CHAPTER XII 



TIMBER — EXOTIC TIMBER TREES SUITABLE FOR PLANTING IN SOUTH AFRICA 

 — INDIGENOUS TIMBER TREES — WET ROT — DRY ROT 



The timber used for building purposes is derived from trees which 

 increase in diameter by the deposition of successive layers of wood, 

 each new layer being deposited immediately under the bark of the tree. 

 As a rule one layer is deposited in the course of each growing season ; 

 though a recurrence, in any one year, of exceptionally warm or moist 

 weather, may result in the formation of a second layer during that year. 

 These layers of wood appear as rings when a tree 

 is cut across, and are called annual rings; by 

 counting them the age of a tree can be approxi- 

 mately estimated. 



If we look at a cross-section of a tree trunk, 



we notice a series of radial lines known as 



Fig. 113. medullary rays. These form lines of weakness so 



that, when the wood dries, splitting takes place 



along them as indicated in Fig. 113. 



Silver Grain. — If the wood be cut into planks parallel, or nearly 

 parallel, to the medullary rays, a beautiful figured appearance known as 

 silver grain is obtained on the surface of the finished timber. Silver- 

 grain is especially marked in the case of hard woods. The method of 

 cutting trees into planks so as to get the silver grain effect is, however, 

 wasteful of timber, and is carried out only in the production of high- 

 class work such as good furniture, etc. 



Heart wood ; Sap wood. — As a tree increases in girth the inner wood 

 becomes denser and harder, and sometimes darker in colour, than the 

 outer wood. The former is called heartwood, and the latter sapwood. 

 It has been customary to specify, in the case of high-class work, that 

 heartwood alone should be used ; but as the demand for timber continues 

 yearly to outstrip the supply, it is becoming increasingly difficult to 



