1^8 SHARP EYES 



will suppose a single spore to have been blown by the 

 wind to the spot upon the lawn. A single mushroom 

 or group is soon seen. Its spores are scattered beneath 

 it. The earth immediately around the stem has been 

 exhausted of the chemical necessities for new growth, 

 and only such spores as have fallen in the outer edge 

 of the circle will find the congenial conditions for vege- 

 tation, leaving the centre bare of growth. And thus 

 the ring enlarges as the interior soil is exhausted, until 

 it occasionally reaches the diameter of several feet in a 

 more or less broken circle. 



