137 
Dr. Wollaston's Essay on Fatty-Rings. 
mushroom {Ag. campestris) ; the narrowest are the most fre- 
quent, and are those of the champignon ( Ag. orcades of Dr. 
Withering). The mushroom accordingly makes circles of 
largest diameter, but those of the champignon are most regu- 
lar. There are, however, as many as three other fungi that 
exhibit the same mode of extension, and produce the same 
effect upon the herbage. These are the Ag. terreus , Ag. pro- 
cerus, and the Lycoperdon bovista, the last of which is far more 
common than the two last mentioned agarics. 
There is one circumstance that may frequently be observed 
respecting these circles, which can satisfactorily be accounted 
for, according to the preceding hypothesis of the cause of 
their increase, and may be considered as a confirmation of its 
truth. Whenever two adjacent circles are found to interfere, 
they not only do not cross each other, but both circles are 
invariably obliterated between the points of contact : at least 
in more than twenty cases, I have seen no one instance to the 
contrary. The exhaustion occasioned by each obstructs the 
progress of the other, and both are starved. 
I think it also not unworthy of observation, that different 
species of fungi appear to require the same nutriment; for 
in a case of interference between one circle of puff-balls and 
another of mushrooms, they did not intersect ; but I cannot 
say positively that I have seen more than one instance. 
I once found that a tree had interrupted the regular pro- 
gress of a circle ; but this appeared to be only a temporary 
impediment, as the extension had proceeded at the usual rate, 
and by passing obliquely from each side into the soil beyond 
the tree, had given the ring the form of a kidney, so that 
