454 



FUNGOUS DISEASES OF PLANTS 



distribution of this fungus, however, is doubtless much more ex- 

 tensive, although the indications are that it is uncommon in regions 

 which are rather dry throughout the summer. 



The sporophores of this fungus have been reported upon various 

 species of maple (Acer), oak (Quercus), elm (Ulmus), basswood 

 (Tilia), willow (Salix), ash (Fraxinus), etc.; therefore it may be ex- 

 pected upon practically any of the deciduous trees. There seems 

 to be no record of its occurrence upon conifers. The tree attacked 



FIG. 223. POLYPORUS SQUAMOSUS, LOWER SURFACE. (After Buller) 



by this fungus dies gradually, and the effect may in general be 

 called a white rot, since there is no marked discoloration, and the 

 presence of the mycelium is to lighten rather than darken the 

 effect. The hyphae probably obtain entrance through wounds, as is 

 the case with most other related fungi. The mycelium then grows 

 upward and downward, first in the central portion of the tree, 

 apparently having little power to affect directly the living portions. 

 It gradually works and spreads outward, killing the young wood 

 doubtless prior to invasion, and finally breaking through the sur- 

 face and producing sporophores after a period of years. 



