Oct. 15. 1914 Heart-Rot of Oaks and Poplars 69 



tawny zones consist of vessels, cells of wood parenchyma, and other 

 elements of the wood in the cells of which a ferruginous amorphous 

 substance has been deposited. These cells are not as strongly attacked 

 by the fungus as are those of the light zones. The rotted wood easily 

 splits into concentric layers, the cleavage usually occurring along the 

 boundary between the white and dark zones. In a tangential view, 

 small, more or less isolated areas of delignified wood fibers may be seen. 

 These delignified fibers are most abundant in the older, rotted portion. 

 In the vicinity of the sporophores the typical cinnamon-brown areas 

 seen in the oak are also present. The rotted wood is soft, almost silky 

 to the touch, is very light in weight, and is easily broken into fragments 

 between the fingers. 



MICROSCOPIC CHARACTERS 



The vessels in the light-colored zone have very thin walls, owing to 

 the action of the fungus; the bordered pits are often eroded until only 

 large irregularly shaped holes are left and the middle lamellae of the 

 vessels and of the wood fibers in this region are dissolved. The wood 

 fibers and some of the adjacent cells are finally delignified and absorbed. 

 The delignification occurs most rapidly along the boundary lines between 

 the light-colored and dark-colored zones, along which the cleavage com- 

 monly occurs. The small amount of delignified fibers present and their 

 rather rapid absorption prevent the formation of the large areas of white 

 cellulose which are so common in the rot produced by this fungus in oak. 

 In the zone of cleavage cobwebby masses of white mycelium occur which 

 fill the vessels and the small cavities left by the absorption of the wood 

 fibers. The medullary rays are readily attacked by the solvents of this 

 fungus and usually have completely disappeared by the time the final 

 stage of the rot is reached. 



ENTRANCE OF THE ROT IN THE HOST 



Polyporus dryophilus, so far as known to the writers, gains entrance in 

 the wood of the host trees only through wounds in which the heartwood 

 is exposed. The most common point of entrance is a broken or dead 

 limb, although in the western and southwestern United States it also 

 frequently enters through fire scars and other basal wounds. 



In Arkansas and eastward, where the species of oaks differ from those 

 in the West and Southwest, the rot caused by this fungus is apparently 

 confined chiefly to the branches and upper portion of the trunk. This 

 may be due to the fact that often there are one or more large dead branches 

 in the crown of the tree, while there are very few on the lower part of the 

 trunk. The fungus has therefore little or no opportunity to enter the 

 bole of the tree below the crown. 



When the fungus enters the stub of a broken limb, it grows downward 

 through heartwood of the stub till it enters the trunk, when it spreads 



