434 THE FUNGI WHICH CAUSE PLANT DISEASE 



advance of the fungus, the nearest hj^phse of which may be several 

 milhmeters distant, and is replaced by a decomposition product. 

 The myceliimi advances through the medullary rays and spreads 

 through spring and summer bands, abstracting the lignin; the 

 middle lamella dissolves and the cells fall apart. Completely 

 rotted wood is straw-colored, very soft, non-resistant. The young 

 hyphae are very fine and require an immersion less for observation. 

 Clamp connections are frequent. The sporophore appears after 

 the destruction of the wood is considerably advanced. 



F. hartigii All. is very similar to, if not identical with, 

 F. igniarius. 



It produces a white rot of firs and spruces. The mycelium is 

 yellowish with numerous branches which may fill the cavities of 

 the bordered pits of the tracheids. The middle lamella is even- 

 tually dissolved, later the inner walls. 



F. robinisB (Murr.) S. & Sy. "• « 



A large fungus with dark rimose surface and tawny hymenium. 

 Pileus hard, woody, dimidiate, ungulate to applanate, 5-25 x 5-50 

 X 2-12 cm.; surface velvety, smooth, soon becoming very rimose 

 and roughened, fulvous to purplish-black, at length dull-black, 

 deeply and broadly concentrically sulcate; mar^ roimded, 

 velvety, fulvous; context hard, woody, concentrically banded, 

 1-3 cm. thick, fulvous; tubes stratose, 0.15-0.5 cm. long, 50 a 

 mm., fulvous, mouths subcircular, edges entire, equaling the 

 tubes in thickness: spores subglobose, smooth, thin-waJled, fer- 

 ruginous, copious, 4-5 ii; cystidia none. 



On black locust causing heart-rot, arising from wound infection 

 of living trees. The very hard wood becomes a soft, yellow to 

 brown mass, spongy when wet. The decay extends out in radical 

 lines from the center, along the large medullary rays, killing the 

 cambium and bark on reaching them. The lignin is first dissolved, 

 later the cellulose. 



The fimgus ceases growth on the death of its host. 



F. marmoratus Berk. (=F. fasciatus [Sw.] Cooke.) 



Pileus hard, woody, dimidiate, applanate to ungulate, convex 

 above, 7-10 x 8-15 x 2-6 cm.; surface finely tomentose, at length 

 glabrous, concentrically sulcate, at first mole-colored, changing 

 to umbrinous, and finally avellaneous with black fasciations; 



