74 MANUAL OF FRUIT DISEASES 



most black line the wood is hard and firm and thus differs 

 from the normal wood only in color. The brown discoloration 

 is caused by decomposition products which diffuse into the 

 bordering healthy tissue. 



Cause of white heart-rot. 



The causal pathogene, Fomes igniarius, is one of the pore- 

 bearing basidiomycetous fungi. It has been known for about 

 two centuries and was formerly used as tinder or touch-wood, or 

 beaten into soft square pieces to be used by surgeons for stop- 

 ping bleeding arteries. It is sometimes called the false tinder- 

 fungus, the true tinder-fungus being a near relative (Polyporus 

 sulphureus). Its life-history is similar to that of other 

 basidiomycetes of this type. The spores, produced on the 

 basidia lining the pores, are matured and disseminated during 

 the early part of the summer. Some of these lodge in wounds 

 where they germinate and, setting up a food relationship with 

 the host, initiate the rot. The most common point of entrance 

 is a knot-hole, or a stub exposed by careless pruning operations. 

 From the germtube, mycelium is developed which grows into 

 the heart-wood of the tree, passing up and down and obtaining 

 the food necessary for its further growth. The rate of spread 

 of the mycelium is dependent on many factors, as, for ex- 

 ample, the breadth of the annular rings and environmental con- 

 ditions. The rate is more constant in the horizontal than in the 

 vertical growth. The mycelium develops abundantly in the 

 wood-parenchyma, medullary-ray cells, and sometimes in the 

 interior of the sap-tubes. It passes as a brown fungal mass from 

 the sapwood into the bark. From here it presses outward and 

 upon reaching a wound or bark fissure begins to form its sporo- 

 phores. These originate by the massing of mycelial threads 

 into a more or less definite form. On the lower surface of this 

 mass a layer of tubes is formed ; in each tube basidia and spores 

 are developed. The spores break away, fall through the tube 

 and out, are caught by the wind and carried to new infection- 



