FAIRY-RING FUNGI. 



481 



the grass in the roots of which it grows, and so causing the brown 

 ring, on which an abundant crop of toad-stool fruits forms in 

 the fall, which by their decay enrich the soil so that it produces 

 a ranker vegetation the next season. 



An appearance which may be called false fairy-rings is occa- 



sionally produced by Physarum cinereum, one of the slime-molds, 



on the leaves and stems of grasses. This fungus grows unnoticed 



on decaying matter in the ground, often creeping out in a regular 



manner from its starting point until a more or less perfect circle 



,Bix or seven feet in diameter is formed, when it suddenly appears 



upon the plants it has grown under, and produces its dusty, ash- 



colored fruit in such abundance as to attract attention from a 



distance. From its mode of life, it does little if any harm to the 



grass, further than to make a little of it unpalatable to animals. 



16. Root-gall fungi of clover, 

 etc.— Galls which vary in size and 

 shape, according to the species ex- 

 amined, are always found in greater 

 or less number on the roots of norm- 

 ally grown leguminosae. They are 

 caused by one or more microscopic 

 fuugijsometimes referred to the genus 

 lihizoUum, widely distributed in the 

 soil where leguminous plan!.? are 

 grown. The fungus penetrates the 

 tender rootlets,especially through the 

 root hairs, giving rise to the develop- 

 ment of galls, but its presence, un- 

 like that of most gall-producing 

 species, appears to be beneficial to the 

 plants attacked, since it seems now 

 fairly demonstrated that the assimilation of atmospheric nitrogen, 

 which has long been attributed to leguminous crops, is effected by 

 them through the agency of these fungi. 



FiQ. 175. 



