28 



the m^xelium spreads through the wood it brings about the chem- 

 ical changes spoken of, extracting substances from the walls. This 

 reduces the volume of the wood and causes the fissures in the cells. 

 But before long the shrinkage becomes so great that larger masses of 

 the wood suddenly break awa}^ from each other at many points 

 throughout the wood. Man}^ small fissures are thus formed, which 

 extend in every direction, both across the rings and within them in a 

 tangential direction. The fissures are very irregular. Sometimes 

 the}^ extend for a short distance within one ring, then cross over into 



another, and so on. They ap- 

 pear both in the spring and sum- 

 mer wood, and not infrequently 

 start in one ring and extend 

 radially through the summer 

 wood of that ring into the 

 spring wood of the next. Often 

 the breaks follow the lines of 

 the medullary rays, but just as 

 often they do not (PL X, fig. 1). 

 The process is evidentl}^ one of 

 drying, for the same result is 

 seen when wood dries, resulting 

 in the formation of fissures, the 

 so-called "checking" of wood. 

 If the fissures are near points 

 where mj^celium of the fungus 

 flourishes, the latter grows into 

 the spaces and fills them com- 

 pletely. Several fissures ma}^ 

 join, forming an irregular longer 

 one. In PL X, fig. 1, a sketch 

 is shown of the cross section of 

 several annual rings, showing- 

 how and where the breaks have 

 formed. As the wood dries 

 more and more the fissures widen and the mycelium keeps step with 

 them. In small fissures it is very evident that the fissure has formed 

 as a break and not by the solvent action of the mycelium. Fig. 2 

 shows such a fissure filled with mycelium. (The same figure is shown 

 at c, PL X, fig. 1.) A glance at the rows of wood cells will show how 

 they have been forced apart, breaking one row. The rows are inclined 

 toward one another, as one would expect them to be. The figure also 

 shows a medullary ray at the right, the walls of which have disappeared. 

 In the cells surrounding the break the mycelium flourishes, and here 

 and there some of the walls are destroyed, making a small hole. 



Fig. 2.— Cross section of Spruce wood showing masses 

 of mycelium of Polyporus pinicola (Swartz) Fr. 



