DEMONS IN EDEN

The Paradox of Plant DiversityBy JONATHAN SILVERTOWN

The University of Chicago Press

Copyright © 2008 Jonathan Silvertown
All right reserved.
ISBN: 978-0-226-75772-8

Contents

Preface..............................................................ix1. An Evolving Eden..................................................12. The Tree of Trees.................................................143. Succulent Isles...................................................284. Demon Mountain....................................................435. The Panama Paradox................................................596. Nix Nitch.........................................................767. Liebig's Revenge..................................................908. Florida!..........................................................1029. New Demons?.......................................................11810. The End of Eden?.................................................13311. Fynbos Finale....................................................149Scientific Names of Plants Mentioned in the Text.....................163Sources and Further Reading..........................................167Index................................................................181

Chapter One

An Evolving Eden

At a spot along the banks of the River Thames, in the southwestern suburbs of London, there "[s]its enthroned in vegetable pride, Imperial Kew by Thames' glittering side." So wrote Erasmus Darwin, Charles's grandfather, in his book-length poem titled The Botanic Garden. The imperial days of Kew Gardens are gone, but in its three hundred acres more than twenty thousand plant species are found in cultivation. It is a good place to sample plant diversity-to wander among the plants and wonder just why there are so many.

Stroll across Kew Green, an oval expanse of grass where cricket is played in summer, toward the imposing iron gates of the garden erected by Sir William Hooker in 1845. Passing through the turnstile, one encounters verdant English lawns interspersed by groves of oaks, bamboos, pines, and other trees in all their variety; spectacular beds of flowers that change kaleidoscopically with the seasons; alpine rockeries that are more richly stocked than the Alps themselves; and even, tucked away in a corner, a little garden displaying a palette of a dozen species of lawn grass, each a subtly different, textured shade of green from its neighbor. And there is much more. The famous Victorian Palm House contains a tropical forest and in its basement are illuminated aquariums stocked with marine algae. The Temperate House has plants from all temperate regions of the globe, but the huge and modern Princess of Wales Conservatory outdoes them all.

Enter by the door at the southern end where pelargoniums and jade plants from South Africa grow just inside. These are familiar as houseplants and the temperature is cool and pleasant, but be ready for more exotic sights. A step farther, through more doors, and you are in the dry tropics among desert plants from the Americas and southern Africa. These are habitats where plants struggle with a physical environment that is blisteringly hot by day, nearly freezing at night, and parched most of the year, and where any unguarded green substance is a moist lunch for a herbivore. Evolutionary adaptations to the twin rigors of an unforgiving physical environment and hostile herbivores are evident in every plant of the desert.

Nestled in a stony bed are the clublike leaves of "living stones" (Lithops species) from the Namib Desert. Most of the plant is buried below ground level, and all that shows are the flattened tops of a pair of clubs, separated by a cleft. The crown of each club is brown and mottled, providing camouflage against a stony background. In fact the mottling hides a window that admits light into the interior of the plant. In the dry season the leaves shrivel, to be replaced by new ones when the rains come. Hiding from herbivores and avoiding the heat of the sun are all very well, but it doesn't do to hide from pollinators. In their season, bright flowers issue from the cleft in the living stones to claim their fifteen minutes of conspicuous fame.

Continue along the path that rises and turns a corner and then, suddenly, you are in a theater of desert plants. The cast is international, but all are actors in the same evolutionary play whose theme is convergent evolution. Center stage is a gigantic specimen of the century plant, Agave americana, from Mexico. It is seated, like a many-tentacled beast, on a platform of rock, its lower leaves arched downward as if drooping under their own weight. Each leaf is like a massive, grey-green spearhead, nearly two meters long and tipped with a needle-sharp spine. In the center of the plant, pointing straight at the sky like a challenge, are three waxy new leaves with freshly hardened spines. This is an awesome plant and only a macho Mexican armed with a machete would tangle with it.

Framing the century plant, its naked limbs tottering to a height of five meters, is a desert tree from Africa, Aloe barberae. Its limbs are striated with a pattern of light brown sinews and each trunk terminates in a rosette of succulent, dark green leaves. This aloe-on-a-stick is an eccentric plant. Some of its more terrestrial relatives are remarkably like Agave, with a rosette of stiff spearlike leaves and no trunk. Aloes in Africa and agaves in America belong to quite different plant families, but they occupy similar habitats on their respective continents and have independently evolved similar life forms. They are an example of convergent evolution. When environmental conditions are similar in different parts of the world, evolution has often fashioned similar looking organisms in each of them, but using different, local starting materials. Thus, in an Old World desert the spiny rosette plants belong to the aloe family, while in deserts of the New World the same growth form has evolved in the agave family. Outward similarities conceal different evolutionary origins.

