MANAGEMENT OF DECAYING TREES 9 



The timber next the pith is described as 

 heartwood or duramen, which is harder and 

 darker in colour than the other portions of the 

 stem. Outside this is the sapwood or albur- 

 num, which may best be described as young 

 or immature wood, and is softer and lighter in 

 weight and colour than the heartwood. Be- 

 tween the sapwood and bark is the cambium, 

 which may be described as a thin and delicate 

 tissue or layer of active cells containing the 

 nutritive materials required as food by the 

 tree. It must be looked upon as the real 

 active part of a tree, in that it transmits the 

 sap from the roots to the crown. 



Everyone knows what the bark is, the main 

 use being to protect the cambium and wood. 

 With the elm, birch, oak, poplar, and such- 

 like rough-barked trees, the bark growth is 

 annually added to from within, the outer 

 surplus layers gradually flaking off owing to 

 pressure caused not only by bark expansion but 

 by the growth of the timber. From smooth- 

 barked trees like the hornbeam and beech a 

 deeply furrowed outer covering is absent, as in 

 these cases the bark itself grows and expands. 



