20 OF WOOD IN GENERAL. 



and constitute what is known as the heart-wood, the cells of which 

 are physiologically dead and serve only a mechanical function, 

 that of supporting the weight of the tree and resisting the lateral 

 strain of the wind. The darker colour of this heart-wood is due 

 to infiltration of chemical substances into the cell walls, but not, 

 in pine, as is sometimes supposed, to any greater thickening, 

 lignification, or filling up of the cells than there is in the sap- 



cs 



l\ 



f/iL 



// 



/////] 



rs J : ' 



i/\ 



''/ : \nt \ \ Vlr 



//Mil 



\ \ i 



5 W 



FIG. 11. Coniferous wood, about natural size. 7', tangential section ; 

 JiS, radial section; CS, cross section; SI'H', spring wood; SH', summer 

 wood. (After Roth.) 



wood. The proportion of sap-wood to heart-wood is always 

 considerable, but it varies in width even in different parts of the 

 same tree, the same year's growth being sometimes sap-wood in 

 one part and heart-wood in another. The width of the annual 

 rings varies from half-an-inch or more near the centre of very 

 quick-grown trees to one-eighth or one-sixth of an inch (3-4 mm.), 

 common widths for the twenty innermost rings in deal, one- 

 twelfth of an inch, a general average width, one-thirtieth 



