Über die Autorin bzw. den Autor

Jonathan Silvertown is professor of ecology at the Open University, Milton Keynes, and is the author of Demons in Eden and editor of Fragile Web.

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An Orchard Invisible

The University of Chicago Press

Contents

{1} An Orchard Invisible SEEDS...................................3{2} First Forms Minute EVOLUTION.................................7{3} Even Beans Do It SEX.........................................21{4} Before the Seed POLLINATION..................................39{5} According to Their Own Kinds INHERITANCE.....................51{6} O Rose, Thou Art Sick! ENEMIES...............................61{7} The Biggest Coconut I Ever See SIZE..........................70{8} Ten Thousand Acorns NUMBER...................................77{9} Luscious Clusters of the Vine FRUIT..........................87{10} Winged Seeds DISPERSAL.......................................101{11} Circumstance Unknown FATE....................................108{12} Fierce Energy GERMINATION....................................118{13} Sorrow's Mysteries POISONS...................................125{14} Ah, Sun-flower! OIL..........................................135{15} John Barleycorn BEER.........................................145{16} Realm of Illusion COFFEE.....................................155{17} Nourishment & Inspiration GASTRONOMY.........................165Scientific Names..................................................177Sources and Further Reading.......................................181Index.............................................................211

Chapter One

SEEDS

A seed hidden in the heart of an apple is an orchard invisible. WELSH PROVERB

Seeds have a mirrored life, the original in nature and another reflected in literature and the imagination. The Welsh proverb simultaneously expresses both the biological potential of seeds and their metaphorical power. The American philosopher and early conservationist Henry David Thoreau, who was fascinated by seeds and inspired by them, wrote, "I have great faith in a seed. Convince me that you have a seed there, and I am prepared to expect wonders." Who cannot wonder that the largest organism on this planet, the giant redwood Sequoiadendron giganteum nicknamed "General Sherman," which weighs roughly the equivalent of a fleet of six Boeing 747-400 Jumbo Jets, germinated more than two thousand years ago from a seed weighing only a six-thousandth of a gram?

This book was once a seed or, to be precise, several. The paper in it is made from wood pulp from northern coniferous forests, grown and replanted from seed; the inks used on its pages and the varnish on its cover contain oils obtained from seeds; but the book grew from another kind of seed also-the seed of an idea. The idea was for a book that would explore the science behind all the familiar things that gardeners, cooks, and everyone else knows about seeds. This book is about why seeds have all the wonderful properties that they do, feeding us, flavoring our food, moistening and protecting our skin, and growing into plants that give us fruits, flowers, fiber, pharmaceuticals, poisons, perfume, protection, and pleasure. I hope to persuade you that reading about seeds is yet another way to enjoy them.

Expect wonders. Orchids have seeds as light as dust that are so ill-provisioned that seedlings spend their first few years as parasites on fungi. The biggest seed, by contrast, weighs twenty kilograms and belongs to the double coconut, a castaway palm whose ancestors were stranded on tiny islands in the Seychelles after these fragments of land were cast aside by India in its geological passage toward Asia. Evolution on a crumb from the Indian landmass produced Earth's largest seed.

The story of seeds, in a nutshell, is a tale of evolution bursting with questions. Its earliest episode concerns how and when the first seed plant evolved from its fern-like ancestors, with other episodes about why seeds have dormancy and what makes them germinate; why some seeds are rich in oil and others in starch; why some seeds are big and others small; why some are poisonous and others palatable. Though science has unraveled much about the whys and wherefores of seeds, there are still mysteries to be solved, including the most fundamental question of all: why do plants bother producing seeds in the first place? Why are plants, or animals for that matter, so hooked on sex?

You can follow the story cover-to-cover in a journey that will take you from a seed's first beginnings in sex and pollination through each step of its life till it ends in your coffee cup or on your plate; or you can dip into the story and sample a crumb of gardening, a grain of genetics, a dose of medicine, a packet of commerce, or a morsel of cooking: all in a seed. This is not a long book (who has time for those, these days?), but do not expect an express route from A to Z. Rather, I have followed a meandering path through the story of seeds and, like the root of a seedling, I have branched into particularly fertile territory wherever I have encountered it. So indulge me, and you will learn, as I did while writing, some fascinating if occasionally tangential connections between seeds and unexpected topics such as the witchcraft trials in seventeenth-century Salem (chapter 6, "O Rose, Thou Are Sick!"), Lyme Disease (chapter 8, "Ten Thousand Acorns"), human color vision (chapter 9, "Luscious Clusters of the Vine"), and the evolution of yeast (chapter 15, "John Barleycorn").

