54 



EXOGENOUS STEM — WOOD. 



Fig. 117. 



transverse section, each zone or circle is usually seen to be separated 

 from that next to it by a well-marked line of demarcation. This liae, 

 as in the Oak (figs. 117, 118), and in the Ash, is indicated by holes 

 which are the openings of large pitted vessels; the remainder of the 

 tissue in the circle being formed by pleurenchyma, with thickened 

 walls and of smaller calibre. In some trees, as the Lime, Hornbeam, 



and Maple, the line is by no means 

 so well marked, as the openings are 

 smaller and more generally diffused; 

 but there is usually a deficiency of 

 >pitted vessels towards the outer part 

 of the circle. In cone-bearing plants, 

 as the Fir, in which the woody layers 

 consist entirely of punctated woody 

 tissue (fig. 49), without any large pit- 

 ted vessels, the line of separation is 

 marked by the pleurenchyma 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, although often 

 inconspicuous, are still visible ; the dry season, during which many 

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



The woody layers vary 

 in their texture at dif- 

 ferent periods. At first 

 the vessels are pervious 

 and full of fluid, but by 

 degrees thickening layers 

 are deposited which con- 

 tract their canal, and 

 sometimes obliterate it. 

 The first-formed layers 

 are those which soonest 

 become thus altered. In 



Pig. 117. Horizontal section of the stem of an oak eiglit years old. T>, Wood, showing 

 concentric circles or zones, separated by points which correspond to the opening of the 

 large pitted 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. 118. 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 m'", r m"", which vary in'extent according to the period when they originated, m. Pith, e c, 

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



Fig. 118. 



