The Triumph of Seeds (29 page)

The allure of fruit begins with biology but endures in countless cultural references, from Chinese symbols for immortality (peach), wealth (grape), and fertility (pomegranate) to the traditional American token of welcome (a pineapple). Strawberries fed Freyja, the Norse goddess of love, while the Greeks paid tribute to Athena as the inventor of olives. In Southeast Asia, the Hindu deity Ganesh
was famously fond of mangos, and the spreading boughs of the bodhi fig shaded both the birth of Vishnu and the enlightenment of the Buddha. (It’s a species so sacred that even taxonomists got the message, giving it the botanical name
Ficus religiosa
.) The bounty of the biblical Eden reflects a long tradition of describing Paradise as a decidedly fruit-laden place. In the words of the poet Hesiod, those fortunate enough to reach Greece’s famed Elysian Fields enjoyed “honey-sweet fruit flourishing thrice a year.” Islamic texts allude to eternal gardens filled with everything from dates, cucumbers, and watermelon to “the quince of Paradise.” Medieval Britons made matters even more plain, referring to the mythical homeland of King Arthur as
Avalon
, from the Welsh for “Island of Apples.” Scholars of etymology trace the very word
Paradise
to a Persian term for a walled enclosure that early Hebrews adopted to mean “fruit garden,” or simply “orchard.” But perhaps the best example of our esteem for fruit comes from English, where any successful venture is considered
fruitful
, and failures are known as
fruitless
.

With fruit so powerfully embedded in language and culture, it’s easy to forget that from a functional standpoint, sweet flesh is simply window dressing for seeds—an elaborate means of travel from here to there. The technical term for fruit dispersal is
endozoochory
, which would sound far more elegant if everyone still spoke ancient Greek: “going abroad within animals.” (We scientists have a great fondness for long mash-ups in dead languages. When a bat moves an
almendro
seed, for example, the proper description is
chiropterochory
: “going abroad with an animal whose hands resemble wings.”) This form of exploitation evolved early in the history of seed dispersal, almost as soon as there were creatures large enough to get the job done. Back in the Carboniferous Period, the forests that Bill DiMichele now studies on the ceilings of coal mines harbored a relatively modest community of insects, amphibians, and early reptiles. But seed ferns and primitive conifers soon pioneered a range of strategies to ensnare them, from small, skinny seed packets, which may have attracted millipedes, to fleshy seeds the size of mangos, which probably
stank like rotten meat, a siren call to ancestral dinosaurs. Echoes of that era remain in the pungent seeds of ancient survivors like ginkgos, whose reek is so strong that many cities ban the planting of female trees. Virtually every ornamental ginkgo in the world is a male, producing nothing more offensive than odorless pollen.

Botanically speaking, early seed plants lacked the specific tissues that would qualify as a true “fruit.” But that didn’t stop them from developing analogs that worked just as well—sweetening the outer layer of a seed coat, for instance, or putting flesh on a nearby stem or bract. Modern conifers and other gymnosperms continue that tradition, as gin drinkers know well from the pulpy, aromatic cones known as juniper “berries.” But while animal dispersal remains
common in gymnosperms, most familiar fruits evolved with the angiosperms—the flowering plants—whose seeds came in a package, by definition. That covering opened up a world of fruity possibilities, and it arose right alongside an explosion in the availability of dispersers. Birds, mammals, and flowering plants all experienced what taxonomists call a
radiation
, a rapid increase in species, immediately following the extinction of the dinosaurs. And while older groups, like lizards, insects, and even fish, continue to disperse seeds, the vast majority of fleshy fruits are meant to attract—and go abroad with—birds and mammals.