In the footlights of the desert stage sits a cluster of barrel cacti from Mexico, the size of overlarge basketballs, that repeat the theme of convergent evolution with another growth form. The barrel shape of the cactus has pleats that allow the plant to expand when it takes up water. An unprotected barrel full of water has as much chance of survival in the desert as a snowball in hell. Barrel cacti are guarded by a thicket of ferocious yellow spines that protect against herbivores, but unfortunately not against people who wish to collect such spectacular plants. The largest specimens at Kew were confiscated from an illegal shipment that was intercepted by UK Customs and Excise.

The cacti of the American deserts have their parallel in Africa in plants belonging to the family Euphorbiaceae. Like barrel cacti and others, many African desert euphorbs have spiny, fluted stems. Indeed, these plants look so much like cacti that they are often mistaken for them, though true cacti are native only in the Americas. The separate evolutionary origins of the cactuslike growth forms in Africa and America are disguised by convergent evolution. Features of their flowers that have not been shaped by convergent evolution tell them apart.

In the wings of the desert stage hangs a third example of evolutionary convergence between the desert flora of Africa and its equivalent in the New World. An African climbing succulent belonging to the milkweed family scrambles up a training wire, but looks no more than a thick wire itself: it is all stem and no leaves. At a height of about two meters, this African plant flops over into the spinous canopy of a South American tree, from which its jointed stems hang vertically downward as if tired of resisting gravity. Perhaps the supporting tree has a sense of dj vu because there is a genus of cacti called Rhipsalis that trails their jointed, leafless, and almost spineless stems in an identical manner in South America.

Paradoxical though it sounds, convergence is one way in which evolution generates diversity. By recreating the same type of plant in different geographical regions, evolution has produced not just one group of cactuslike plants but two: one in the New World and another in the Old. Rosette plants and succulent, leafless climbers have likewise been duplicated. Thus, geographic barriers between regions have caused whole floras to evolve in isolation from one another, enriching the global flora. How local biodiversity can suffer when these barriers are breached by plants being taken from one place to another we shall see later.

Leave the dry tropical zone by a flight of steps that gives a parting view of the desert theater from backstage, and pass through a glass door. You are enveloped by a hot, clammy fog. There is a hiss overhead from the vaporizers that keep the humidity of this part of the conservatory at a level more to the liking of its inhabitants. Through the mist you decipher a sign that reads "ZONE 1: Moist Tropics." This is another environment with another whole set of species, collected together here from all over the Old and New World tropics. In nature, moist tropical environments contain more plant species than any other on Earth. Just three of the most spectacular species to be found at Kew illustrate the role of insects in the evolution of plant diversity. There is a good view of the first one over the balcony just ahead. Look down from this vantage point into a pool of clear water where the giant water lily Victoria amazonica reigns. Each lily pad is like a giant's frying pan, up to two and one-half meters across, with an upturned rim that helps it float. The stalk and lower surface of the leaf are armed with large spines. This plant's enemies are aquatic and attack from beneath, while its pollinators are aerial creatures.

Victoria amazonica has flowers that befit a giant. They are about a foot across and also protected by spines when in bud. Sir Ghillean Prance, a former director of Kew, discovered how the giant lily is pollinated in the wild. Its secret attraction for pollinators is in the heart of the flower where there is a hollow chamber with the stigma (female part) perched in its base and a ring around the interior of the chamber roof stocked with starch and sugar. Flower buds develop below the surface and when ready to open find their way upward toward a window of light between the lily pads. In the daylight hours before its nocturnal debut the flower stalk pushes the bud, still tightly closed, clear of the water surface where it waits for sunset. As the light fades, the flowers slowly open and release a sweet fragrance: an entire lake of unfolding Amazon lilies is a breathtaking sight. Enticed by the perfume, and guided by white petals that shine like beacons in the moonlight, scarab beetles mob the flowers and crawl through a tunnel into the inner chamber where they feed on the starch and sugar to be found there. At about midnight the flowers close, trapping the feasting beetles inside, and soon afterward the petals begin a blushing transformation of color. By midafternoon of the following day, the flowers that closed with white petals have reopened in velvet-purple, but the beetles are still held prisoner because the tunnel from the central chamber of each flower remains blocked. At sunset the tunnel is unblocked and as they scramble to escape, the beetles, sticky from their honeyed meal, become covered in pollen from anthers that have now opened in the lining of the escape tunnel. As they fly off to another night of sweet imprisonment, the beetles carry with them pollen that they will deposit on the stigma in the chambered heart of another flower.

