The Triumph of Seeds (12 page)

Calm water stretched away below in glints and ripples of tide as I walked down the short path. I couldn’t help stopping for an early lunch and found an open place where I could soak up as much warmth as possible from the winter sunlight, a rarity in our neck of the woods. Before I’d even unwrapped my sandwich, however, I spotted the object of my quest, peeking out from a crack in the rock beside me. To be honest, I knew I’d find it without much effort. I often led field trips to this site, and had once looked on proudly as the majority of my botany students ignored a passing group of orcas to focus on these tiny plants. (Having grown up on the island, they were familiar with whales, but this was their first spike moss!)

I knelt down for a closer look. Spike mosses trace their ancestry straight back to the giants of the coal forests. While this plant grew only a few inches tall, the leaves pressed to its little stem would have looked right at home on the fossils I’d seen in New Mexico. But what the spike moss knows sets it apart from almost every other spore plant that has ever lived. I pinched off the tip of a branch and held it up in the sunlight, squinting hard. Then I rubbed my eyes and sighed. It was time to admit that I’d reached a point in life where I could no longer enjoy the pleasures of spore viewing if I forgot to bring my reading glasses.

What I was looking for came into clear focus back at the Raccoon Shack, with the help of a dissecting microscope. There the spores practically glowed, tucked into speckled golden pouches at the base of each leaf. But it’s not unusual for tiny things to look beautiful
under magnification. What made these spores remarkable was their size, or rather, their sizes. Low down on the stem each spore looked bulky and smooth-edged, like a big river stone, but near the branch tips they were minuscule, spilling out of their golden sacs like smears of reddish dust. The spike mosses know something that ancestral seed plants also had to learn: how to separate the sexes. The large spores are female, the
precursors to eggs, and the small ones are male, the beginnings of sperm. This system not only increases genetic mixing, it allows the plants to start “packing a lunch,” investing their energy in the female spores destined to produce a new plant. While both the males and females must still travel off and grow into gametophytes, and they still require water for their swimming sperm, this clever adaptation evolved at least four times in the spore plants. And on one of those occasions, it led to seeds.

Like unearthing a perfect fossil, looking at spike mosses is a glimpse into the past. As modern ambassadors of an ancient line, their mismatched spores mirror a critical
step in the evolution of seeds. With the sexes separated, it becomes much easier to imagine the rest of the story. Over time, early seed plants learned not to cast off their female spores but to cling to them, letting the eggs develop right there on the tops of their leaves. Male spores continued dispersing and with a few tweaks became windborne grains of pollen. When that pollen landed on an egg, the plant suddenly found itself in possession of all the basic elements of a seed: a fertilized baby that could be protected, provisioned, and sent off to grow directly into the next generation. This system gave seed plants immediate advantages whenever the weather turned dry. Where spores required water for their swimming sperm and moisture-loving gametophytes, seed plants could reproduce on a gust of wind. And their durable, well-stocked offspring landed in the soil prepared to wait for just the right conditions in which to germinate and grow.

The fossil record for seed evolution remains hazy, but as with spike mosses, other modern plants help fill in the gaps. Most people recognize the ginkgo tree as a popular ornamental, or the source of
herbal elixirs sold to boost memory and improve blood flow. But it’s also the sole survivor of an early seed-plant family whose pollen still produce swimming sperm, a holdover from the spore era. A group of palm-like trees called cycads also retain this trait, and one of them boasts sperm so huge they can be seen by the naked eye. (Festooned with thousands of waving tails, the sperm of the
chigua
from coastal Colombia exceed those of any other plant or animal.) Together with the conifers and a handful of lesser-known species, these plants make up the
gymnosperms
, or “naked seeds,” so named because their seeds mature unadorned on the
surface of leaves or cone scales.

Gymnosperms dominated the world’s flora from the dry periods of the Carboniferous all the way through the time of the dinosaurs, and they remain extremely common today. Anyone who has enjoyed pine nuts on a plate of pesto is familiar with naked seeds. So are the billion or so people who live in or around temperate forests, where pines, firs, hemlocks, spruce, cedar, cypress, kauri, and other conifers still cover more land area than any other plants. But while they may be widespread, these venerable trees and shrubs long ago passed the crown of plant diversity down to a younger group of seed innovators.

T
he final major step in seed evolution occurred when a few gymnosperms learned to cover up. They did it in much the same way people do after a bath, and for similar reasons. At three years old, my son Noah still uses the blue plastic tub we bought when he was an infant. He can climb out on his own now, but when he does I wrap him up immediately in a big fluffy towel. I do this not out of some prudish aversion to nudity, but because his little naked body seems so vulnerable. For me, it triggers an instinctive parental response to protect and nurture. While plants don’t run around making conscious decisions about towels, the same evolutionary drive led one line of gymnosperms to wrap their naked seeds, folding up the underlying leaf to enclose the developing egg. Botanists call this chamber the
carpel
and the plants that have one are known as
angiosperms
, Latin for “seeds in a vessel.”

I didn’t see any fossil angiosperms in New Mexico. “Wrong conference,” one attendee told me gruffly. The rocks were wrong, too, off by several major geologic time periods. While it sounds like a simple, even obvious step to wrap a protective leaf around a seed, angiosperms didn’t work it out until the early Cretaceous, after naked seeds had been commonplace for more than 160 million years. To put that into perspective, the entire diversity of placental mammals, from rodents and bats to whales, aardvarks, and monkeys, has evolved in a time period less than a third as long. Botanists still puzzle over this delay, but no one disputes that putting seeds in a vessel turned out to be a good idea. Once established, angiosperms spread so fast that Darwin considered their rise an “abominable mystery” that threatened his concept of
measured, incremental change. They now make up the vast majority of all plant life, and their seeds dominate the discussions in this book.

