HEART-WOOD AND SPLINT-WOOD 127 



verse sections are scarcely visible to the naked eye, while in 

 oak, beech and other kinds of timber, many of them are several 

 cells thick, and in transverse sections appear as distinct light- 

 coloured radial bands (in, Fig. 64). In true radial longitudinal 

 sections, when seen at all, they appear as transverse bands of 

 variable vertical diameter running from the pith outwards 

 (Fig. 62), the primary rays have the greatest vertical breadth. 

 In longitudinal sections cut obliquely to the radius of the 

 stem small portions only are visible as bran-like spots. 



The cells of the medullary rays are brick-shaped, generally with 

 thick pitted walls and living contents, which they often retain for 

 a long time. They conduct various food-products manufactured 

 in the leaves, and in winter starch and various food-substances 

 are stored in them for use in the following season. Air circulates 

 to all parts of the wood and bast in the intercellular spaces be- 

 tween the medullary ray cells. 



{d) Heart-wood and splint- wood. — In the old stems of oak, 

 walnut, larch, yew and other trees, the wood of the annual rings 

 in the centre of the tree is heavier, harder, darker in colour, and 

 drier than that of the younger rings near the cambium : this 

 dark wood is known as heart-wood or duramen, while the light- 

 coloured softer wood surrounding it is termed splint-wood, sap- 

 wood or alburnum. The width of the splint-wood or the number 

 of annual rings over which it extends is not the same in all trees, 

 nor is it always the same in the same species of the same age. 



The splint-wood is the part which conducts the ' sap ' and many 

 of its parenchymatous cells are still living : starch, sugar and 

 other compounds readily attacked by fungi are generally stored in 

 it, and from its liability to rot it is valueless as timber. 



The heart-wood acts as a strong support for the rest of the 

 tree : its vessels no longer conduct water and the parenchyma 

 of the wood and medullary rays have lost their living contents. 

 Various gummy and resinous compounds block up the cell- 

 cavities and in some cases calcium carbonate is present in 



