170 PARASITIC FUNGI 



cells 5 to 8 fi in diameter, lives in and feeds upon the 

 layers of living skin cells. These living skin cells are 

 not on the actual skin surface, which consists of dead 

 cells constantly produced by division of the living 

 cells below the surface and rubbed off. The growth of 

 the fungus often causes local inflammation, and the 

 raising of the skin in circular pustules corresponding 

 with the centres of infection. The mycelium grows 

 outward from these centres, the central parts dying 

 off and thus producing the ring-shaped structures 

 characteristic of the disease. 



Ring-shaped structures formed in this way are pro- 

 duced by many fungi, both parasites and saprophytes, 

 for instance the " fairy rings " often seen in meadows, 

 and also sometimes by the higher plants, for instance 

 grasses, which similarly spread outwards from a 

 centre by vegetative growth, the central parts dying 

 off. The cause of the dying off of the older central 

 parts of the plant may be either the exhaustion of the 

 food supply, or the production of substances which 

 act as a poison to the plant itself, in the same way 

 that the accumulation of alcohol and carbon dioxide 

 in fermenting wort eventually stops the growth of the 

 yeast plant. 



Another form of ringworm is caused by Microsporon, 

 which attacks the living bases of the hairs of the skin, 

 in which it spreads, forming a mycelium of rectangular 

 cells, and causing the hair to break off short. The 

 mycelium then breaks out to the surface of the hair, 

 where it forms abundant spores (2 fj, in diameter) by 

 thickening of its cell walls. These spores are easily 

 brushed off and render the disease extremely con- 

 tagious. A number of allied species live on dogs, 

 cats, horses, etc., and these are contagious to 



