FUNGI. 27 



can be no heartrot in trees too young to form heart- 

 wood; but even after the differentiation of the wood 

 into heartwood and sapwood has begun, the heartwood 

 is for a time more or less immune from attack. After 

 a number of years, however, the heartwood has changed 

 so much in character that it becomes proper food for 

 heartwood-destroying fungi. The relative extent of 

 decay by Echinodontium tinctorum is far greater in 

 slow-growing, suppressed white firs than in thrifty ones, 

 in part, perhaps, because the heartwood of thrifty- 

 growing trees differs in character from that of sup- 

 pressed trees. 



The decay is due to the action of the fungus hyphae 

 on the cell-walls of the wood. They use certain com- 

 ponents of the cell-walls for food and leave the rest, 

 so that the structure of the cell-w T alls is broken down. 

 Wood recently invaded by the hyphae does not yet show 

 actual decay ; although it is already under the influence 

 of the " advance guard " of the fungus, it appears sound 

 or only slightly changed in color or consistency. It is 

 only after prolonged action of the mycelium that the 

 wood structure actually breaks down. Thus, the wood 

 separating the dry-rot pockets in incense cedar caused 

 by Poly poms amarus (p. 46) looks perfectly sound, 

 although it must, of course, contain hyphae which con- 

 nect the mycelium of one pocket with another; it has 

 lost nothing of its strength, while the wood in the 

 pockets is completely destroyed. In the development 

 of the mycelium of the Indian paint fungus in white 

 fir the hyphae advance steadily, not in leaps, as that 

 of Polyporiis amarus in incense cedar. The part of the 



