The Triumph of Seeds (30 page)

But while Darwin may have been more excited about the craters and finches, his botanical diligence in the Galapagos paid off handsomely. Over the next five weeks he managed to gather and preserve 173 species, nearly a quarter of the known flora. And in the years ahead, those plants would add a critical dimension to his ideas about evolution. Because Darwin’s thoughts on how species arose had in fact started with a question anyone might ask on a journey as long and far-flung as the
Beagle
’s: Why do plants and animals occur where they do? Scholars still argue about how much the Galapagos influenced Darwin’s thinking, but as early as his fifth day in the islands he jotted down a revealing note: “I certainly recognise [
sic
] S. America in ornithology, would a botanist?” Clearly, he was already wondering where the ancestors of the Galapagos flora had come from. As it turns out, among those wretched-looking weeds from Chatham Island was a plant capable of answering that question perfectly. When his friend Joseph Hooker examined the specimen a few years later, he immediately noticed both similarities to, and differences from, its South American cousin. Botanists now call it
Gossypium darwinii
—Darwin’s cotton—and the story of how it reached the Galapagos is a powerful example of just how far seeds can travel, why they do it, and what happens when they get there.

The scientists who study cotton didn’t have to invent a Latin name for the genus; they simply adopted “
Gossypium
” straight from the Romans. Cotton was a well-known fabric throughout the ancient world. The armies of Alexander the Great brought the first examples back from India, and it soon spread around the Mediterranean and south to the Arabian Peninsula (where people called it
qutun
, the source of its name in English). The Aztecs and Incas had cotton, too, as did the Arawak Indians encountered by Christopher Columbus. Wherever he went ashore in the Caribbean, he found people weaving it into everything from fishing nets and hammocks to women’s skirts that he described as,
“just big enough to cover their nature but nothing else.” Columbus mentioned Arawak cotton nineteen times in the log from his first voyage, noting,
“They do not plant it by hand, for it grows naturally in the fields like roses.”

F
IGURE
13.1.  Cotton (
Gossypium
spp.). Lined up end to end, the fibers from a single cotton boll can stretch more than twenty miles. Woven together into yarn, they anchor an industry that shaped the history of empires, the Industrial Revolution, and the American Civil War. A full boll is pictured above, with fuzzed and shorn seeds shown below. I
LLUSTRATION
© 2014
BY
S
UZANNE
O
LIVE
.

That same observation could have been made in tropical places around the world, where more than forty different species of cotton
grow wild. Some have simple seeds, but wherever the pips are plumed, local people have learned to spin those fibers into thread. Cotton now reigns as the most popular fabric in the world, anchoring a $425 billion industry that makes it the most valuable nonfood crop in history. It’s so ubiquitous we have to be reminded that it didn’t evolve to be woven into togas, turbans, hammocks, and t-shirts. The elaborate fluff engulfing a cotton seed arose with a different purpose in mind—helping baby plants ride the wind.

To understand the concept of wind dispersal, simply let your lawn grow long and weedy in the springtime, and take a walk there with the nearest child. Since his first days on two legs, our son Noah has been fond of plucking ripe dandelion heads and holding them upward with an irresistible, one-word request: “Blow!” By my count, one modest puff can send over two hundred seeds suddenly airborne, drifting down in a delight of tiny parachutes. Try to catch them, and you’ll soon find yourself running five, ten, or even twenty feet (six meters) from the mother plant. On a windy day, they will escape you completely. With dandelions now common from London to Tokyo to Cape Town, this ritual has become a universal lesson in botanical aerodynamics, the architectural upshot of seed and breeze. For dandelions, the trick lies in a delicate spindle tufted with lint—symmetrical, flexible, and perfectly spaced for maximum drift. Cotton stays aloft with a different design, and to understand it I decided to do something that few people have bothered with since Eli Whitney invented his famous cotton gin: pick apart a cotton boll by hand.

In the wild, cotton plants grow as perennial shrubs or small trees, with angular branches and hairy, gray-green leaves. Domestic varieties are fast-growing annuals—shorter in stature, but otherwise much the same. They all belong to the mallow family, a large group that includes Congo jute and okra, but is best known for showy garden flowers like hollyhocks and hibiscus. Cotton has beautiful flowers, too, with papery, lemon yellow petals surrounding a purple center. The fruit forms a round pod, or boll, that bursts and inverts as it ripens, revealing plumes of white down that make cotton fields look like a harvest of snowballs or fantastical sheep. In the fourteenth century, English traveler Sir John Mandeville astonished readers back home with descriptions of an Asian tree that bore gourd-like fruits bursting with tiny lambs. It’s unclear whether he meant cotton, which he described more accurately elsewhere, but the
idea took hold. Embellished versions of the story soon attributed cotton to “vegetable lambs,” and illustrators pictured them stretching their fuzzy necks down from the branch tips to graze.

F
IGURE
13.2.  Stories from medieval traveler Sir John Mandeville and others led to the myth that cotton came from “vegetable lambs,” woolly botanical creatures harvested from the fruits of an Asian tree. Anonymous (c. seventeenth century). W
IKIMEDIA
C
OMMONS
.

