In the early 1970s Chevalier and his colleagues figured out how to inoculate the roots of tree saplings with truffle spores, thus increasing the chances that the trees would produce truffles. When a truffle spore comes into contact with one of the hair-thin rootlets that fan out under an oak, it sends out long, microscopically thin filaments that wrap around the root. Under magnification the root tip comes to look fuzzy and slightly swollen, like the end of a cotton swab. It is part of both truffle and tree. Ordinary white mushrooms colonize dead plant matter, which is why they are readily grown on compost and on an industrial scale.
Truffles are nicer.
No one has ever managed to grow a truffle in the lab, without a tree. At some point, possibly when the web under the tree reaches a certain density, and typically in May, a sex act with profound culinary consequences takes place. Whether two different filaments, one male and one female, have to coalesce, or whether an edible black truffle fruit springs forth from a single hermaphroditic filament is not clear. Later, in the heat of August and September, the young truffles go on a growth tear that brings their weight to an ounce or two and sometimes to as much as a pound. Then they slow down and start to ripen.