230 
Journal of Agricultural Research 
Vol. XI, No, 5 
and the perennial plants such as Gutierrezia sarothrae and Artemisia 
frigida , all of which grow in the native sod. The fruiting bodies of the 
fungus occur in this stimulated zone. The soil in the area just inside 
zone 3 (fig. 3) is permeated with filaments of the fungus from a depth of 
about 8 cm. to a depth of something over 30 cm. During years of 
normal or subnormal rainfall the soil of this zone is dry, and since water 
does not penetrate this soil, this zone is marked by dead or dying plants, 
or by the absence of a plant cover of any kind. Inside this bare zone, 
where the mycelium has died and partly disappeared and where moisture 
can penetrate more freely, the vegetation which is now composed largely 
of ruderals (fig. 13), is as luxuriant as is permitted by the soil-moisture 
supply and the increase in available nitrogenous material. The vege¬ 
tation on the inside of the ring (zone 1) does not differ in composition and 
appearance from that outside (zone 5), although it has been destroyed 
and reestablished. 
Fig. 14.—A bisect of a ring of A garicus tabularis in a wheat field in 1915, a year of ample mois¬ 
ture supply. 
The effect of the fairy ring caused by A garicus tabularis on the native 
vegetation varies greatly with the moisture supply. The appearance of 
the ring is entirely different during a dry year (PI. 16) and during a wet 
year (PI. 19), and no description can be given which will apply at all 
times. The zones of greatest potential stimulation of plant growth due 
to available nitrogenous material may be brown and dry, owing to the 
lack of available soil moisture. The dead zone is only an expression of 
the extreme drouth produced by the combination of dry weather and 
unfavorable soil conditions. If rain is continuous and well distributed, 
the vegetation does not die, and a ring in which the bare zone has all 
but disappeared is the result. This condition was produced artificially 
in 1916 (PI. 23, B). But since such a combination of favorable condi¬ 
tions seldom occurs in eastern Colorado the bare zone is well marked 
during normal years and can be distinguished even during wet years. 
The bare zone originates in the outer stimulated zone. During summer 
or late fall, or during dry years, this is changed to a withered condition, 
and if drouth is prolonged, to a bare condition due to the death of the 
grasses. 
