350 OHIO EXPERIMENT STATION: BULLETIN 214 
formed is properly protected by dressing, opens the way for spores 
of these fungi which cause timber decay to obtaina start and thus 
eventually to invade the heartwood of the interior. For dressing cut. 
off branches, asphaltum is admirable; in its lack gas tar is good, and 
either is better than ordinary paints. 
Fig. 24. A wound parasite (Pleurotus ulmarius) on the trunk of a 
maple tree. (After Freeman, Minnesota Plant Diseases). 
There is always to be borne in mind that the protection of the 
woody cylinder of trees depends on its being covered by the living 
layer of sap wood. Every branch of considerable size connects 
directly with the extensive heart cylinder; we thus see that the 
wound fungi which attack the heart wood are the timber decays and 
their presence emphasizes the need for care in providing protection 
for all wounds, especially those caused by pruning. i 
Any decay becoming established inthe dead heartwood may 
extend for long distances through this dead wood and in the end so 
destroy it as to be ina position to invade the external or sapwood _ 
layer. 
