20 STEM. 



The inside of the bark of such stems has 

 also the technical name of Liher. 



The Heart-wood was, when young, Albur- 

 num, and afterwards changed its nature, by- 

 becoming the receptacle of certain secretions 

 peculiar to the species. 



Hence the greater durability of Heart-wood 

 than of Sap-wood. While the latter is newly 

 formed empty tissue, almost as perishable as 

 bark itself, the former is protected against de- 

 struction by the introduction of secretions that 

 become solid matter, which is often insoluble 

 in water, and never permeable to air. • 



The secretions by which Heart-wood is soli- 

 dified are prepared in the leaves, whence they 

 are sent downwards through the bark, and from 

 the bark communicated to the central part of 

 the stem. 



The channels through which this communi- 

 cation takes place are called Medullary Rays, 

 or Silver Grain. 



Medullary rays are plates of cellular tissue, 

 in a very compressed state, passing from the 

 pith into the bark. They are what form the 

 cross grain of most of our ornamental woods. 



The wood itself is composed of tubes con- 

 sisting of woody fibre and vascular tissue, im- 

 bedded longitudinally in cellular substance. 



