SAP-ROTS OF SPECIES OF DECIDUOUS TREES. 



59 



results iii their gradual destruction, ultimately causing the death of 

 the entire tree. The injury to the wood is only local, occurring gen- 

 erally near the base of the trunk. The fungus is said to enter the 

 trunk through injuries near the ground line or through wounds on the 

 roots, and " spreads upward through the entire wood, reaching, in 

 specimens observed, the height of 10 feet. The entire wood of the 

 lower portion of the trunk becomes thoroughly infected before the 

 fungus obtains sufficient vigor 

 to produce its external fruiting 

 bodies. In this condition the 

 wood is rendered very brittle, 

 and the tree thus affected is 

 poorly fitted to withstand the 

 force of the wind in severe 

 storms'' (38). 



The diseased wood is char- 

 acterized by the appearance of 

 numerous localized pockets, 

 separated from one another by 

 wood which the fungus has 

 evidently not been able to de- 

 stroy. Heald states that the 

 fungus is probably only a facul- 

 tative parasite and "is not 

 able to attack young, healthy 

 trees, but that it can become 

 parasitic on older trees in which 

 the vitality has been consid- 

 erably lowered, or that have 

 reached the maximum of their 

 development." 



The writers have repeatedly 

 observed this form of decay in 

 the cottonwood, but in their 

 experience it usually starts 

 near the base of the trunk in 

 large wounds caused by fire or 

 otherwise. On that account they are not inclined to call this decay 

 of the cottonwood a disease in the sense in which the decays induced 

 by Fomes igniarius, F. fraxinopJtilus, and others are diseases. There 

 are a large number of species of fungi which, like Fomes applanatus, 

 grow on dead wood and which may now and then grow on living 

 trees. All of these, including Fomes applanatus, can grow just as 

 well and apparently better on wood after it has been cut from liv- 



149 



Fig. 8. — A dead stub of a maple tree bearing fruiting 

 bodies of the sap-rot fungus (Fomes applanatus). 



