Like many small organisms, fungi are often overlooked, but their planetary significance is outsize. Plants managed to leave water and grow on land only because of their collaboration with fungi, which acted as their root systems for millions of years. Even today, roughly 90 percent of plants and nearly all the world’s trees depend on fungi, which supply crucial minerals by breaking down rock and other substances. They can also be a scourge, eradicating forests — Dutch elm disease and chestnut blight are fungi — and killing humans. (Romans used to pray to Robigus, the god of mildew, to guard their crops against plagues.) At times, they even seem to think. When Japanese researchers released slime molds into mazes modeled on Tokyo’s streets, the molds found the most efficient route between the city’s urban hubs in a day, instinctively recreating a set of paths almost identical to the existing rail network. When put in a miniature floor map of Ikea, they quickly found the shortest route to the exit.

“Entangled Life” is full of these sorts of details, but it’s also deeply philosophical: a living argument for interdependence. Without fungi, matter wouldn’t decay; the planet would be buried under layers of dead and unrotted trees and vegetation. If we had a fungi-specific X-ray vision, we would see, Sheldrake writes, “sprawling interlaced webs” strung along coral reefs in the ocean and twining intimately within “plant and animal bodies both alive and dead, rubbish dumps, carpets, floorboards, old books in libraries, specks of house dust and in canvases of old master paintings hanging in museums.”

The idea of fungi as metaphor for life has lately entered the zeitgeist, seeded in part by the forest scientist Suzanne Simard, who discovered that trees are connected through a mycelial network, the “Wood-Wide Web.” There was also the surprise hit 2019 documentary “Fantastic Fungi,” an effusive tribute that felt a bit like being cornered at a party by the stoned guy who’s really, really into mushrooms. But where “Fantastic Fungi” fell decidedly into the old-school, ’shroom-head camp, Sheldrake’s book is more embracing and more optimistic. Sheldrake describes mycelium as “ecological connective tissue, the living seam by which much of the world is stitched into relation.” At a time when the planet seems to be falling apart — or, rather, is being actively dismembered — the idea that we are bound together by an infinite number of invisible threads is so beautiful it almost makes your teeth ache.

Sheldrake is adept at channeling this longing for connection. After reading “Entangled Life” in lockdown, the couture designer Iris Van Herpen was moved to create a collection inspired by fungi, featuring a dress pleated like a chanterelle and bodices made of snaking silk tendrils modeled on hyphae, the thin, mobile strands that fungi use to explore the world. Hermès, Adidas and Lululemon have all embraced animal-free “mycelial leather,” and designers have started selling biodegradable furniture made from the stuff. The HBO series “The Last of Us,” about a cordyceps fungus that turns humans into zombies (based on a real species that hijacks the brains and bodies of ants), drew around 32 million viewers per episode. Retail stores have followed the trend, too. This spring brought an explosion of toadstool-print clothes and décor — shirts, wallpaper, throw pillows, dinner plates — plus mushroom-shaped table lamps, poufs and bedside tables.