It is very easy to find the world’s most extensive marketplace – just find your nearest forest, field or garden, and look underground.

The planet’s land plants are engaged in an ancient alliance with the so-called “AM fungi” that grow into their roots. One plant might be colonised by many fungi, and a single fungus could connect up to many plants. The fungi harvest nutrients like phosphorus and nitrogen from the soil and channel them to their hosts. In return, the plants provide the fungi with the sugars and carbohydrates they need to grow.

This symbiotic partnership covers the planet in green. It’s common to 80 percent of land plants, and is credited with driving the evolution of this group some 470 million years ago. Now, Toby Kiers from Vrije University in Amsterdam has found that plants and fungi have maintained their grand alliance by setting up a strong market economy.

All natural coalitions are vulnerable to cheats. Any individual could withhold nutrients from one or more of its partners, benefiting from their contribution while giving nothing back. But Kiers found that at least one plant – barrel clover – can ...