PROPERTIES OF WOOD. 



43 



The diameter of the whole cell becomes less, and the opening, or 

 lumen, of the cell becomes larger. 



(2) Thick-walled cells shrink more than thin-walled cells, that 

 is, summer cells more than spring cells. This is due to the fact that 

 they contain more shrinkable substance. The thicker the wall, the 

 more the shrinkage. 



Consider the effects of these 

 changes; ordinarily a log 

 when drying begins to "check'' 

 at the end. This is to be ex- 



plained thus: Inasmuch as 

 evaporation takes place faster 

 from a cross than from a lon- 

 gitudinal section, because at 



the cross-section all the cells are cut open, it is to be expected that 

 the end of a piece of timber, Fig. 36, A, will shrink first. This 

 would tend to make the end fibers bend toward the center of the 

 piece as in B, Fig. 36. But the fibers are stiff and resist this bend- 

 ing with the result that the end splits or "checks" as in C, Fig. 36. 

 But later, as the rest of the timber dries out and shrinks, it becomes 

 of equal thickness again and the "checks" tend to close. 



(3) For some reason, which has not been discovered, the cells or 

 fibers of wood do not shrink in length to any appreciable extent. 



Fig-. 35. How Cell Walls Shrink. 



Fig-. 36. The Shrinkage and Checking- at the End of a Beam. 



This is as true of the cells of pith rays, which run radially in the 

 log, as of the ordinary cells, which run longitudinally in it. 



In addition to "checking" at the end, logs ordinarily show the 

 effect of shrinkage by splitting open radially, as in Fig. 37. This is 



