380 



DISEASES OF CULTIVATED PLANTS 



When the object is to grow healthy trees for whatever pur- 

 pose, all diseased trees should be cut down and burned at 

 once, as when the mycelium has once gained an entrance it 

 extends in the trunk from year to year. 



Polyporus dryadeus (Fries.). This is our largest species of 

 Polyporus, and is not infrequently met with on oak trunks ; 

 the largest specimens usually occur near the ground-line, but 

 it also springs from points where branches have died or been 



'""" ' --'-i;'- i 



FIG. 117. Polyporus betulimis. Fungus and section of same, reduced. 



broken off. According to Hartig, during the decomposition 

 of the wood by the mycelium, oblong yellowish or whitish 

 blotches appear in the otherwise sound heart-wood. The 

 cell-walls of these white blotches have been converted into 

 cellulose, and have become isolated by the solution of the 

 middle lamella. These patches are gradually dissolved, and 

 holes surrounded by hard wood are formed. 



Sporophore bracket-shaped or hoof-shaped, 7-12 in. across, 

 attached by a broad base, becoming corky in substance, 



