Fridge Fungus

It occured to me the other day that I have no idea what species these are:

White mushrooms, closed-cup, flat, chestnut, button, champignon, portabello, portabella, portabellini ... these are the common mushrooms found in supermarkets (and fridges) all over the UK. But what are they? And do they exist in the wild? 

Surprisingly, it turns out that they are all one species - Agaricus bisporus. 

What we call closed-cup or button mushrooms are the immature stage of the mushroom, which commonly grows white or brown. The Portabello or flat mushroom is the mature fungus. 

And yes, it does grow wild in grasslands across Eurasia and North America although it is not a common mushroom at all. It grows in rich soils, usually near horse manure or compost piles. As such, it's most commonly found in pasture where cows and other cattle feed. The fact that you so rarely see it in the wild means that it's easily confused with the Field or Meadow Mushroom (Agaricus campestris) which is also edible and tastes pretty much the same. In fact, they are so similar that A. bisporus ws only recognised as a separate species in 1923. 

A. bisporus is cultivated in more than 70 countries and is one of the most commonly and widely consumed mushrooms in the world. In the USA, the white button form alone accounts for about 90% of all mushrooms sold. 

So now you know.