DECAY OF STRUCTURAL TIMBER. 65 



The explanation for this form of sap decay is briefly as follows: 

 When the stick of wood (fig. 11) was cut it was perfectly green and 

 full of water. It was piled so that it was exposed on all sides to the 

 wind and sun. As a result of this exposure the outer layers of the wood 

 fiber dried very rapidly, and the amount of water left in the outer 

 layers w T as insufficient for the development of the wood-destroying 

 fungus. As a result of the drying, small season checks formed on 

 the outside, one of which will be noticed in the middle at the top of 

 the photograph. The season check penetrated into the interior for 

 an inch or two ; then some of the spores of the fungus lodged in it and 

 there was still sufficient moisture to enable the spore to germinate 

 and grow. The fungus then rapidly spread through the wet sapwood 

 near the bottom of the season check, bringing about the decay shown 

 in the figure. This decay would have gone on until all of the sapwood 

 was destroyed, and fruiting bodies would then have formed on the 

 outside in one of the season checks. 



A very familiar instance of this kind of internal decay can be found 

 in any forest where the stumps of trees are examined a year or more 

 after the trees have been felled. The cut surface of the stump will 

 appear season checked, but otherwise sound. When bored into, 

 however, the wood within about a half inch of the upper surface of 

 the stump will usually be found completely decayed. The realiza- 

 tion that sap-rot in timber is due almost wholly to the action of fungi 

 the spores of which enter through season checks and develop in the 

 sapwood when it is not allowed to dry out is one of the most impor- 

 tant practical considerations in connection with the entire study of 

 the diseases of woods. 



SUSCEPTIBILITY TO DECAY. 



The same species of wood will show differing tendencies toward 

 decay, according to the localities in which the trees are grown. This 

 is well illustrated by the difference in lasting power which is found 

 in so-called "hill" and " bottom" red oak (Quercus rubra~L.). Hill 

 red oak grows very much more slowly than bottom red oak, forming 

 a denser grained, heavier wood, which is less permeable by water. In 

 figure 10 the results of an extensive experiment comparing hill and 

 bottom red oak are shown. This figure shows the results of exposing 

 some 2,400 pieces of hill and bottom red oak. One hundred pieces 

 were cut every month, beginning in January, 1903, and continuing 

 until December, 1904. Two tracts of land were selected one on 

 which the hill variety grew and the other some 50 miles away, on 

 which a dense bottom red-oak stand was growing. All of the tim- 

 bers were piled in a similar manner and were left for observation. 



149 



