Oct. 29,1917 
Fungus Fairy Rings 
241 
would result in the separation of the zones shown in figure 3 (at A). 
Outward growth would be slow, approximately 12 cm. a year on an 
average. This progress would not be regular, but would be wavelike, 
being comparatively rapid, 30 to 60 cm. during favorable years and very 
slow or none at all during unfavorable years. The sod would first be 
stimulated by the increase of available nitrogen, owing to the reduction 
of the organic matter of the soil, then killed by insufficient soil moisture 
in the area of dense mycelium. As this mycelium began to decay, the 
native ruderals would again invade and develop luxuriantly, owing to 
the abundant supply of readily available nitrogenous material. These 
ruderals would give way to short-lived grasses, and these in turn to the 
short grasses of the natural sod. To use a figure already employed by 
Ritzema Bos (1901), the spread of the mycelium would resemble the 
spread of a flame started by dropping a match into dry grass. Even if 
the grass were reestablished at a distance of a meter or so behind the 
flame, the chances of the flame's striking back would be very remote. 
We may think of the flame as representing the active mycelium and the 
grass as representing the organic matter of the soil on which the mycelium 
feeds. 
The fairy ring starts from the point of germination of the fungus spore 
and spreads outward at approximately an equal rate in all directions. 
In case of an obstacle, such as an ant hill, or another fairy ring, growth 
stops at this point. In the case of an ant hill the ring will close in around 
it as soon as it is passed and tend to regain its original complete form. 
In case two rings meet they do not continue, but are exterminated at the 
line of contact. As the fungus filaments spread outward they consume 
a portion of the organic matter of the soil. Carbohydrates are con¬ 
sumed, and the proteid portion is consumed or changed into amino acids 
and then to ammonia. The fungus advances and never recedes, since 
the young growing filaments are always advancing into new soil and 
since only the old filaments lie on the inside of the ring where the avail¬ 
able portion of the organic matter of the soil has already been con¬ 
sumed by the fungus. 
The effect of the fungus filaments on the soil is to reduce a part of the 
organic matter to ammonia, which is combined to form ammoniacal salts 
or is converted by bacteria into nitrites and later into nitrates. When 
the mycelium dies, it is reduced by bacterial action to ammonia, which 
may later be built up into nitrates. The increase in available nitro¬ 
genous material in the soil occupied by the young mycelium stimulates 
the growth of the grasses or other plants, which consequently make 
greater demands on the soil moisture. When this is once exhausted 
(in Agaricus tabularis ), the mass of fungus filaments prevents the pene¬ 
tration of rain water. The intense drouth to which plants are thus 
subjected kills off the buffalo and grama grasses and the other plants 
which may be associated with them. The area is thus left bare for the 