Whichever way you read the book, you will find a recurring theme: evolution constantly invents new uses for old devices. This happens because of how evolution works, creeping gradually and without direction from one solution to life's challenges to another. The achievements of the process of evolution are astonishing, and to some people unbelievable, but we didn't get here overnight. The two millennia it takes a seed to produce a tree the size of General Sherman is a mere fleeting instant in the 360-million-year history of seed plants. Laozi, a Chinese philosopher writing at about the time General Sherman was just a six-milligram seed, got it right when he reflected, "To see things in the seed, that is genius."

Chapter Two

EVOLUTION

Organic Life beneath the shoreless waves Was born and nurs'd in Ocean's pearly caves; First forms minute, unseen by spheric glass, Move on the mud, or pierce the watery mass; These, as successive generations bloom, New powers acquire, and larger limbs assume; Whence countless groups of vegetation spring, And breathing realms of fin, and feet, and wing. ERASMUS DARWIN, FROM The Temple of Nature

Charles Darwin's visionary grandfather Erasmus was ridiculed by his contemporaries for his evolutionary views. The family crest that he designed and blazoned on his carriage boldly declared "ex omnia conchis," or "everything from seashells." It may have been Erasmus's little joke, but he was decades ahead of his time and fundamentally right: life did first evolve in the sea. Where, though, did seed plants originate? There are indeed some seed plants that live in the sea, but the seagrasses (as they are known) live in shallow coastal waters and have terrestrial ancestors. Seagrasses are mere parvenus to a marine existence, still paddling in the muddy shallows where they can avoid the heavy-hitters among marine plants-the algae.

Even though seed plants first evolved on land, that does not mean that we can ignore the marine origins of land plants themselves. Quite to the contrary-although evolution took the plant out of the sea it couldn't so easily take the sea out of the plant. Or, as E. H. Corner put it in his classic book The Life of Plants, land plants "are made from a sea recipe." From a sea recipe, evolution cooked up something brand-new to serve the demands of life on land: an embryo in a box, which we call a seed. In fact the box contains not just an embryo, but also a food store placed there by mother, so a seed is really an embryo in a picnic basket.

Seeds were the ultimate refinement of plant adaptation to terrestrial life. So what preceded them and how did they evolve from a sea recipe? A comparison between plants and animals is illuminating. Among animals, successful colonization of the land from the sea was achieved several times independently by vertebrates, molluscs, and arthropods (insects and crustacea), but among plants there was only one, solitary successful colonization. All land plants, which include mosses, ferns, horsetails, gymnosperms (conifers, cycads, and related groups), and flowering plants, descend from a single ancestor that first made the transition from sea to land. There must have been failed attempts too, but we do not know how many.

That the transition from sea to land was successful only once among plants is no doubt testament to the many obstacles to survival and reproduction that life on land would have presented to a marine alga. Indeed, the differences between terrestrial and marine environments that affect plants are so numerous that E. H. Corner declined to list them in his book, saying "A list would cover many pages and the active mind must be spared the tedium." A praiseworthy attitude summed up by another botanist who penned the lines:

There should be no monotony In studying your botany; It helps to train And spur the brain-Unless you haven't gotany.

If only all botanical authors were so considerate to their readers. However, there is a small list of just two obstacles I must bore you with. In a terrestrial environment, how can sperm swim and how can fertilized eggs avoid drying out?

Land plants tackle these problems in a variety of ways. Mosses and ferns, for example, cannot really be said to have solved the first problem at all, as they require damp conditions for sexual reproduction. In these groups sperm need a film of moisture to enable them to swim from male organs to female ones. This restricts the distribution of these plants to habitats that are at least occasionally wet. In ferns, the large, leafy plants that we are so familiar with are asexual and produce no eggs or sperm. Instead, they produce tiny, dust-like spores. When shed, these germinate to produce a microscopic sexual stage that leads an independent existence. This in turn produces the eggs and sperm. After fertilization, the resulting embryo takes root and develops into the large leafy plant we recognize as a fern. Some marine algae also have separate asexual and sexual stages, and this arrangement must also have been present in the ancestor of all land plants.

In the sixteenth century it was commonly and erroneously believed that ferns must reproduce by seed. But where were the seeds? Surely, since all plants grew from seed and the seeds of ferns could not be found, fern seed must be invisible! At that time herbalists believed that plants betrayed their medicinal uses in the shapes of their leaves and flowers, so kidney vetch was good for kidney complaints and liverwort benefited the liver. It was but a natural extension of the doctrine of signatures, as this herbal system was known, to assume that carrying invisible fern seed would confer invisibility upon the bearer.

Of course a problem stood between any herbalist peddling this theory and a huge money-making opportunity: how do you obtain fern seed? There was a way. Fern seed could be collected on the stroke of midnight on Midsummer Night's eve, but only by catching it as it fell from the plant onto a stack of twelve pewter plates. It would pass through the first eleven, but be trapped by the twelfth. Not everyone believed this, of course. In Shakespeare's play King Henry IV, written in 1597, a thief called Gadshill attempts to recruit an accomplice to a robbery by telling him that they will "steal as in a castle, cocksure; we have the receipt of fern-seed, we walk invisible." To which he receives the reply, "Nay, by my faith, I think you are more beholding to the night than to fern-seed for your walking invisible." No one nowadays believes in the existence of fern seed, but homeopaths still believe in the power of remedies containing herbal extracts that have been diluted to invisibility, so perhaps we shouldn't mock our credulous forebears.

