42 DISEASES OF DECIDUOUS FOREST TREES. 



phores are almost perfectly white when they first form, but gradually 

 turn darker, and in old age are light brown. The upper surface is 

 very soft and hirsute, and near the point of insertion is much cracked, 

 having the appearance of weathered corn pith. One of the most 

 striking features of the sporophore is the manner in which the rough, 

 hairy upper surface extends over the rounded edge on to the lower 

 side of the sporophore for one-eighth to one-fourth inch or more ; the 

 lower surface is the same color as the top. The pores are very irregu- 

 lar in shape, with uneven and jagged edges, giving the whole lower 

 surface a somewhat spiny appearance. The sporophore never attains 

 any great age, because it is attacked with great avidity by various 

 insects which destroy it rapidly. 



HEART-ROT CAUSED BY FOMES NIGRICANS. 



A brown heart-rot of deciduous trees, especially of the yellow birch 

 (Betula lutea Michx. f.), willow (Salix sp.), and aspen (Populus tremu- 

 loides Michx.), is due to Fomes nigricans Fr. This fungus is found 

 frequently in the Northern States from Maine to Oregon and occurs 

 most often on the yellow birch. Its method of attacking trees, its 

 rate of development, and its spread from tree to tree are very much the 

 same as those described for Fomes igniarius. 



The fungus is a parasite in the sense that it attacks the heartwood 

 of the tree, gaining entrance through some wound. The decayed 

 heartwood is reddish brown in color, very soft, and has very much the 

 uppearance of wood destroyed by Fomes igniarius. Lindroth (48) 

 in a recent description of the changes which this fungus brings about 

 in birch wood notes a number of distinct regions of destruction. In 

 the trees examined by the writers they have not been able to differ- 

 entiate such distinct zones. Plate VI, figure 1, shows a cross section 

 of a thrifty birch tree, in which it will be noted that the destruction 

 is more or less uniform, beginning at the center of the trunk and ex- 

 tending outward almost to the bark. The decayed wood is spongy 

 and will not powder when rubbed between the fingers. 



The most striking feature in connection with this type of heart-rot 

 is the extent to which the decay involves the whole trunk. It starts 

 at the center and progresses outward, gradually involves the sapwood, 

 and ultimately reaches the bark, killing the tree. A similar progress 

 of decay involving the sapwood has already been noted for Fomes 

 igniarius. This type of disease is very striking in such trees as the 

 paper birch (Betula papyrifera Marsh.). The minute changes which 

 take place in wood destroyed by Fomes nigricans are of less interest 

 in this connection; they have been fully studied and described by 

 Lindroth in the article already mentioned (48). 



149 



