6 THE DECAY OF TIES IN STORAGE 



in the wood, the rate of decay may vary considerably with different 

 organisms, depending on the amount of moisture present. For in- 

 stance, there are many fungi which thrive only under humid forest 

 conditions and will not develop on partially seasoned timber in storage, 

 while others can tolerate a considerable amount of drying and still 

 continue to decay the wood. It is safe to say that most of the fungi 

 which rot structural timbers are quite resistant to drying when once 

 established in the wood. 



The term "dry rot" should not be used indiscriminately, as is 

 rapidly becoming the case among laymen. While the writer thinks 

 the designation should be retained, it should be limited in its scope 

 so as to apply only to the decay produced by Merulius lachrymans and 

 its close relatives, which are mainly of economic importance in con- 

 nection with the decay of timbers in buildings. Even in this case the 

 term is more or less of a misnomer, as these organisms always start 

 under moist conditions. They have the capacity, however, of develop- 

 ing porous water-conducting strands which may spread from the 

 moister portions of a building to dry timbers, whose decay is furthered 

 by the moisture which the fungus carries. 



Air. A certain amount of air in the wood is absolutely necessary 

 for decay to take place. The fungi require it in their growth. When 

 wood is saturated the air in the wood cells is displaced by water and 

 fungus growth is impossible. 



The opinion widely prevalent among laymen that alternate wetting 

 and drying is necessary for decay has developed through observation 

 of the way wood decays in exposed situations. For instance, take the 

 case of an infected railway tie partly embedded in soil. During a 

 very dry season there may not be sufficient moisture in the wood 

 to permit decay, and the fungus will remain dormant. On the advent 

 of rains, if only sufficient moisture falls to put the tie in good moisture 

 condition, it begins to rot rapidly again and will continue to do so as 

 long as the moisture and temperature are favorable. If, on the other 

 hand, the rainy period is long continued, the tie may become saturated 

 and decay will be retarded again until the stick dries out sufficiently 

 to admit the necessary amount of air. Thus, in the alternation of 

 wet and dry conditions, there occurs at some point intermediate between 

 the dry and wet ranges a condition at which decay is at its maximum. 

 If the moisture were held at this optimum point it can readily be seen 

 that the stick would decay much more quickly than under the alternat- 

 ing conditions. Therefore it is only under fluctuating climatic condi- 

 tions that alternation becomes of advantage. 



Temperature. In general, the fungi under consideration grow best 

 between 75 degrees and 90 degrees F. They will all grow at much 

 lower temperatures, but much more slowly. The most severe winter 

 conditions do not kill them. They merely cease growth and remain in 



