PSALLIOTA CAMPESTRIS 367 



effect upon wheat plants in an arable field as upon wild grass plants 

 in a natural sod (cf. Figs. 132 and 133). 



In many well-grazed pastures in England, Psalliota campestris 

 does not form fairy rings which can be clearly discerned, and the 

 mushrooms occur either singly or in ill-defined associations. Such 

 pastures, however, often produce mushrooms year after year. 

 Under these conditions is the mycelium perennial, living on from 

 year to year in the turf ? Or must we suppose that the pastures 

 are annually re-infected from spores in such a way that each 

 mycelium, in a single season, develops to maturity, produces a single 



FIG. 132. A vertical section through a fairy ring caused by the mycelium of 

 Psalliota (= Agaricus) tabularis, in grass in eastern Colorado, U.S.A. The 

 mycelium, which has produced a fruit-body in the outer stimulated zone, is 

 progressing from right to left. Drawn by H. L. Shantz and B. L. Piemeisel. 

 Courtesy of the United States Department of Agriculture. 



crop of fruit-bodies, and then dies from exhaustion ? Doubtless, 

 new infections of the turf take place occasionally. However, there 

 is reason to suppose that a mycelium, when once it has been pro- 

 duced, often lives on from year to year just as it does in fairy rings 

 but without giving rise to a ring. Ring- formation is only made 

 possible by the turf being so even in its quality that the mycelium 

 can develop centrifugally in an equal manner along all radii. In 

 many fields the turf is organically uneven, owing to the occurrence 

 of weeds, irregular grazing by animals, uneven drainage, uneven 

 deposits of excrement or manure, heterogeneous character of the 

 soil, or to the presence in it of various organisms other than 

 herbage plants. In such fields the mycelium of Psalliota campestris 

 must penetrate through the turf in an irregular manner ; but there 

 is no reason why this irregular penetration should not often be 

 continued perennially just as it is in fairy rings. I am therefore 



