THE FUNGI WHICH CAUSE PLANT DISEASE 429 



infection. The mycelium grows mainly in the heart wood but it 

 may gain entrance through the sap wood or encroach upon the 

 sap wood from the heart wood. Its growth may continue after 

 the death of the host. In early stages it follows the medullary 



Fig. 



308. — A dead beech tree with sporophores of F. fomenta- 

 rius. After von Schrenk and SpaiUding. 



rays. The completely rotted wood is white to light yellow and in 

 it the mycelium abounds in the large vessels and the medullary 

 rays. The walls of the afTected wood cells are thin and the middle 

 lamella is often wholly lacking, due to solution of the lignin. 



F. fomentarius (L.) Fr."' " 



Pileus hard, woody, ungulate, concave below, 7-9 x 8-10 x 3-10 

 cm. ; surface finely tomentose to glabrous, isabelline to avellaneous 



