You can eat a spinach leaf. You can eat a lettuce leaf. Why not a magnolia leaf? Why not leaves of grass? Why can't you eat any old leaf off the sidewalk?

It sounds like a simple question, the work of a 6-year-old or a general interest reporter short on ideas. But the mind, not unheartlike, wants what the mind wants. We wanted to know why you can eat some leaves and not others.

So we asked ethnobotanist Ashley Glenn, who studies the interactions between plants and people. (Note: It's complicated.) First, though, we apologized for our dumb question. Not so dumb, says the expert.

"This is a great question to learn a lot of things about plants, because it covers evolution, chemistry, and culture," Glenn tells Urbo.

"Why are there more than a thousand species of plants with edible leaves and I've only tried six of them?"

Okay! This clearly isn't the light salad of a subject we expected. It's a deep, hearty stew, rich with forgotten greens. But that's getting ahead of ourselves.

Here's what we learned when we asked why you can't eat just any leaf:

1. The First Thing: Evolution

You can't eat grass or magnolia leaves because they're not edible, you might say. That only rephrases our question. What makes them inedible?

This is where plant physiology and anatomy come in, which is as good as saying it's where evolution comes in, because evolution is the force that selects our traits—plants and people both.

People begin to cook their food. They band together into societies. They build tools, they discover medicines; they innovate, they survive, they reproduce, they continue.

Plants have their own version of adaptability.

"Since plants can't move or claw or bite, they have to come up with a way of communicating and a way of deterring herbivory," Glenn says. "[A plant] has to uptake nutrients, it has to attract pollinators, it has to deter herbivores, it has to survive drought, and it has to do all these things without moving or articulating in any way. So what plants do is develop chemicals. That is their whole way of interacting with the world."

Botanists divide these chemicals into two general categories: Primary and secondary compounds. Primary compounds make up the essential structures of the plant. They're the stuff of cell walls, chlorophyll, sugars—the clay of the pot, not the soothing tea inside.

And among the primary compounds, there's one substance that provides the clearest and the most boring answer to our question: cellulose. The sad truth is that people just can't digest cellulose. We lack the enzymes.

Oddly enough, no vertebrate can digest cellulose, or at least, not alone. Ruminants like cows keep at least one of their stomachs stocked with friendly bacteria that break down the cellulose in their grass-heavy diet for them. Even termites rely on symbiotes to help them digest wood. It's bonkers.

So if a leaf is too full of cellulose, humans shouldn't eat it. You'll waste more calories chewing than you'll absorb from the non-digestible substance. Take pine needles (which are technically leaves): They're packed with cellulose and no good to eat.

When a leaf contains indigestible primary compounds, it doesn't enter people's systems in the first place. It's the secondary compounds that really make leaves interesting—and dangerous.

The Second Thing: Chemistry

Plants don't need secondary compounds to exist in the simplest sense of the word. They produce these chemicals to communicate, to repel things that might eat them—things like us. Or else they make substances that attract pollinators or seed dispersers: Again, sometimes, things like us.

"Secondary compounds are anything that is a poison or a colorant to make it more attractive to an insect, or nectar, or things like this," says Glenn.

Many of these compounds are toxic in certain doses. Some of those same compounds are toxic in one dose and medicinal in another. Either way, poisonous plants tend to warn would-be herbivores away with bitter flavors, Glenn explains.

"We are hardwired to not like bitter leaves, because that's a signal they're not safe," she says. "That's something that we evolved as a survival strategy. There's no doubt there were humans who loved the taste of bitter, and they died. Because they ate poison."

That's evolution for you: A bitter pill to swallow.

Green Deane is a professional forager who runs the startlingly comprehensive wild foods website Eat the Weeds. He warns that there's only one way to know which untamed leaf is poisonous and which one will spice up a boring salad, and that's hard-won expertise.

"There are leaves that taste bad but are edible, and there are leaves that taste good that are not edible," Deane says. "Size isn't a factor, age isn't always, nor location, nor animal or bird interests, et cetera. Each species has to be taken on its own values."