The best way to appreciate fruit diversity involves a simple experiment that most of us undertake several times a week: shopping for groceries. Even a tropical rainforest can’t compare to the density of species arrayed in a typical supermarket produce aisle. My home-town grocery store has been on the same block since 1929, strategically located between two other longtime businesses—the drugstore and the local tavern. On a recent spring morning, the shelves boasted seventy-one varieties of fresh fruit from thirty-nine distinct species. The smallest was a blueberry no larger than my thumbnail. Produce departments now stock them year round, but in the wilds of North America, where blueberries evolved, their ripening coincides with fall bird migrations and the pre-hibernation fruit-gorging
of bears. At the other end of the spectrum, I found a bin of watermelons that weighed up to fifteen pounds (seven kilograms) each. Their
tsamma
melon ancestors ripen during the dry season in southern Africa, providing essential water for everything from antelopes to hyenas to people. Every fruit in the store told some version of the same story—a wild scenario made commonplace by the efficiencies of modern agriculture. Of course, many tree fruits are now propagated by cuttings, and most of the seeds in the store would eventually end up in someone’s compost bin or septic system, but their mere presence demonstrated the success of the fruiting strategy. The everyday bonanza of grocery-store produce is something like extreme dispersal in action—the fruits on display had come from as far away as Italy, Chile, and New Zealand. But they were not only well traveled. They also served as a lesson on the range of ways that flowering plants make fruit, from the obvious—like the sweet flesh of an apple—to the ones that most people haven’t really thought about—like the juice-filled hairs inside an orange, or the strawberry, whose shape and flavor come from a swollen flower base, which is why the seeds perch so oddly on the
outside
of the fruit.

The interplay between fruits and their dispersers affects every partner in the dance. It influences dietary habits and migration patterns as well as the timing of reproduction—for both the animals and the plants involved. But the adaptations can be much more specific. The teeth of fruit bats, for example, evolved from insect-eating cleavers into a bite with angular surfaces designed to crush and pulverize. Guenons and vervet monkeys have special pouches that stretch from their cheeks down the sides of their necks, allowing them to stuff in huge loads of fruit for safe consumption later. Startle one of these monkeys at a fruiting tree, and your last sight of it will be a distinctly bulging face leaping away through the canopy. Fruit-eating birds have developed everything from wider beaks and flexible throats to shorter intestines for processing abundant fruit supplies quickly. Parrots counter the toxins in unripe fruits by gobbling up clay rich in
kaolinite
, the same mineral that formed
the original basis for the stomach-soothing tonic Kaopectate. I’ve watched Cedar Waxwings eat clay, too, which is hardly their only fruity distinction. They digest berries so quickly that their droppings are still sweet (which led them to develop unique rectums that absorb sugars just as well as their intestines do).

Plants, in turn, have learned to tailor their strategies to attract
particular types of dispersers. Birds appreciate prominent splashes of red or black (raspberry, blackberry, cranberry, black currant, hawthorn, holly, or yew), but odor is more important for drawing in animals that are color-blind (elephants), nocturnal (bats), or whose noses are sharper than their eyes (tortoises, opossums). The pits and pips at the centers of fruit boast some of the hardest seed coats in nature, tough enough to withstand scraping, chewing, and the chemical scour of digestion. In fact, being eaten by a disperser enhances the germination of fruit seeds twice as often as reducing it. When an elephant chews the fragrant fruit of a South African marula tree, its tremendous teeth loosen woody plugs in the pit, an essential step that later allows the seed to imbibe water and sprout. The exact benefit isn’t always so clear, but digestion boosts the germination of everything from the cherries devoured by bears to the prickly-pear cactus preferred by the Galapagos tortoise. Some combination of chemical change and physical abrasion probably helps to break the seeds’ dormancy, and then there’s the end result: depositing those seeds in a warm pile of
fertilizing dung. In some cases, other creatures then gather up the seeds and disperse them farther. Tree squirrels do it with the marula nuts from elephant piles, and deer mice scatterhoard the chokecherries and dogwood seeds they find in bear poop. But the most notorious example of this process takes us back to the world of gourmet coffee, where people pay up to $300 per pound ($650 per kilogram) for coffee beans plucked from the turds of the Asian palm civet. A single cup in a trendy Manhattan café can set you back nearly $100. That high price tag has inspired lucrative spin-offs from the poop of other species known to nosh coffee berries: elephants in Thailand, Peruvian coatis, and a turkey-like Brazilian bird called
the Dusky-legged Guan. (Unfortunately, the civet coffee boom has also resulted in cruel schemes to force-feed caged animals. When the topic came up during my visit to Slate Coffee Bar in Seattle, barista Brandon Paul Weaver dismissed the whole craze with a memorable line: “Coffee from assholes, for assholes.”)

