THE BLACK ZONES FORMED BY WOOD- 

 DESTROYING FUNGI. 



Arthur S. Rhoads. 



IXTRODUCTIOX. 



Various phases of pathological study have occupied the 

 attentiou of botanists at different times. Before the causes 

 of diseases in plants where known the general external appear- 

 ance of the affected organ was described. Later most of the 

 attention was directed toward the discovery of the parasitic 

 organisms which cause the derangements, and incidentally 

 the study of the physiologic responses of the host plant w^as 

 begnm. Within the last few years many students of the 

 subject have examined various morphological changes Avhich 

 occur in diseased plants, first dealing almost entirely with 

 the gross anatomic appearance, but later making more minute 

 histological and cytological investigations. Woody tissue 

 when invaded by fungi, however, is just beginning to be 

 studied carefully. ^Moreover, comparative studies are always 

 helpful in deciding general principles. Only as we become 

 acquainted with many examples of cytological and histo- 

 logical changes can we arrive at the truth regarding the reac- 

 tion of the host plant to parasitic invasion. The j^i'^ctical 

 value of such results can hardly be doubted. 



To combat a disease successfully we should know as much 

 as possible about the causes. We should therefore under- 

 stand the pathologic reactions of the diseased plant. The 

 relations between parasitic fungi and their host plants are of 

 various kinds. Here we largely confine our attention to the 

 effects of the fungus upon its host and, among these, we are 