Societies that depend on the immediate environment for survival develop this expertise, which encompasses the vast fields of folk medicine and dietary specialization. In the industrialized world, with a few Deane-like exceptions, we've let that expertise lapse in favor of bland spinach in resealable plastic.

The Third Thing: Culture

In his illuminating guidebook/food-system manifesto, Eat Your Greens: The Surprising Power of Homegrown Leaf Crops, author Dave Kennedy addresses the cultural tipping point he feels the industrialized food system has reached.

"Despite the the frantic efforts of an overheated persuasion industry, there is more genuine enthusiasm for raising chickens in backyards than there is for new fast food menus," he writes.

Kennedy sees homegrown leafy green vegetables as a "third pillar" in an emerging hybrid food system that bends toward local sources without entirely abandoning the industrial approach. In short, Kennedy is trying to change the culture. He wants us to eat new types of leaves, which is an ancient and ongoing project in many societies; the third reason people don't eat a given leaf is that their culture doesn't see that leaf as food, not yet, whether it's edible or not.

"There's so much diversity in how people use plants, and the stuff that survives gets carried on because it helps people live and have more kids, and they carry their traditional knowledge down with them," says Glenn. "That's why we're a spice-loving species, because spices are really good for you."

Glenn has studied herbal medicine in communities from Amazonian Peru to the Dakotas. Through her work with international nonprofit Sacred Seeds, she has helped to preserve life-saving plants from around the world. And while localized diets depend on local plant life, Glenn says she's observed a key similarity from place to place, from culture to culture: People keep trying new leaves, growing the dietary library for future generations.

"People all the time are trying leaves," she says. "Always have, always will. It's usually some expert in the community who will nibble a little bit of the leaf to see the effect of it. People are constantly innovating."

But not all people innovate in the same ways. In the U.S., we remain largely dependent on a top-down food system that's powered by industrial farming, backyard chickens notwithstanding. As a result of decades of this approach, the storehouse of traditional knowledge, of local diet and folk medicine, has been depleted in mainstream U.S. life.

"With leaf morphology the way it is, it's really easy to confuse plants in a lot of cases," Glenn says. "You need an expert, and we're losing experts."

The secret answer: You can eat way more leaves than you thought.

So there are some solid reasons you can't just eat any leaf: Some have too much cellulose, others are poisonous, and others simply aren't considered food in your local culture. Then there are the leaves you can't even touch—ones that are spiky or sticky or make you itch: powerful deterrents all. But there's an even more compelling answer, which is that you can eat a lot more plants than you've probably tried.

We asked Kennedy to explain—but first we offered another quick apology for our silly question about why you can't eat every leaf in the world.

"Well, I would probably put it more positively," Kennedy tells Urbo. '"Why are there more than a thousand species of plants with edible leaves and I've only tried six of them?'"

It's a worthwhile question. Monoculture leads to vulnerability; vulnerability leads to pesticides and chemical fertilizers, which lead to dependence and a brittle food supply. Meanwhile, a narrower diet limits the range of nutrients we consume.

"Diversity in what we grow and what we eat is very protective," Kennedy says. "Having multiple sources for essential nutrients is insurance, and eating modest amounts of a much wider variety of foods protects against toxins."

Besides, Kennedy says, there's the dirt itself to consider.

"Diversity is also key to maintaining healthy, productive soils, in the event our grandchildren might enjoy eating," he says.

There's more to the well-rounded dinner table than spinach and kale. Try growing chaya or red toon or whatever feels at home in your climate. Eat your sweet potato leaves. See Kennedy's website, Leaf for Life, for more information on edible leaf crops you can plant in your own backyard (if they're not growing there already).

And lest you think you're getting out of an article like this without a quote from the poet-farmer Wendell Berry, here's this: "Eaters ... must understand that eating takes place inescapably in the world, that it is inescapably an agricultural act, and how we eat determines, to a considerable extent, how the world is used."

You can't eat just any leaf. But you can eat lots more leaves. You should.

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