HYMENOMYCETES 



371 



central-stemmed species of Polyporus are not uncommon 

 in tropical countries, but are rare in Europe. A few 

 species of Polyporus spring from sclerotia; in one Australian 

 form, Polyporus mylittae, the sclerotium is sometimes as 



FIG. 114. Poly fonts betulinus, a parasite on birches, i, sporophore 

 with incurved margin, and showing the hymenium on its lower surface ; 

 2, section of thick, fleshy sporophore with incurved margin, and very short 

 tubes on its under surface. Reduced. 



large as a child's head, and is as hard and dense as the 

 hardest wood when dry. Central-stemmed forms of this 

 family are best represented in the northern hemisphere by 

 the genus Boletus. The species are usually large, grow on 



