weave themselves through the gaps between plant cells in an intimate brocade and help to defend plants against disease.

Mushrooms are a fungus’s way to entreat the more-than-fungal world, from wind to squirrel, to assist with the dispersal of spores, or to prevent it from interfering with this process.

Mycelium describes the most common of fungal habits, better thought of not as a thing but as a process: an exploratory, irregular tendency.

Unsustainable agricultural practices reduce the ability of plants to form relationships with the beneficial fungi on which they depend. The widespread use of antifungal chemicals has led to an unprecedented rise in new fungal superbugs that threaten both human and plant health. As humans disperse disease-causing fungi, we create new opportunities for their evolution.

many fungi can live within the roots of a single plant, and many plants can connect with a single fungal network. In this way a variety of substances, from nutrients to signaling compounds, can pass between plants via fungal connections.

Tricked out of our expectations, we fall back on our senses. What’s astonishing is the gulf between what we expect to find and what we find when we actually look.

Many scientific concepts—from time to chemical bonds to genes to species—lack stable definitions but remain helpful categories to think with.

There was something embarrassing about admitting that the tangle of our unfounded conjectures, fantasies, and metaphors might have helped shape our research. Regardless, imagination forms part of the everyday business of inquiring.

wondered what it was like to be a fungus. I found myself underground, surrounded by growing tips surging across one another. Schools of globular animals grazing—plant roots and their hustle—the Wild West of the soil—all those bandits, brigands, loners, crapshooters. The soil was a horizonless external gut—digestion and salvage everywhere—flocks of bacteria surfing on waves of electrical charge—chemical weather systems—subterranean highways—slimy infective embrace—seething intimate contact on all sides.

LICHENS ARE PLACES where an organism unravels into an ecosystem and where an ecosystem congeals into an organism. They flicker between “wholes” and “collections of parts.”

Lichens are a product less of their parts than of the exchanges between those parts. Lichens are stabilized networks of relationships; they never stop lichenizing; they are verbs as well as nouns.

lichens don’t contain microbiomes. They are microbiomes, packed with fungi and bacteria besides the two established players.

we have yet to find any lichen that matches the traditional definition of one fungus and one alga.”

“The human binary view has made it difficult to ask questions that aren’t binary,” he explained. “Our strictures about sexuality make it difficult to ask questions about sexuality, and so on. We ask questions from the perspective of our cultural context. And this makes it extremely difficult to ask questions about complex symbioses like lichens because we think of ourselves as autonomous individuals and so find it hard to relate.”

it is no longer possible to conceive of any organism—humans included—as distinct from the microbial communities they share a body with. The biological identity of most organisms can’t be pried apart from the life of their microbial symbionts.

the study of symbiosis reveals that life is full of hybrid life-forms, such as lichens, which are composed of several different organisms. Indeed, all plants, fungi, and animals, including ourselves, are composite beings to some extent: Eukaryotic cells are hybrids, and we all inhabit bodies that we share with a multitude of microbes without which we could not grow, behave, and reproduce as we do.

It was only by striking up new relationships with fungi that algae were able to make it onto land. These early alliances evolved into what we now call “mycorrhizal relationships.” Today, more than ninety percent of all plant species depend on mycorrhizal fungi.

photosynthesis isn’t enough to support life. Plants and fungi need more than a source of energy. Water and minerals must be scavenged from the ground—full of textures and micropores, electrically charged cavities, and labyrinthine rotscapes. Fungi are deft rangers in this wilderness and can forage in a way that plants can’t. By hosting fungi within their roots, plants gain hugely improved access to these sources of nutrients. They, too, get fed. By partnering, plants gain a prosthetic fungus, and fungi gain a prosthetic plant.

In their relationship, plants and mycorrhizal fungi enact a polarity: Plant shoots engage with the light and air, while the fungi and plant roots engage with the solid ground. Plants pack up light and carbon dioxide into sugars and lipids. Mycorrhizal fungi unpack nutrients bound up in rock and decomposing material.

They are stationed at the entry point of carbon into terrestrial life cycles and stitch the atmosphere into relation with the ground.

What we call “plants” are in fact fungi that have evolved to farm algae, and algae that have evolved to farm fungi.

Neither plants nor fungi inherit each other. They inherit a tendency to associate, but they conduct what are, by the standards of many other ancient symbioses, open relationships. As in the earliest days of life on land, plants form their relationships depending on who’s around. The same goes for fungi. Though this might be a limitation—a plant seed that finds no compatible fungi is unlikely to survive—the ability to reform their relationships, or evolve entirely new ones, can allow partners to respond to changing circumstances.

even with their own root systems, almost all plants still depend on mycorrhizal fungi to manage their underground lives.

Most modern crop varieties have been developed with little thought for their ability to form high-functioning mycorrhizal relationships. We’ve bred strains of wheat to grow fast when they are given lots of fertilizer, and ended up with “spoiled” plants that have almost lost the ability to cooperate with fungi.

Animal microbiomes tend to be dominated by bacteria, while plant microbiomes tend to be dominated by fungi.