SAP-WOOD AND BARK. 



85 



pressed outwards by the continuous growth of the enclosed wood, the 

 outer portion loses all vitality ; the epiderm with adjacent dead tissue 

 splits under the action of the weather and in time is cast off, 

 but before this happens, a new envelope formed by the cork tissue 

 is always present. 



The latest-formed outer rings of wood are familiarly known as sap- 

 wood, technically alburnum. It is the medium by which processes vital 

 to the tree are carried on, such as the conveyance of the water and 

 food-stuffs taken up by the roots to the extremities of the branches, 

 the foliage, etc. The sap-wood is of little value as timber, being 



urn 



Fig. 47. 3, Transverse 4, longitudinal section of parts of a medullary ray M, with pits p 

 from first year's growth of Pinu-s excelsn. X 150. 



soft in texture and decaying rapidly on exposure, but when saturated 

 with resin its heating power is very great. When the older formed 

 w r ood-cells cease to perform vital functions, they become changed and 

 the original cavity is filled up with mineral substances deposited by the 

 upward current from the roots ; it then forms the heart-wood, and 

 so far as the tree itself is concerned, it is practically dead, the 

 living part being represented by a shell enclosing it. Evidence of 

 this may be seen in old Yew and Pine trees still in active growth, 

 but whose trunks are hollow through the decay and removal of the 

 heart-wood. 



As previously stated, the anatomical structure of coniferous wood is 

 the chief factor in determining its STRENGTH and ELASTICITY. By 

 Strength is understood the amount of resistance the wood offers to 

 the separation of the fibre of which it is composed, by any external 

 force applied to it. The ultimate strength is usually measured by 

 the force in pounds per square inch of section which must be 

 exerted in order to break it ; transverse strength is the most 

 important, it measures the resistance which wood offers to breakage 

 by a force acting at a right angle to the grain.* Among the 

 strongest European coniferous trees are the Larch, Spruce Fir, Scots 

 * Sclilich, Manual of Forestry, Vol. V. p. 46. 



