42 WOOD AND FOREST. 



The lighter kinds have the most water in the sap-wood, thus 

 sycamore has more than hickory. 



Curiously enough, a tree contains about as much water in winter 

 as in summer. The water is held there, it is supposed, by capillary 

 attraction, since the cells are inactive, so that at all times the water 

 in wood keeps the cell walls distended. 



THE SHRINKAGE OF WOOD. 



When a tree is cut down, its water at once begins to evaporate. 

 This process is called "seasoning." 1 In drying, the free water within 

 the cells keeps the cell walls saturated; but when all the free water 

 has been removed, the cell walls begin to yield up their moisture. 

 Water will not flow out of wood unless it is forced out by heat, as 

 when green wood is put on a fire. Ordinarily it evaporates slowly. 



The water evaporates faster from some kinds of wood than from 

 other kinds, e. g., from white pine than from oak, from small pieces 

 than from large, and from end grain than from a longitudinal sec- 

 tion ; and it also evaporates faster in high than in low temperatures. 



Evaporation affects wood in three respects, weight, strength, and 

 size. The weight is reduced, the strength is increased, and shrinkage 

 takes place. The reduction in weight and increase in strength, im- 

 portant as they are, are of less importance than the shrinkage, which 

 often involves warping and other distortions. The water in wood 

 affects its size by keeping the cell walls distended. 



If all the cells of a piece of wood were the same size, and had 

 walls the same thickness, and all ran in the same direction, then the 

 shrinkage would be uniform. But, as we have seen, the structure of 

 wood is not homogeneous. Some cellular elements are large, some 

 small, some have thick walls, some thin walls, some run longitudinally 

 and some (the pith rays) run radially. The effects will be various 

 in differently shaped pieces of wood but they can easily be accounted 

 for if one bears in mind these three facts : ( 1 ) that the shrinkage is 

 in the cell wall, and therefore (2) that the thick-walled cells shrink 

 more than thin-walled cells and (3) that the cells do not shrink 

 much, if any, lengthwise. 



(1) The shrinkage of wood takes place in the walls of the cells 

 that compose it, that is, the cell walls become thinner, as indicated 

 by the dotted lines in Fig. 35, which is a cross-section of a single cell. 



*See Handwork in Wood, Chapter III. 



