The largest living thing on Earth is a fungus
In the Malheur National Forest in Oregon a honey fungus spreads through the soil across roughly four square miles. Genetic testing showed it is all one individual, and the low estimate of its age is 2,400 years — a single organism larger than any whale or tree.
Almost none of it is visible. All you see in autumn is a scatter of ordinary mushrooms; the creature itself is the MYCELIUM, a mat of threads finer than cotton running through the ground. Every MUSHROOM and TOADSTOOL works this way — the thing on the surface is fruit, as an apple is to an apple tree, and the FUNGUS is underfoot.
A fairy ring is a clock
Mycelium spreads outwards from wherever it started, at a fairly steady rate, exhausting the food behind it as it goes. The mushrooms come up at the growing edge — which is a circle. That is the whole of the fairy RING: not a mystery, just the rim of something round and buried.
Because the spread is steady, the size gives you the age. Rings a few metres across are a decade or two old; there are rings in Europe hundreds of metres wide that are reckoned to have been growing for several centuries, quietly, in the same field. Folklore made them the dancing floors of fairies and warned children not to step inside. The truth is arguably better: you are standing on something enormously old.
Seven trillion spores from one puffball
Fungi do not make seeds. They make SPORES — single cells, small enough to drift on air — and they make them in numbers that are hard to take seriously. A giant PUFFBALL is estimated to release around seven trillion. Step on a ripe one and the brown smoke that comes out is the spores leaving.
An ordinary field mushroom manages billions, dropped from between the GILLS under its CAP. The gills pack a great deal of spore-making surface under a small roof, and that is the whole reason so many fungi are built as a STEM holding a CAP over a set of pleats. Almost every spore fails. With those numbers, it does not matter.
The one that looks like dinner
The death cap causes the great majority of fatal mushroom poisonings worldwide, and the reason is that it looks thoroughly unremarkable — greenish-white, pleasant-smelling, easily taken for an EDIBLE species by someone who is fairly sure. Cooking does nothing to it. The toxin attacks the liver, and the symptoms often hold off for six to twelve hours, by which point serious damage is done.
Hence the forager's rule: never eat a wild mushroom unless somebody who really knows has looked at it. A POISON species and a safe one can be the same colour, the same size, growing side by side. The CHANTERELLE, SHIITAKE, BUTTON and TRUFFLE here are all good eating; the point is being certain which is on the plate.
The useful ones, and the rest of the grid
Two fungi on this list are in most kitchens already. YEAST is a single-celled fungus that eats sugar and breathes out carbon dioxide, which is the entire reason a loaf of bread rises. MOULD is doing something similar on the slice you left too long — spreading a mycelium into it and fruiting in blue-green fuzz.
The rest of the grid is the woodland version: the DAMP corners of a FOREST or WOODLAND where fungi do the job nothing else will, breaking dead timber down. DECAY has a bad name, but a forest that could not rot would bury itself in its own fallen branches within a century.