Ferns and mosses may lack seeds and their reproductive cycles may be reminiscent of that of some marine algae, but there is one feature of their life cycle that they share with other land plants and which distinguishes all of them from algae: all land plants produce a multicellular embryo that is retained within maternal tissue. For this reason land plants as a group are known as embryophytes (i.e., plants with embryos). Precisely when during its development the embryo is released varies a great deal between species, but even the most negligent mother among terrestrial organisms never behaves the way many marine animals and plant=s do, which is to squirt their eggs and sperm into the environment and forget about them. Amphibians (frogs, toads, and salamanders) come close to this strategy, but of course they return to water to reproduce. It is no coincidence that land plants as a group can be defined as embryophytes. Parental care of the embryo is essential for successful reproduction on land.

The retention of fertilized eggs within maternal tissue where the developing embryo could be protected from desiccation was a crucial evolutionary step in the colonization of land from the sea, but free-living sexual stages like those of ferns still needed a wet environment to reproduce. Because we are looking back at the evolutionary history of the seed, it is almost impossible to resist describing what I am about to reveal of that history as "the next step" in seed evolution. But, though evolution seems to have a direction when we look back from the vantage point of the present, it doesn't follow purposeful steps-rather, it wanders from one chanced-upon solution to the next with no aim whatsoever. One particular path among the many wanderings of evolution led to the evolution of the seed. Others led to modern representatives of plants like mosses and ferns that have no seeds, and yet others led to extinction for plants like giant club mosses and seed ferns.

This caution issued, what happened next on the particular path that we are following to the evolution of the seed liberated plants from a dependence on a watery environment for sex. The large, robust plant, instead of shedding its female spores, retained them within its tissue where they became a protected, tiny sex machine. Of course there were consequences of this for male spores. Once the female had become cloistered, males had to find their way to the egg by a different route. Swimming would no longer do it. Male spores were already equipped for aerial dispersal, though, so all that had to happen was for these to delay the liberation of their sperm until they reached the vicinity of the egg. Thus the male spore became a pollen grain.

The earliest seed plants found in the fossil record appeared in the Devonian period about 360 million years ago. They belonged to the group known as gymnosperms, whose living members include ginkgos, cycads, and conifers. The name "gymnosperm" is derived from the Greek for "naked seed," because seeds of these plants are not enclosed in an ovary. Incidentally, the word "gymnast" has the same root: classical Greek gymnasts performed naked. The maidenhair tree, Ginkgo biloba, is a living fossil gymnosperm whose reproductive system still contains more than a flavor of the sea recipe from which embryophytes evolved.

Ginkgo biloba is the last remaining representative of an ancient and once more numerous group of gymnosperms. Ginkgo ancestors are found in Permian fossil deposits 280 million years old. G. biloba was first found by a Western botanist in monastery grounds in China, but it now grows in botanical gardens and parks all over the world. This living fossil is a great survivor. One specimen survived the blast of the atomic bomb dropped on Hiroshima in 1945, even though it was only 1.1 kilometers from the epicenter of the explosion. The species is also extremely pollution tolerant. Many streets in New York City are planted with ginkgos, but only with males, because females produce mature seeds that have an unpleasant smell like rancid butter. No doubt the smell pleased the dinosaurs that once fed upon its seeds, but it is a deterrent to humans who have replaced them as dispersal agents. If you can find a mature female tree in spring you will see that its naked, unfertilized seeds dangle suggestively in pairs on the end of long stalks.

Male ginkgo trees produce wind-dispersed pollen grains that each contain an undeveloped male. When an unfertilized seed (called an ovule) on a female tree is ready for pollination, a drop of mucilage is exuded from a small pore in its tip. The mucilage is later retracted so that any wind-borne ginkgo pollen that has become trapped in the mucilage is drawn into a chamber inside the ovule. Males gather inside the chamber, still immature, each in his tiny flying saucer of a pollen grain. A marriage is now predestined between one of the waiting juvenile males and the equally immature female, but first they must mature and then there will be a contest between sperm. Had he known about it when he wrote his book-length poem about plant sex, The Loves of the Plants, Erasmus Darwin would undoubtedly have risen to the very heights of poesy in honor of this betrothal between juveniles with its prenuptial contest.

(Continues...)

Excerpted from An Orchard Invisibleby Jonathan Silvertown Copyright © 2009 by Jonathan Wendell Silvertown. Excerpted by permission.
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