Like Mandeville, I come from a cool, rainy island where the idea of growing cotton sounds extremely exotic. Unlike a medieval Englishman, however, I didn’t have to travel to India to find some in its natural state. Modern craft stores offer raw cotton bolls at a very
reasonable price, still attached to the branch. They’re meant for wreaths and flower arrangements, but each one holds an incredible tale of seed evolution for any plant adventurer brave enough to attempt a dissection. Armed with tweezers, a pocketknife, and a pair of sharp microscope probes, I clipped a medium-sized boll from the branch and headed for the Raccoon Shack.

Stem side down on my desk, the boll did indeed resemble a sheep, its tufted white back as soft as old flannel. But when I pressed the fuzz tight between my fingers I felt the nubs of seeds deep inside, completely buried in fibers. The boll measured three inches by two (seven and a half by five centimeters) and weighed an eighth of an ounce (four grams), a size oddly similar to the small wren I had taken apart on the same desktop while researching my book on feathers. It, too, had been light, compact, and designed to fly. Plucking the bird involved two solid hours of painstaking tweezer-work—gripping, tugging, and sorting through more than 1,200 tiny plumes. That was child’s play. After less than a minute of hand-ginning cotton, I realized I didn’t stand a chance of disentangling even a single lone fiber. They weaved and snaked together so tightly I couldn’t pull on one without yanking knots in scores, if not hundreds, of others. My plan to neatly tease apart seed from felt collapsed. I had hoped not only to gin the cotton, but also to count, sort, and measure individual fibers, just as I had done with wren feathers. In the end, I resorted to scissors, and even then it took dozens of hacking snips to cut through the woolly mat. The final result was a snowdrift of tangled cotton and a pile of pathetic seeds, still furred with uneven clumps of fiber. The analogy of a poorly shorn flock sprang inescapably to mind.

Under a microscope, the source of my problems became clear. At the surface of each seed, dense fuzz sprouted like close-cropped turf, so thick I couldn’t make out where the seed coat ended and where the fluff began. A glance at the cotton entry in Derek Bewley’s seed encyclopedia told me why: they were one and the same. When it comes to seed coats, plants like cotton don’t follow the rules. Instead
of protection, a cotton seed devotes its outermost layer to the promise of dispersal. Flight (and flotation, as we will soon learn) has provided an evolutionary incentive great enough to transform individual cells from microscopic blips into epic filaments over two inches (five centimeters) long. It’s no wonder they’re hard to untangle. With every hair only one cell wide, cotton seeds the size of a split pea can easily grow a downy coat of more than 20,000 fibers. With thirty-two seeds in the average fruit, that makes every boll a tangle of more than half a million strands. Lined up end to end, they would stretch more than twenty miles (thirty-two kilometers).

It’s said that Eli Whitney took inspiration for his famous machine from the sight of a barnyard cat pouncing on a chicken. The bird squawked and scooted off, leaving the cat with tufts of feathers clinging to each claw. Cotton gins work the same way, clawing fiber from seed with hooks fixed to great spinning drums. Whitney’s 1793 patent application pictured a humble wooden box with a hand-crank and a single roller. That technology quickly advanced through the ages of steam and electricity, all the way up to modern computerized behemoths that can sort, clean, dry, and press a 500-pound (227 kilogram) bale in less than two minutes. Through it all, the evolutionary intent of cotton remained abundantly clear; drifting and spinning in wispy clouds around every gin in what one nineteenth-century observer called a “furious snowstorm.” The single-celled hairs on cotton seeds combine maximum surface area with minimum—indeed barely perceptible—weight. Whether sent aloft by wind or machine, the fluff stays airborne and moves around exactly as the seed intended.

Dispersal by wind leads to what biologists call a “seed shadow.” Breezes are fickle, and even the best aerodynamics can only keep something flying for so long. The result is a predictable pattern: most seeds drop to earth fairly close to the mother plant, falling in a dense cluster that thins to scattered, then occasional, and then rare the farther away you look. Just where that seed shadow ends remains an open question, since truly long-distance events occur too infrequently to study. One of the few attempts to track seeds in
the atmosphere focused on a weedy aster called Canadian fleabane. Using a remote-controlled airplane equipped with sticky seed traps, researchers found fleabane plumes ascending on updrafts to at least
375 feet (120 meters). From there, even a modest wind can move them tens or hundreds of miles or kilometers. But we know that seeds can waft much higher and farther. In the Himalaya, unidentified wind-blown seeds have been found in rocky crevices at 22,000 feet (6,700 meters), far above the realm where plants can grow. No one knows how far they’ve traveled, but enough of them arrive to form the basis of a food chain: fungi rot the seeds, springtails eat the fungi, and tiny spiders
prey upon the springtails.

The best evidence for long-distance dispersal, however, does not come from mountaintops or from combing the upper atmosphere for the occasional seed. It’s found in the very patterns that so fascinated Charles Darwin during his journey on the
Beagle
—the distribution of species—and in the simple fact that plants growing in remote places, like cotton on the Galapagos Islands, couldn’t have gotten there by any other means. Darwin’s notes don’t reveal exactly when he started thinking about seed dispersal, but within a few years of the
Beagle
’s return to England, he began immersing everything from celery seeds to whole asparagus plants in flasks and tanks of seawater. Most species germinated well after a month, and some lasted longer than four, but he was disappointed when only a few seeds managed to stay afloat past the first few days. Still, he calculated dispersal distances of at least
300 miles (483 kilometers) on an average Atlantic current, and he pondered the idea of wind-dispersed seeds floating up on distant beaches, drying out, and tumbling inland on the breeze.