Mycorrhizae are actually a fungus. They exist as very tiny, almost or even entirely microscopic, threads called hyphae. The hyphae are all interconnected into a net-like web called a mycelium, which measures hundreds or thousands of miles—all packed into a tiny area around the plant.
The mycelium of a single mycorrhiza, in turn, can extend outward, connect multiple plants (even plants of different species!), and even connect with other mycorrhizae to form a Frankenstein-like underground mash-up called a common mycorrhizal network.
In a common mycorrhizal network, it’s hard to tell where one mycorrhiza ends and another begins. Because of this vast network, a single plant can be connected to a completely different species of plant halfway across a forest!
Mycorrhizae actually connect to plants in two ways. One form, called ectomycorrhizae, simply surrounds the outside of the roots. Another form, called endomycorrhizae, actually grows inside of the plant—their hyphae squeeze in between the cell wall and the cell membranes of the roots (sort of like wedging themselves in between a bicycle tire and the inner tube).
Under normal conditions, you’re not likely to see mycorrhizae because they’re so small. But every once in a while, something amazing happens: the mycorrhizae will reproduce and send up fruiting bodies that produce spores—we call them mushrooms! Some of these mushrooms are even edible, like truffles or chanterelles.