One of the manifestations of fairy ring growth is a necrotic zone—an area in which grass or other plant life has withered or died. These zones are caused by the mycelia which, during a very dry year, coat the roots of grasses and other herbs in meadows. After some time they are removed by biotic factors from the ground, at which stage a zone on the surface soil becomes visible. Patterns other than the basic ring or arc are also possible: circles, doubled arcs, sickle-shaped arcs, and other complicated formations are also formed by this process. Fungi can deplete the soil of readily available nutrients such as nitrogen, causing plants growing within the circle to be stressed which leads to plant discoloration. Some fungi also produce chemicals which act like hormones called gibberellins, which affect plant growth, causing rapid luxuriant growth.