The Forest Unseen: A Year's Watch in Nature (17 page)

Despite the nobility of the tick’s quest and my admiration for her armor and weaponry, I feel a strong need to flick her away or pinch her with my fingernails. Such disgust may come from somewhere deeper than mere learned precaution. Fear of ticks is etched in my nervous system by the experience of many, many lifetimes. Our battle with questing ticks is at least sixty thousand times older than the Arthurian legends. We’ve been scratching and tweaking at ticks through our entire history as
Homo sapiens
, back to our early primate days when we chattered and groomed one another, to our time spent as itchy insectivores and, further, to our reptilian origins when ticks evolved ninety million years ago. The Grail tires after so many millions of years of pursuit. I skirt the viburnum bush as I leave.

W
e are on the cusp of summer. For the last two weeks, the temperature and humidity have edged higher day by day, and the heat has slowed my walk to the mandala; the time for the vigorous, warming hikes of winter has long passed. In the forest, the abundance of animal life is impressive, especially when contrasted with the quiet of winter. Birdsong streams from all directions. The air is busy with the continual passage of small midges, mosquitoes, wasps, and bees. Ants crawl over the litter, several dozen visible at any moment within the mandala’s circle. Furry jumping spiders also ply the forest floor, and millipedes trundle through gaps in the litter. Above, the tree canopy is dense and multilayered. The leaves have matured from the light, airy green of spring to summer’s more substantial, deeper hues. Photosynthesis in this thick canopy of leaves is now at full capacity, harvesting the energy that forms the foundation of the forest ecosystem.

At ground level, the spring ephemeral wildflowers have mostly faded away. The plants that are left are all shade specialists, growing slowly in the dim understory. Of these species, the ferns are the most abundant and conspicuous. As I look across the forest floor, I see ferns emerging every meter or so across the whole mountainside.

Christmas fern fronds as long as my forearm arch like jaunty hat plumes on the southern edge of the mandala. Their fresh growth reaches over last year’s fronds, which, although they are still attached to the fern’s base, lie prostrate and dying. The old fronds stayed green
through the winter and spring, giving the plant a photosynthetic boost before the emergence of this year’s new growth. The hardiness of the Christmas fern gave European settlers greenery with which to decorate their winter festivals, and the fern was named to honor this use. The new growth on these plants emerged in the mandala in April, pushing through the litter as silvery, tightly coiled fronds called fiddleheads. As the coils unfurled, their central stems lengthened and leaflets grew, producing elegant, tapered feathers.

The leaflets at the tips of the highest-reaching fronds are pinched and shrunken. Instead of presenting the sun with a broad surface for photosynthesis, these attenuated leaflets carry two rows of discs on their undersides. The discs are about as wide as peppercorns. Like a skullcap pressed onto a head of curls, each disc has a mass of brown fuzz protruding from its edges. When I peer through the hand lens, the mass of curls is transformed into a carpet of dark snakes. Each snake’s body is divided into sand-colored segments with wide mahogany edges. The snakes’ mouths hold fat clusters of golden globes. I see no movement today, but previously I have seen the snakes rear up, then snap back, spitting the globes high into the air.

The globes are the fern’s spores, each one with the makings of a new fern enclosed in its tough coat. The snakes are botanical catapults, designed to hurl spores skyward. The snakes’ segments are cells with unevenly thickened walls, and this unevenness powers the motion. On sunny days, the water that lines the cells evaporates, increasing surface tension in the remaining water. Because the cells are so small, the increasing surface tension is strong enough to bend them, arching the snake upward. As the snake rears, it scoops up a mass of spores, preparing to launch them. More water evaporates and the tension increases, bending the snake further. Snap! The tension breaks and the cell walls’ pent-up energy flicks the spores away. When the sun shines directly on a ripe frond, water evaporates rapidly from the snakes’ cells, sending spores flying like popcorn from hot oil. To the naked eye these escaping spores look like puffs of smoke. Under a lens, the action
is more dramatic: the barrage of snapping catapults looks like a battlefield reenactment.

