SECT. m. POLYPOREI. 265 



ance, and a vast variety of characters ; besides, the sub- 

 stance itself varies in density and colour. The stem 

 also may be long or short, sometimes wanting alto- 

 gether, when the pileus or cap is attached to the surface 

 on which the fungus is growing. The growth of indi- 

 vidual fungi, whether Polyporei or Agarics, is centri- 

 fugal, that is, they spread from the centre of the pileus, 

 as in the Polyporus fraxineus, which involves every 

 stick and blade of grass it meets with as it increases 

 in diameter, and continues to increase for years, till it 

 is occasionally a yard across. 



The fructiferous surface in the higher fungi is essen- 

 tially turned away from the light, yet, although in 

 many of the lower Agarics it is uppermost and exposed, 

 such is the tendency to produce the fructification on 

 the lower side, especially in the Polyporei, that if the 

 position of the plant be reversed the hymenium or 

 fructiferous surface is gradually obliterated and a new 

 one is formed on the other side. 



The Polyporei abound in the tropical forests, but 

 species are found in all latitudes. The higher fungi are 

 more or less plentiful in forests everywhere, and every 

 genus of trees seems to have one or more species of 

 fungus peculiar to itself. The Boleti, a genus of the 

 Polyporei, which are thick fleshy fungi of various forms, 

 and for the most part brilliantly coloured, grow under 

 trees in the temperate zones, sometimes in conspicuous 

 circles. When a slice of the Boletus luridus, cyanes- 

 cens, or other species is exposed to the air, the white 

 fleshy part acquires a blue tint in consequence of the 

 action of ozone upon the acetate of aniline, which was 

 ascertained by Dr. Phipson to be a constituent of 

 these fungi. According to M. Dutrochet more heat is 

 evolved by the Boletus seneus than by any other vege- 

 table except the Arum. 



The Polyporei destroy decaying trees and timber, 



