240 CAUSES OF FERTILITY AND STERILITY (chai*. 



well seen in the development of "fairy rings" in 

 pastures. "Fairy rings" are circles of dark -green 

 grass, common enough in poor pastures, which are 

 found to extend their size every year, leaving the grass 

 within the ring of a lighter colour and of generally 

 poorer aspect than that outside. On examining the 

 soil immediately outside a ring, it is found to be full of 

 the mycelium of one or two common species of fungi, 

 but the mycelium rarely occurs in the soil beneath the 

 ring itself, and never in that within the ring. The 

 ring appears to be dependent on the growth of 

 the fungus, which starts at one point and draws 

 upon the humus reserves contained in the soil. 

 Having consumed whatever humus is available, the 

 mycelium must proceed into the annular area of soil 

 immediately, round the first patch, thus from year to 

 year it spreads outward. After the death of the 

 fungus, there is left behind in the soil it has just 

 occupied a quantity of organic matter, which readily 

 decays and becomes available for plant nutrition; 

 thus a ring of luxuriant vegetation immediately 

 follows the death of the fungus. In other words, 

 the humus of the soil, slow to decay and nitrify in 

 the usual way, is changed into material undergoing 

 rapid change by its preliminary conversion into the 

 tissue of the fungus. At the same time, as the 

 supply of rapidly acting plant food has been solely 

 derived from the soil, the ultimate result is the im- 

 poverishment of the soil within the ring by the develop- 

 ment of the fungi and the subsequent luxuriant growth 

 of grass. 



The following figures relate to the composition of 

 the soil (mean of five examples) within, on, and outside 

 fairy rings : — 