From an evolutionary standpoint, the leap from spores to gymnosperms was the paramount step for seeds. Bill DiMichele laments our tendency to focus on angiosperms. “It misses the story,” he told me. “There just happen to be a lot of them.” But there’s no doubt that wrapping those naked seeds refined the system and opened a range of new opportunities. After all, a towel is just the beginning. Noah’s wardrobe trends toward striped pajamas, but people can cover up their nakedness with whatever they want: shorts and a Hawaiian shirt, a cocktail dress, or even a suit of armor. Seed coverings soon evolved from simple leaf tissue into the dizzying array of structures we know collectively as fruit. Like clothing, fruit can be protective but it can also attract, giving angiosperms a powerful way to hoodwink animals into dispersing their babies. (We will explore the ties that bind fruits, seeds, and animals, including people, in
Chapter 12
.)

Even more important than the evolution of fruit, however, was the way that covering seeds affected pollination. With the egg hidden away inside its vessel, wind became a less reliable tool for pollen delivery. Instead, angiosperms turned increasingly to animals, and particularly insects, to move pollen from flower to flower. Colorful
petals, nectar, fragrance—all the allure we associate with flowers—developed in response to this need, transforming pollination from a random wind splatter into one of nature’s most precise (and beautiful) methods for mixing genes. This system helped propel the rapid diversification that so mystified Darwin, and it also gave rise to another name for angiosperms:
“the flowering plants.”

In nature, the flowering plants put sex, seeds, and dispersal on full display, spurring not only their own evolution but also that of the animals and insects with which they became so entwined. In most cases, the diversity of dispersers, consumers, parasites—and, most especially, pollinators—rose right alongside that of the plants they depended upon. But the evolution of floral sex has also proved vital to people. Without the ability to manipulate pollination and save the result as a durable seed, it’s hard to imagine our ancestors ever succeeding in agriculture. Author and food activist Michael Pollan takes the case a step further, calling the practice of plant breeding “a series of
experiments in coevolution” that has changed both plants and people forever. Pollan has argued that human desires for sweetness, nourishment, beauty, or even intoxication have become encoded in the genetics of our crops. Selecting for these traits both pleases us and benefits the plants as we dutifully disperse them from their original habitats to gardens and farm fields across the globe. But our intimacy with seed plants fills more than our bellies—it also feeds the human imagination. The knowledge we’ve gained from this long relationship may be our deepest reservoir of insight into the workings of nature. Without it, the most famous experiment in history might never have taken place.

CHAPTER FIVE

Mendel’s Spores

The various forms of Peas selected for crossing showed differences in length and color of the stem; in the size and form of the leaves; in the position, color, size of the flowers; in the length of the flower stalk; in the color, form, and size of the pods; in the form and size of the seeds . . .

—Gregor Mendel,
Experiments in Plant Hybridization
(1866)

“P
lant the peas on President’s Day.” Living with an avid gardener, I had come to know this adage as both mantra and command. For Eliza, sowing peas marked the much-anticipated start of a new season, and the soil in her garden was always fresh-turned and waiting well in advance. This year we both had plans for peas, but as I yanked another clump of grass from the Raccoon Shack’s overgrown flowerbed, it was quite clear that mine would be late. Considering that I still had to bring in fresh soil and devise some kind of protection from the chickens, let alone order the seeds, I’d be lucky to get them in by Palm Sunday. Still, that schedule probably put me closer to planting day in Brünn, Moravia (now Brno, Czech Republic), where the famous garden I hoped to evoke was still locked away under snow.

Weather aside, Gregor Mendel had a lot of things going for him when he coaxed his first row of pea shoots to life
in the spring of 1856. His abbot, Cyrill Napp, ran the Augustinian Monastery of St. Thomas more like a research university than a cloister, encouraging his monks in their studies of everything from botany and astronomy to folk music, linguistics, and philosophy. They enjoyed good food, an excellent library, and ample time to pursue their research. For Mendel, the abbot went so far as to build a dedicated greenhouse and turn over use of the orangery and a wide swathe of the monastery gardens. But the young monk also benefited from millions of years of evolution, because without the unique characteristics of seeds, making his famous discoveries would have been a lot more challenging, if not impossible.

Try to picture the father of modern genetics conducting his experiments on spore plants. He would have spent every day on his hands and knees in the mud, searching for tiny gametophytes and hopelessly trying to corral their sperm and eggs. How could one possibly control the breeding of plants whose sex happens out of sight in the soil, with microscopic,
free-swimming sperm? Spore plants simply don’t lend themselves to manipulation, which is why the handful of ferns and mosses that have ever been domesticated remain essentially unchanged from their wild ancestors. (It’s worth noting another spore trait that prevents them from being particularly useful to people: because they don’t “pack a lunch” for their babies, spores have no nutritional value. People might nibble on the occasional leaf of a spore plant, but—with very few exceptions—you can’t make bread, porridge, or
anything else from the spores themselves.)

Mendel never considered studying ferns or mosses. As a farmer’s son, he knew enough about plants to realize that such a haphazard mating scheme could never teach him about heredity. But he did try his hand with mice, reportedly stopping only after the local bishop found it unseemly for a monk’s quarters to be filled with cages of rapidly multiplying rodents. When he finally settled on peas, Mendel found a system ideally suited to his experiments. Hand-pollinating
the flowers allowed him to play matchmaker, selecting exactly which plants he wanted to cross and then watching how their traits were passed down. Unlike spores, the seeds of his pea plants united genes from both parents into something that could be easily sorted, examined, and counted. And unlike mice, they lived outdoors, smelled sweet, and even provided a tasty surplus for the monastery kitchen.