EXOGENOUS STEM. WOOD. 



large porous vessels, the line of separation is marked by the pleuren- 

 chyma becoming dense and often 

 coloured. In some kinds of wood, 

 as Sumach, the zones are separated 

 by a marked development of cellular 

 tissue. The separation between the 

 zones is said to be owing to the 

 interruption in the growth of the 

 tree during autumn and winter, and 

 hence it is well defined in trees of 

 temperate and cold climates. But 

 even in tropical trees, the lines, al- 

 though often inconspicuous, are still 

 visible; the dry season, during which 99 



many of them lose their leaves, being their season of repose. 



78. The woody layers vary in their texture at different periods. At 

 first the vessels are pervious and full of fluid, but by degrees thicken- 

 ing layers are deposited which contract their canal, and sometimes 

 obliterate it. The first- 

 formed layers are those 

 which soonest become thus 

 altered. In old trees, there 

 is a marked division be- 

 tween the central Heart - 

 wood or Duramen (durus, 

 hard), and the external 

 Sap-wood or Alburnum 

 (albus, white); the former 

 being hard and dense, and 

 often coloured, with its 

 tubes dry and thickened; while the latter is less dense, is of a pale 

 colour, and has its tubes permeable by fluids. The difference of colour 

 between these two kinds of woods is often very visible. In the Ebony 

 tree, the duramen, or perfect- wood, is black, and is the part used for 

 furniture, &c.; the alburnum is pale: in the Beech, the heart- woodis light- 

 brown; in the Oak, deep-brown: in Judas tree, yellow: in Guaiacum, 

 greenish. The alteration in colour is frequent in tropical trees. In 

 those of temperate climates, called white-wood, as the Willow and Pop- 



Fig. 99. Horizontal section of the stem of an oak eight years old. 6, Wood, showing con- 

 centric circles or zones, separated by points which correspond to the opening of the large porous 

 vessels, or Bothrenchyma. e, Bark, showing also eight concentric circles, thinner and less 

 distinct. The wood and bark are traversed by medullary rays, some of which extend from the 

 bark to the pith, and others reach only a certain way inwards. 



Fig. 100. Horizontal section of two woody bundles of Cork-oak, separated from each other 

 by the medullary ray, r m'. The two primary bundles are divided by secondary rays, r m" 

 r ml", r m'"', which vary in extent according to the period when they originated, m, Pith, e c, 

 Cellular envelope, y, Corky envelope, which is highly developed and exhibits several layers. 