One insect's meat is another insect's poison. Thus, a diversity of tastes among pollinating insects can engender the evolution of a diversity of flowers among plants. While sweet perfume attracts beetles in search of nectar, the rank smell of rotting flesh brings flies. What is that smell? It seems to be coming from another pool, upstream of the lily pond. Cascading from a tree is a climber with large heart-shaped leaves that some joker seems to have hung with surreal saxophones made of flimsy white cloth spattered with dried blood.

Few artists can match evolution's flare for the bizarre and those who try usually plagiarize nature. But this is no artefact. These are pelican flowers, belonging to a vine from South America. The face of the flower is heart-shaped in outline, about half a meter from top to bottom, with an eighty-centimeter-long tassel hanging from its lower lip. Just above the center of the flower is an opening stained a deep magenta, and all around it the same bloody hue flecks its creamy fabric. This is a lure for flies. A plant mimic of a murder scene, honed by evolution into the quintessence of a fly's wildest desires, reproduced in full smell-o-rama. The flower is like a flared horn and its magenta opening is the entrance to a tube that bulges and then goes through a U-shaped bend, like a saxophone. Where the mouthpiece of the saxophone would be, the tube narrows to a stalk by which it hangs from the vine.

One final plant to see before leaving the conservatory is the titan arum, which has one of the biggest, smelliest flowers on Earth. Perhaps fortunately the titan arum isn't actually flowering right now because otherwise there would be a long line of visitors waiting to see (and smell) this rare event, patiently filing past its stinking, two-meter-tall flower spike, guarded by a specially assigned policeman from the Kew Gardens Constabulary wearing a gas mask. The leaf of this plant (not leaves, for it has but one) is not noisome but is just as spectacular as its inflorescence. There are three of these plants in the conservatory at the moment, each in a massive plant pot. Out of each pot sprouts a stout leaf stalk that would respectably serve a medium-sized tree for a trunk. The stalks lean at about thirty degrees from vertical and are perfectly smooth, dark green in color with an attractive pattern of lighter green splotches. At a height of three meters or so the stalk branches into leaflet-bearing ribs, each rib dividing into two, then four, then eight. The result is a shallow-domed canopy like an upturned saucer covered in a single layer of foliage. This leaf feeds an underground tuber that will grow to thirty kilograms in weight before it is ready to flower. To what extreme lengths some plants are driven by the evolutionary imperative to reproduce!

The giant water lily, the pelican flower, and the titan arum are spectacular examples of how pollinators can drive the evolution of flowers in weird and wonderful directions. Though these plants have exceptionally large flowers, the significance of insects in their evolution is by no means exceptional. Interactions with pollinating insects have been a major diversifying force in the evolution of the flowering plants and almost certainly account for why this group as a whole is so numerous while the conifers, which are wind pollinated, number fewer than a thousand living species.

A visit to the Princess of Wales Conservatory gives a tantalizing taste of the wonderful diversity of the plant world. Though unusual species grab the attention, there isn't any plant, not even the garden-variety daisy or dandelion that is heedlessly trodden underfoot, that doesn't have an evolutionary story to tell. Some of them will be heard later in this book. All are part of a single evolutionary drama-the story of how the green branches of the evolutionary tree sprouted and grew into the diversity of plant species we enjoy today.

Botanical gardens have an important role in researching and preserving botanical diversity, nowhere more so than Kew. But Kew also has a unique place in the history of evolutionary thought owing to the part it played in the development of Charles Darwin's ideas on evolution. Darwin lived and wrote On the Origin of Species not twenty-five miles from Kew at Downe, in Kent. The link between the two was Joseph Hooker, who in the mid-nineteenth century assisted and eventually succeeded his father as director at Kew. He was a confidant of Charles Darwin and a key figure in the development of Darwin's theory.

Like Darwin, who had undertaken a round-the-world voyage in his early twenties, Joseph Hooker at a similar age was assistant surgeon and botanist on a four-year expedition to the Southern Hemisphere. Soon after Hooker's return, Darwin seems to have recognized him as a kindred spirit, addressing him quite early in their correspondence as a "co-circum-wanderer and fellow labourer." In 1844 Joseph Hooker was the first scientist to whom Darwin confessed his conviction that species were not separately created and immutable, but rather had evolved one from another and shared a common descent. Darwin wrote in a letter to Hooker that, after much research, "[a]t last gleams of light have come, and I am almost convinced (quite contrary to [the] opinion I started with) that species are not (it is like confessing a murder) immutable."

(Continues...)

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