ALBURNUM 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 ligneous librc, vasi- 

 form tissue, and ducts (33,/"). The fust gives strength and solid- 

 ity to the trunk, and determines the direction of the cleavage. 



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

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

 the outer parts of the zone. The former are distinguished by the large size of 

 their open ends, while the woody fibres are more minute and compact. This cir- 

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

 of the stem. 



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

 years growth ; a, pith, b, bark, c, medullary rays, rf, woody bundles of fibre and vessels ; 

 2, stem of three years' growth, a, pith, e, bark, 6, c, rf, successive annual layers ; 4, a, pith, b, 

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

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

 in the cellular tissue ; 5, a, cellular tissue, 6, spiral vessels on inner side of c, dotted ducts, d, 

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



203. The outer and more recent portion of the layers constitutes the albur- 

 num {albus, white), or sap-wood. This is usually of 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 ascends. The interior layers of the alburnum gradually harden by 

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

 the passage of fluids through them. Thus the dlramln (chtrus, hard), or heart- 

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

 men which is useful in the arts. 



