126 HEART-WOOD 



latter are extracted and afford useful dyes, e.g. the logwood 

 (haematoxylin) obtained from Hczmatoxylon campechianum (Tropi- 

 cal America). The employment of mahogany, walnut, etc., in 

 cabinet-work is largely due to the rich colouration of the heart- 

 wood and the high polish which its hard character enables it to 

 take. In the Ebony-tree (Diospyros) the living sap-wood is white 

 and even soft, the ebony of commerce being the mature, very 

 hard, and jet-black heart-wood. In some cases (e.g. Beech) little 

 heart-wood is formed, most of the xylem remaining functional. 



The impregnating substances are often antiseptic, and prevent 

 decay by inhibiting the development of Fungi and Bacteria, thus 

 increasing the durability of the wood. Teak (Tectona grandis) 

 owes its great value as a tropical timber to the presence of an 

 oil which renders it immune from the depredations of wood-boring 

 insects ; it is also the cause of its peculiar scent. The liability 

 of many Willows to develop hollow trunks at an early stage may 

 be attributed to the absence of antiseptic substances from the 

 heart-wood. 



The cavities of the water-conducting elements in the heart- 

 wood are frequently blocked in various ways, most commonly 

 by the ingrowth of structures known as tyloses (Fig. 61). These 

 are bladder-like intrusions through the pits, from the wood- 

 parenchyma cells, into the vessels, and are sometimes so numerous 

 as to fill the latter completely with a false tissue resembling 

 parenchyma. They are bounded by the extended thin-walled 

 pit-membrane, which undergoes a certain amount of surface 

 growth, and occasionally becomes thickened and lignified (e.g. 

 False Acacia, Robinia pseudocode). Each of the young tyloses 

 is living, containing cytoplasm, cell-sap, and sometimes also a 

 nucleus; but when they have reached their full size both the 

 tyloses and the wood-parenchyma cells * of the heart-wood die, 

 so that the whole of the latter consists of dead elements. Tyloses 

 are also produced in herbaceous stems (e.g. Vegetable Marrow, 

 Fig. 61), but here their function is obscure. The plugging of 

 the vessels of the heart -wood is, however, not always effected 

 in this way, since in other cases mineral deposits (lime in the 

 Elm) may take their place. 



1 The latter are invariably dead in the mature heart-wood, even when 

 no tyloses are formed. 



