CAUSES OF DISEASE. 13 



Smaller wounds in pines are, as a rule, soon covered 

 over by pitch, which very effectively prevents the dry- 

 ing out of the exposed sapwood and the germination 

 and growth of fungus spores. The less resinous coni- 

 fers and our broadleaf trees lack this protection, and 

 even the pines are unable to cover very large wounds 

 completely. The sapwood dies, dries out, and checks. 

 Spores (p. 28) of parasitic fungi enter the cracks, ger- 

 minate, and infect the heartwood. Proper moisture 

 and temperature are the necessary conditions for ger- 

 mination of the spores; but infection can only take 

 place if the spores land where the mycelium resulting 

 from germination finds the proper food for further 

 development. The spore of a heartwood-inhabiting 

 fungus, such as Polyporus amarus, for instance, must 

 be carried on to an incense cedar, since the fungus can 

 not attack any other tree. If it lands on the bark it 

 may germinate, but the resulting mycelium is unable 

 to penetrate into the heartwood. Only when a spore is 

 deposited on a broken-off branch stub, which allows the 

 mycelium to grow through it into the heartwood of 

 the bole, or when a spore falls onto an open wound 

 large and deep enough to offer an entrance into the 

 heartwood either directly or through cracks, can infec- 

 tion take place. Thus certain diseases of the heartwood 

 (rot or decay) can very frequently be traced directly 

 to fire scars, lightning scars, spike tops, stubs of 

 broken limbs and branches, and even to the crotches of 

 forked trees, where the swaying of the two forks in the 

 wind mechanically destroys the tissues. Moisture nat- 



