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It was somewhat new to me that mushrooms usually aren't individual organisms, but are merely the visible bodies of a bunch of fungi living in the soil. I know that mushrooms emit spores to reproduce, but what has been bizarre to me is how fairy rings form. Why do the fruiting bodies arrange themselves in a more or less circular shape, as opposed to the random scattering one would expect from wind-borne spores?

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Furthermore, our normal ideas about what an organism is, and where its boundaries lie, are difficult to apply to these fungi. –  Jay Bazuzi Dec 25 '11 at 20:42
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When a fungal spore germinates in a suitable location, the growing mycelium will spread underground in all directions. In the ideal situation, the result is that the mycelium will become circular. Over time, the center of the mycelium will die out whereas the newly formed mycelium (underground) will develop the familiar mushrooms above ground and this will result in a fairy ring.

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In addition, the mycelia (the underground mass of hyphae which constitutes the bulk of the fungus) expand outwards because they decompose organic matter in soil as they go, leaving very little organic matter in the soil in the interior of the ring. My Campbell & Reece textbook tells me that they can expand outwards at about 30 cm/yr. (1)

(1) Campbell, N.A., Reece, J.B. et al., Biology, 8th edition, Pearson, 2008

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