34 CoUefje of Forestry 



than the heartwood, a coiKlition often resulting in hollow- 

 trees. This condition generally occnrs from the absence of 

 oxygen in the sajiwood in conseqnenee of its large amount of 

 water. As soon as the moisture ratio between the heartwood 

 and the sapwood is upset, as in dead or felled wood, the sap- 

 wood which (owing to its greater abundance of food ma- 

 terials) is by far the more favorable medium for the growth 

 of fungi and is decomposed much more quickly than the 

 heartwood.'^ Von Schrenk (1000, ]). 48) has observed the 

 readiness with which the sporophores of many wood-destroy- 

 ing fungi can be induced to form wherever a wound is made 

 in the diseased trunk, thus admitting air. As we now knoAv, 

 the decomposition of woody tissue occurs through the com- 

 bined action of fungal hyjihae (by virtue of their enzyme 

 excretions), water, oxygen, and a favorable temperature. The 

 entrance of air alone does not destroy the woody tissue and in 

 the absence of sufficient air and moisture the fungal threads, 

 which decompose the wood, cannot function. 



Decomposition very seldom occurs from small surface 

 wounds in trees or in the cut ends of small branches because 

 here the injured woody portions are sufficiently protected by 

 the absence of air in consequence of the large water content 

 of the sapwood. The conditions are otherwise where the 

 deeper-hang tissues of larger limbs or the trunk are exposed, 

 since these are better aerated. In the latter case fungal 

 hyphae penetrate, following the vessels and pith rays, and 

 ramify in the center of the stem. The result is that tyloses 

 are formed in the vicinity of the wounded areas where the 

 wood is still living. Later, after the death of the parts at- 

 tacked, the browning of the contents of the parenchyma cells 

 (formation of decomposition products) occurs. As has been 

 shown, both of these formations are to be seen most frequently 

 in the regions immediately adjoining the infected areas, 



"Compare the investigations of Tuzson (1903, p. IG) and those of 

 Miineh (1010-) whieli agree in the essential points. 



