A MUSHROOM 83 



by the threads, and a small group of fruiting bodies is 

 formed. As the available food in this part of the soil is 

 used up, the older threads die and the younger branches 

 push out into the surrounding soil, where they produce a 

 new crop of fruiting bodies in a ring about the place where 

 the first ones grew. The growth outward from the original 

 center continues from year to year, and each successive ring 

 of fruiting bodies is a little wider than the preceding ring. 



111. Relationships of Mushrooms. The mushrooms, with some 

 related forms, are classed as basidium fungi. The rusts are also often 

 classed among the basidium fungi, because their winter spores, together 

 with the small, few-celled plants to which the winter spores give rise, 

 are thought to be comparable with the basidia of mushrooms and with 

 the projections growing from the basidia that bear the spores. If 

 this notion is correct, the sporidia of a rust would correspond to the 

 spores of a mushroom. Aside from the rusts (and the smuts, which 

 are probably related to the rusts), there are about 12,000 known species 

 of basidium fungi. 



FIG. 40. Specimens of a bracket fungus (Fomes applanatus). At the left 

 is shown one with its under surface exposed. Photograph .by E. T. Harper. 



