44 



WOOD AND FOREST. 



to be explained by two factors, (1) the disposition of the pith (or 

 medullary) rays, and (2) the arrangement of the wood in annual rings. 



(1) The cells of the pith rays, as we have seen in Chapter I, run 

 at right angles to the direction of the mass of wood fibers, and since 

 they shrink according to the same laws that other cells do, viz., by 

 the cell wall becoming thinner but not shorter, the strain of their 



shrinkage is contrary to that of 

 the main cells. The pith rays, 

 which consist of a number of 

 cells one above the other, 

 tend to shrink parallel to the 

 length of the wood, and what- 

 ever little longitudinal shrinkage 

 there is in a board is probably 

 due mostly to the shrinkage of 

 the pith rays. But because the 

 cells of pith rays do not appre- 

 ciably shrink in their length, this 

 fact tends to prevent the main 

 body of wood from shrinking 

 radially, and the result is that 

 wood shrinks less radially than 



tangentially. Tangentially is the only way left for it to shrink. The 

 pith rays may be compared to the ribs of a folding fan, which keep 

 the radius of unaltered length while permitting comparative freedom 

 for circumferential contraction. 



(2) It is evident that since summer wood shrinks more than 

 spring wood, this fact will interfere with the even shrinkage of the 

 log. Consider first the tangen- 

 tial shrinkage. If a section of a 



single annual ring of green wood 

 of the shape A B C D, in Fig. 

 38, is dried and the mass shrinks 

 according to the thickness of the 



Fig. 37. The Shrinkage and Splitting 

 of a Log. 



Fig. 38. Diagram to Show the Greater 



Shrinkage of Summer Cells, A, B, than 



of Spring Cells, C, D. 



cell walls, it will assume the 

 shape A' B' C' D'. When a num- 

 ber of rings together shrink, the tangential shrinkage of the summer 

 wood tends to contract the adjoining rings of spring wood more than 

 they would naturally shrink of themselves. Since there is more of 



