Differentiation 229 



absorbs food from the organic matter in the soil. When it has a good 

 supply of this, and other conditions are also favorable, the mycelial mate- 

 rial is mobilized into a rounded mass just below the surface of the ground. 

 This develops into a "button" and then into the familiar mushroom fructi- 

 fication, with its high degree of differentiation. This is composed not of 

 a mass of cells that are attached in a firm tissue, as in the higher plants, 

 but of a body of tangled hyphae, free to slide past each other. As the 

 stalk increases in length, these hyphae tend to be oriented parallel to 

 its axis, though at the base and in the pileus ("umbrella") they remain 

 much tangled. Growth takes place primarily by elongation of the cells of 

 the hyphae and is entirely at the expense of food already available in the 

 mycelium. How, from such an apparent chaos of snarled threads, the 

 very precisely formed fruiting body of the fungus grows and differen- 

 tiates poses the same difficult problem as does the development of the 

 sorocarp in the Acrasiaceae, and one lying at the heart of the morpho- 

 genetic process. Bonner, Kane, and Levey (1956) have reexamined the 

 development of Agaricus and confirmed and extended the results of the 

 classic studies of de Bary, Atkinson, and Magnus, but the problem has 

 attracted relatively little attention in recent years. The development of 

 these fungus fruiting bodies, however, offers to the student of the prob- 

 lems of differentiation and form determination some of the most promising 

 material available for his work. 



