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- 