The catapults’ dependence on the sun’s drying power allows the ferns to launch spores only on dry days, when the spores have a good chance of traveling far. Today, the air is thick with humidity, the sky is darkening gray, and thunder growls in the distance. This is an inauspicious time for traveling spores; they risk being washed out of the air, and so the catapults lie motionless.

Like an animal’s egg or sperm cell, each spore carries a shuffled deck of exactly half its parent’s genes. But unlike an egg or sperm, a spore will land, then germinate without uniting with another spore. This is the first hint that the life cycle of plants is radically different from our own. Sex for animals is a quick two-step: make the sex cells by halving your genetic library, then fuse egg and sperm into a new animal. Just two stages, a simple rotation. But ferns are up to something strange. No frond emerges when the spore germinates. Instead, a small “lily pad” grows, spreading out its flat body until it is the size of a small coin.

The lily-pad fern makes its own food and lives as a separate individual. After a few months or years, swellings appear on its skin. Some swellings are like blisters; others are like tiny chimneys. The blisters expand, then on a rainy day they burst and release sperm cells. These spin through surface water, sniffing for chemicals from an egg sitting at the base of a chimney. Each chimney’s core is filled with chemicals that bind and destroy sperm cells of the wrong species. Suitable sperm face no such bar and swim to the egg. The two cells then fuse. The resulting embryo develops into a new Christmas fern that will eventually grow large enough to fling spores from the tips of its arching fronds. The fern’s life cycle therefore has four steps: spore, lily pad, egg or sperm, and large fern.

On the other side of the mandala a rattlesnake fern adds some interesting twists to this life cycle. Its leaves spread low over the litter in a lacy fan that is about as wide as the span of my hand. A spike rises
from the center of the fan, reaching twice as high as the leaves. At the top of the spike, several dozen millimeter-wide capsules cluster on small side branches. These capsules shake spores from vertical slits in their sides. The spores germinate and grow not into lily pads but into subterranean tubers, like miniature potatoes. These tubers have no chlorophyll and depend on a fungus for food. After several years of growth, the tuber makes sperm and eggs, which will then produce another rattlesnake fern.

Once grown, the adult rattlesnake fern continues to exchange nutrients with the fungus. Some rattlesnake ferns take this mutualistic relationship to its extreme, never lifting their fronds above the leaf litter. These individuals grow and produce spores entirely belowground, nourished by their fungal collaborators.

Both species of fern in the mandala alternate between two life-forms: the large spore-bearing plant and a smaller egg-and-sperm factory, the lily pad or tuber. This alternation between different identities is hard for humans to grasp, and the sex life of ferns remained a mystery until the 1850s. The obvious reproductive structures, the spores that blew away in the wind like pollen or seeds, were unlike any other sex cell. Botanists called ferns and their similarly confusing kin, the mosses, cryptogams or “hidden sex” plants, applying a terminological Band-Aid to the bothersome mystery. The confusion lifted once sperm and egg cells were discovered swimming in the watery surfaces of tiny lily pads.

The fern’s reproductive methods are well suited to life in protected, wet places, but ferns fare poorly under drier, more rigorous conditions. Without moisture for their sperm to swim in, ferns cannot reproduce. In addition, their lily-pad stage offers little protection or nourishment for embryos. Flowering plants have broken free from these constraints by modifying the fern’s life cycle. Instead of releasing spores into the wind, flowering plants produce spores that are retained within the flower’s tissues. These spores grow into protected miniature lily pads that then make eggs and sperm. Thus the fern’s independent lily pad
was shrunk to a few cells buried inside the flower. This freed flowering plants from the need to find a watery nook in which to reproduce. Deserts, rocky ridges, and dry hillsides were no longer barriers to plant sex, nor were dry spells or sunny days. Whittling down and retaining the lily pad also allowed flowers to nurture their offspring, passing nourishment to them, wrapping them in protective seed coats, and holding them aloft inside fruits to catch the wind or be picked up by a passing seed-dispersing bird.

