MORPHOLOGY 



133 



activity throughout the whole of the wood, extending even to the 

 pith ; such wood is called splint wood : the Beech (Fagus sylvatica) 

 may be quoted as an example. In other trees which form heart-wood, 

 the living elements die after a certain time, so that only dead tissues 

 ure found at a certain distance from the cambium. Before the death 

 of the living cells, they usually produce certain substances, such as 

 tannin and gums, which permeate the cell walls of the surrounding 

 elements, and also partially close their cavities. The tannins impart 

 to the dead wood a distinct colour, often very characteristic, 

 especially when it has been transformed into wood dyes, or so-called 

 XYLOCHROME. The tannin in the woody walls acts as a preservative 

 against decay, while the gums close the functionless water-courses of 

 the dead wood. The dead portion of the wood of a stem is called 



the HEART-WOOD Or DURAMEN, in 



contrast to the living SAP-WOOD 

 or ALBURNUM. Usually the splint 

 or sap wood is at once distinguish- 

 able from the heart-wood by its 

 lighter colour. In some stems, 

 however, the heart-wood does not 

 change its colour. In that case, 

 as the protecting materials are 

 generally absent, it is liable to 

 decay, and then, as so often occurs 

 in the Willow, the stem becomes 

 hollow. 



The sap-wood is limited, ac- 

 cording to the kind of wood, to 

 a larger or smaller number of the 



younger annual rings, and tO it FIG. 142. Transverse section of a vessel fron 



falls the task of water conduction. 



the heart- wood of Robinia Pscwlt'cttcia, closed 

 by tyloses]; at a, a is shown the connection 

 between the tyloses and the cells from which 

 they have been formed, (x 300.) 



' The distinction between sap- and 

 heart- wood is sharpest where the latter is 

 dark-coloured, as in the Oak, with its brown heart- wood, and in species of Diospyms, 

 whose black heart- wood furnishes. ebony. The darker the heart- wood, the harder 

 and more durable it usually is. The following may be mentioned as examples of 

 woods which yield dyes and colouring principles Haematoxylon campcchianum, L. 

 (Campeachy wood, logwood), with a red heart-wood from which H^MATOXYLIN is 

 extracted ; Pterocarpus santalinus, L. fil. (red sandal-wood), from the heart-wood 

 of which SANTALIN is obtained ; Caesalpinia brasiliensis, L., and G. ecltinata, Lam. 

 (Brazil wood, Pernambuco wood), with a red heart-wood which supplies BRASILIA ; 

 and the Alsage Orange, Madura aurantiaca, Nutt. (yellow Brazil wood), which has 

 a yellow heart- wood from which MORIN is derived. Inorganic substances may also 

 be deposited in the duramen ; thus calcium carbonate is found especially in the 

 vessels of the Elm and the 'Beech, while silicic acid occurs in those of the Teak 

 ( Tectona grandis). 



TYLOSES (Fig. 142) are also instrumental in closing the water-courses of the 



