposts - with scents extracted from flowers, rotten wood and even from faeces.

The battle is not evenly matched, for the insects themselves - many of whom visit a variety of plants - are under less pressure to retain an accurate fit with their partner than are the flowers. The orchids evolved well after insect pollination began and have had to adapt to the needs of their partners, rather than the other way around. Some insects - many bees included - are quite catholic in their tastes and some orchids are indifferent as to who moves their pollen, as long as somebody does. A few species are visited by more than a hundred different insects, while only around half of all orchids are more or less faithful to a single pollinator. To become too closely connected to a particular insect is risky. Darwin himself speculated that the giant orchid of Madagascar would disappear if its specialised pollinator died out, and he may have been right.

The orchids face a higher risk of failure if they cannot find a pollinator than do animals in the same predicament, for an insect can always try another kind of flower if its prime source of food becomes too rare or too mean. Many flowers - those of orchids included - are in fact visited by several pollinators, even if particular species do tend to concentrate on similar insects; on long-tongued bee-flies and long-tongued flies, or on tiny bees, flies and beetles, each of which picks up the pollen on its legs. Even the bees that pick up their own sexual scents from an orchid are less dependent than they seem. A certain South American species has become naturalised in Florida, where its host does not grow. It finds its chemicals instead in aromatic plants such as basil and allspice when it chews their leaves and extracts the smelly substances. The bee pollinates a wide variety of local plants, which reciprocate with nectar rather than with an aphrodisiac. Once again, the insect has more freedom of action than does its partner.

As a result, the two parties are often less entangled than Darwin imagined. A shift in one is not always matched by an equivalent move by the other, with deeper flower trumped by longer tongue. Molecular trees of plants and pollinators suggest that the insects have instead often switched to species with shallower flowers from which nectar can be sucked with less effort.

The orchid’s ability to force its ally to serve its selfish interests is further limited because such gorgeous beings are often rare and scattered among other species. Make life too hard and the insect will sip elsewhere. Infidelity by the pollinator is bad news for the orchid as it may fail to export its own genes and in addition it may get pollen from the wrong species. Nevertheless, not all the pollinators have been promiscuous, for fossil water lilies from ninety million years ago have flowers quite like those of their modern ancestors as evidence that their association with beetles is ancient indeed.

 

The pressure for sex often causes natural selection to run away with itself. Like many showy animals, birds and butterflies included, there are lots of different orchids. Twenty-five thousand kinds are known, compared with no more than a hundred or so species of wild roses (which are happy to attract almost any insect that might pass by). Most of the barriers to gene exchange among the orchids are held in the brains of their pollinators. As a result, the fertile minds of gardeners have been able to generate thousands of hybrid forms by getting round the ancient bond between flower and insect with a simple paintbrush. Their success shows how fine the balance of barriers to the movement of DNA must be. A tiny shift can change the equation of flower and pollinator and make a new species. In some cases a mutation that changes colour from a hue attractive to bees to another favoured by birds has started a new species in a single step. In the same way, in orchids pollinated by scent-seeking bees, a subtle shift in the proportions of each constituent can attract different kinds of bee, which means that physically identical plants may in fact be distinct entities that never exchange genes.

Orchids bolster Darwin’s case that species arise through the action of natural selection and he soon realised that their diversity had been driven by the vagaries of insect behaviour. He was much less certain of the origin of flowers themselves, which he called ‘an abominable mystery’ and a ‘perplexing phenomenon’. The mystery has been cleared up and the orchids have helped.

Plants colonised the land more than four hundred million years before the present. Those pioneers had no flowers and neither did the huge forests of giant ferns that covered large parts of the planet a hundred million years later. The fern forests declined and the dinosaurs flourished for an age in a flowerless world. Not until the first flowers of all, perhaps a hundred and fifty million years ago, did the conflict between insect and plant begin. It led to an explosion of change in both parties. Their joint transformation was spectacular, for more than three hundred thousand species of flowering plants have evolved, together with several times that number of insects.

The oldest fossil flower comes from a famous bed close to the estuary of the Yellow River in China. It dates from around a hundred and twenty-five million years ago, at the time when the white cliffs of Dover were being formed in a shallow sea. It looked rather like a water lily and floated in fresh water with its small flowers above the surface. For tens of millions of years such structures remained modest, but sixty-five million years ago - just as the dinosaurs left the stage - the world burst into bloom.

The orchids played their part in beautifying an unpeopled world. A distinctive pollen sac attached to a stingless bee has been found in twenty-million-year-old amber from the Dominican Republic. That orchid’s modern relatives use just the same group of insects to transfer their male cells. The molecular clock suggests that orchids as a whole originated around the time of the extinction of the dinosaurs. Their massive radiation happened just after that memorable event and was accompanied by parallel change in the insects that pollinate them.

The great blooming was evidence of an early skirmish in the war between orchid and insect. Conflict between plants and pollinators has gone on ever since. It is expensive and never more so than when it escalates. The Soviet Union collapsed under the financial pressures of its attempts to match the power of the Americans and for centuries Britain and France spent a third of their wealth in mutual conflict. In war, as in love and business, lavish display is a test of merit. A military parade intimidates the enemy and a costly publicity campaign is a sign of a high-class company. The medium becomes the message, the powerful stay in charge, cheats go bankrupt and, for most of the time, truthful ostentation prevails. The best signals are too expensive to copy, which is why McDonald’s sues anyone who imitates their golden arches and why Japanese Yakuza gangsters cut off their fingers.