The reproductive innovations of flowering plants allowed them to become by far the most diverse group of plants alive today. There are over a quarter of a million species of flowering plant but just over ten thousand fern species. When flowering plants evolved, about one hundred million years ago, many species of ancient ferns and other nonflowering plants were displaced, outcompeted by the newcomers. It would be a mistake, however, to dismiss modern ferns as primitive leftovers. Recent studies of DNA show that modern ferns evolved and diversified
after
the rise of the flowering plants. As the flowering plants took over, they caused ancient ferns to disappear, but they also inadvertently produced the ideal conditions for a new dynasty of ferns: moist mandalas in which shade-loving Christmas and rattlesnake ferns can thrive.

A
gusty wind is clearing the sky of the clouds that have drizzled rain all week. The first sunshine in days is breaking through small gaps in the tree canopy, checkering the mandala with patches of light and shadow. The slick surfaces of
Hepatica
leaves flash as the sunlight catches them. Other plant species lack
Hepatica
’s shine but glow with various shades of green. After so many days under dull skies, the mandala’s colors seem particularly vivid. The forest also sounds more alive. From all around me comes a gentle hum, the combined buzzes of thousands of insect wings, like the sound of a distant beehive.

Although it is midmorning and the sun has been out for hours, two snails lie exposed on the damp leaf litter. They have likely been here since before sunrise, twisted together in a mating tangle. Their horn-colored shells face each other, aperture to aperture, and their bodies are fused in a knot of gray-and-white flesh. These snails are locked in a difficult negotiation and exchange. Instead of transferring sperm from male to female, as most animals do, the snails are moving sperm both ways. Each individual is both a donor and a recipient of sperm, male and female united in one body.

Hermaphroditism creates a complex economic problem: how to make sure that the reproductive exchange between partners is fair. For snails, as for most organisms, sperm is cheap to produce but eggs are expensive. In unisexual animals these different costs generally favor selectivity on the
part of females and unselective promiscuity on the part of males, especially in species where males contribute nothing to raising the young. In hermaphrodites, though, discernment and promiscuity are joined in one body, and mating becomes fraught, with each individual being cautious about receiving sperm while simultaneously trying to inseminate its partner.

Snails that detect a whiff of disease on their mates will refuse to express their female side, giving but not receiving sperm. Snails that find uninfected mates, however, will readily accept sperm from their partners. This selectivity may help snails pick out genetically superior sperm for their limited supply of eggs. Hermaphrodites are also sensitive to their larger social context. When they live in an area with few potential mates, they express both their male and female sides, but in a more crowded situation their femininity is dimmed and they act more like males, freely giving sperm but saving their eggs for only the best partners. The situation is yet more complex if mates have already bred and accepted sperm from another individual. One partner may then refuse another altogether, causing the rejected partner to attempt to mate forcibly, shoving sperm packets at the unwilling partner. Love triangles are fraught; love hexagons are war zones.

War is no metaphor. In some species, snail mating tensions have ballooned into armed conflict: bony darts shoot from one mate to another, sperm-destroying glands neutralize unwanted maleness, and muscles jostle sperm and eggs into battle lines. Even the length of the snails’ embrace may be a result of sexual conflict. The snails palpate each other with their tentacles, circle, slowly edge into position, always ready to pull back or realign. What the snails are assessing at each stage is unknown, but their extended courtship and copulation is choreographed like cautious diplomacy, a prenuptial conference over the terms of the union. Such languor must surely be costly. The snails in the mandala have lain with their bodies mostly unshelled for over half an hour, easy prey for birds or other predators.

Hermaphroditism is an unusual sexual system for animals, most of which divide male and female functions into separate bodies. Yet all
land snails are hermaphrodites, as are some sea-dwelling mollusks and a scattering of other invertebrates. The sexuality of the snails in the mandala has more in common with springtime flowers than it does with the birds or the bees. All the mandala’s spring ephemerals and trees are hermaphrodites, and many of them combine male and female in a single flower. This diversity of sexual systems is puzzling. Why should wrens be boys and girls, but the trees that the wrens live in be boy-girls? The beetles that the wrens feed to their nestlings are either male or female, but the snails that arrive in the same beakful of food are all hermaphrodites.