ALEURNUai AND DURAMEN. 



79 



number of layers, if counted at the base, will correctly indicate 

 the age of the tree. "; 



202. Each woody layer is composed _of hgneous fibre, vasi- 

 form tissue, and ducts ( 33, /). The fii-st gives strength and sohd- 



ity to the trunk, and determines the direction of the cleava"-e 



& V 



a. The ducts^ra always first formed and lie in the inner part next the centre, 

 while tlie fibres arc produced towards the end of the season, and are deposited in 

 the outer parts of the zone.";. The fomier are distinguished .'l)y the large size of 

 tlieir open ends, while the woody fibres are more minute and compac^' This cu-- 

 cumstance renders the limits of each layer distinctly perceptible in a cross' section 

 of the stem. 



c c c c dc dc 4 a bcdcUcit 



FIG. 26. - Sections of wood ; 1, 2, 3, horizontal, 4, .5, vertical. 1, Exogenous stem of one 

 year's growth ; a, pith, 6, bark, c, medullar>' rays, d, woody bundles of fibre and vessels ; 

 2, stem of three years' growth, a, pith, e, bark, 6, c, d, successive ammal layers ; 4, a, pith, h, 

 spiral vessels of the medullary sheath, c, doited ducts, d, woody fibre, e, bark ; .3, Endogen- 

 ous stem, exhibiting the bundles of woody fibre, spiral vessels, and ducts, irregularly disposed 

 in the cellular tissue ; 5, a, cellular tissue, b. spiral vessels on iimer side of c, duUed ducts, d, 

 woody fibre on the exterior side. 6, Laticiferous vessels of the bark. 



203. The outer and more recent portion of the layers congtitutc^he ALnrn- 

 NUM (aZ6i«s, white), or sap-woodj This is usually/6f a softer structure and lighter 

 color than the rest of the wood, and it is through the vessels of these layers alone, 

 that the sap ascendsj The interior hsycrs of the alburnun^' gradually harden by 

 the deposition of solid secretions in their vessels, until they can no longer allow 

 the passage of fluids through tliem. Thus the duramen {dunis, hard), or Ircart- 

 wood is formed, the texture of which is firm and durable. It is oulv the dura 

 men which is useful in the arts.) 



