144 BOTANY part i 



the splint or sap wood is at once distinguishable 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, according to the kind of wood, to a 

 larger or smaller number of the younger annual rings, and to it falls 

 the task of water conduction. 



The distinction bet^veen sap- and heart-wood is sharpest where the latter is 

 dark-coloured, as in the Oak, with its brown heart- wood, and in s-pecies of Diospyros, 

 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 princijiles — Ilaematoxylon eampecliianum, L. 

 (Campeachy wood, logwood), with a red heart- wood from which haematoxylin is 

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

 of which SANTALIN is obtained ; Cacsalpinia hrasilicnsis, L., and C. eckinata, Lam. 

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

 and the Alsage Orange, Madura aurantiaca, Nntt. (yellow Brazilwood), 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 Beach, while amorphous silicic acid occurs in those of 

 Teak {Tectona rjrandis). The closing membrane of the bordered pits in the heart- 

 wood is pushed to one side, so that the torus presses against the opening of the pit 

 and completely closes it. According to H. Mayr (^^^) resin does not penetrate the 

 walls of wood cells under normal conditions ; the wood of Conifers only becomes 

 resinous through the impregnation of the cell walls with resin after they have 

 become dried up through wounds or other causes. The resin-ducts of Conifers may 

 also be closed by the formation of tyloses. 



The elements of secondary growth differ in Gymnosperms and 

 Dicotyledons. The vascular strands of Gymnosperms are composed 

 almost exclusively of tracheides (Fig. 1.51 A). These are provided 

 with bordered pits which are situated, for the most part, on their 

 radial walls. The ti-acheides of the spring-wood (/) have larger 

 cavities than those formed later (s). Parenchyma is also present in 

 the wood, though in relatively small amount ; in some Abietineae 

 resin-passages occur in it (Fig. 151 //). 



Except in the Gnetaceae, true vessels are not found in the secondary growth, 

 nor in the primary vascular portions of the bundles of Gymnosperms. The wood 

 produced by tlie cambium consists of radial rows of tracheides, the number of 

 which is occasionally doubled by the radial division of a cambium cell (Fig. 

 151 A, a). The tracheides are often over a millimetre long, much longer than 

 the cambium cells from which they are developed. They attain this length by 

 a subsequent growth, during which their growing ends become pushed in between 

 one another. In addition to the tracheides, small amounts of wood parenchyma 

 are also produced in Gymnosperms by a transverse division of the cambium cells. 

 It is in the parenchymatous cell rows of the wood of Pines, Spruce-Firs, and 

 Larches that the schizogenous resin-ducts are produced (Fig. 151 A, h). In other 



