ALBURNUM AND DURAMEN. 



19 



niunber 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 first 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. 



b c d c (1 c d 



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



year's Towth ; a, pith, 6, bark, c, medullary rays, rf, woody bundles of fibre and vessels; 

 2, stem of three years' growth, o, pith, e, bark, b, c, d, 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, exhibiting the bundles of woody fibre, spiral vessels, and ducts, irregularly disposed 

 in the cellular tissue ; 5, o, cellular tissue, b, spiral vessels on inner side of c, doUed 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 albumum gradually harden by 

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

 tlie passage of fluids through them. Thus the duramen (dwus, hard), or heart- 

 wood is formed, the texture of whicli is firm .".nd durable. It is only the dura 

 men which is useful in the arts. 