Dispersal by way of tasty flesh occurs in nearly a third of all plant families, from cycads to squashes to citrus. The strategy evolved again and again in different settings, because when it works, the results are dramatic. A thirsty brown hyena, for example, can eat eighteen
tsamma
melons in a single night, dropping their seeds over a home range as large as 150 square miles (400 square kilometers). Brown bears in a blueberry patch do even better, munching their way through 16,000 of the tiny fruits in a matter of hours. Since each berry contains an average of thirty-three seeds, that puts the blueberry dispersal rate of a single hungry bear at more than half a million seeds per day. Examples abound, and whole scientific careers have been happily devoted to exploring the nuances. But the fact remains that most seeds travel by other means.

For the vast majority of plants, seed dispersal represents the one moment of mobility in an otherwise stationary life. It determines what grows where, a fundamental organizing principle of ecosystems. As such, the process carries great evolutionary consequence, and seed plants have been coming up with variations on the theme for close to 400 million years. Fruit offers a reward, but other seeds simply hitchhike, using hooks, spines, or stickiness to catch a free ride on the
exterior
of an animal. (The commercial form of this strategy is known as Velcro, a product inspired by the way burdock seeds stuck to the fur of the inventor’s dog.) Some seeds launch from exploding pods, while others drop into water and drift with the tide. For many people, the most familiar dispersal experience comes from the poke of grass seeds lodged in a sock. They work their way inward with every step, eventually becoming so unbearable you’re forced to stop, pluck them out, and throw them on the ground. Mission accomplished.

In Costa Rica, José and I learned that
almendros
move around with help from the wings of bats. But long before bats even existed, seeds had learned to grow wings of their own. Whether gliding, twirling, wafting, or soaring, riding the wind is the most ancient form of seed dispersal, and it remains the most common. With all that practice, plants have developed means of flight (and in some cases, flotation) that transport seeds in quantities, and over distances, never dreamed of by bats, bears, or birds. The results do more than organize the placement of shrub, herb, grass, and tree. They give us yet another chapter in the long story of seeds and people—how papery wings and bits of fluff influenced everything from aeronautics and fashion to the history of industry, the British Empire, and the American Civil War. And like so many biological tales, the best place to start lies in the notebooks and journals of a certain young naturalist in the Galapagos Islands.

CHAPTER THIRTEEN

By Wind and Wave

The vegetable life does not content itself with casting from the flower or the tree a single seed, but it fills the air and earth with a prodigality of seeds, that, if thousands perish, thousands may plant themselves, that hundreds may come up, that tens may live to maturity, that, at least, one may replace the parent
.

—Ralph Waldo Emerson,

Essays: Second Series
(1844)

A
s a young man, Charles Darwin didn’t much care for plants. When he set forth as naturalist on the HMS
Beagle
, botany trailed a distant third behind his passions for geology and zoology. He described himself as
“a man who hardly knows a Daisy from a Dandelion,” and even his mentor at Cambridge, who recommended him for the job, admitted,
“he is no botanist.” Later in life, botanical studies came to dominate Darwin’s research, and he wrote whole books about carnivorous plants, climbing plants, the structure of flowers, and the pollination of orchids. But as the
Beagle
slowly wended its way around the coast of South America, Darwin pursued his botanical collections mostly as a matter of duty, and even considered throwing the specimens away. So when the ship finally made landfall in the Galapagos, it’s not surprising that he devoted most of
his attention to the volcanoes, lava fields, tortoises, and odd birds. A single passing comment in his field notebook seemed to sum up Darwin’s thoughts on the flora:
“Brazil without big trees.” Of his first day ashore, on Chatham Island, he later wrote: “Although I diligently tried to collect as many plants as possible, I succeeded in getting only ten kinds; and such wretched-looking little weeds would have better become an
arctic, than an equatorial Flora.”