CONIFEROUS WOOD 



17 



annual rings and from one to three or more inclies across, is of 

 lighter colours and is known as the sap-wood or alburnum. Many 

 of its cells are still in a sufficiently active state of vitaHty to store 

 up starch, at least in winter, though growth is confined to the 

 outermost layer of all, the cambium. The inner rings are darter 

 and constitute the heart-wood or duramen, the cells of which are 

 physiologically dead and serve only the mechanical function, 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, 



'BS 



TTTT77 



Fig. IL—Comferous wood, about natural size. TS^ tangential section ; RS, radial 

 section ; CS, cross section; SPW, spnng wood j SW^ summer wood, (After Roth.) 



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

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

 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 

 <3[uick-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 

 (0-7 mm.), an average for the twenty outermost rings, and even 



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