It is the Mycelium, the main body of the fungus, which forms the circles as it spreads out as web of strands underground at a rate of 6 - 12 inches a year (although in Germany it has been suggested that it is caused by witches dancing in a circle on the grass on Walpurgis night).
Although this fungus is edible, it is best avoided because the deadly poisonous species Clytocybe rivulosa.
Spore PrintTo take a spore print,
As the spore of the fairy ring champignon are white I used black paper for this print. I greatly increased the contrast of the scan to show the pattern.
Gilbert White's JournalI'm convinced that if the Rev. Gilbert White (1720-93) was alive today he would publish his Natural History of Selborne (1789) as a web site. He made the most of the communications technology of his day, taking advantage of the stagecoach network to mail his observations to friends and getting yearbooks printed with blank columns for his observations of birds, flowers, insects and the weather.
Here's what he had to say about the fairy ring fungus;
The cause, occasion, call it what you will, of fairy rings, subsists in the turf, & is conveyable with it: for the turf of my garden-walks, brought from the down above, abounds with those appearances, which vary their shape, & shift situation continually, discovering themselves now in circles, now in segments, & sometimes in irregular-patches, & spots. Wherever they obtain, puff-balls abound; the seeds of which are doubtless also brought in the turf.
Journals, 15th Oct. 1780
Related LinkGilbert White's House & Selborne
The Selborne Society, founded in 1885 to commemerate Gilbert White.