84 



ORGANOGRAPHY. 



carpenters, because it is to their presence that many woods, 

 such as the plane and sycamore, owe their peculiar lustre. 

 As new layers of wood are formed in successive years, new 

 additions are made to the ends of the medullary rays, by 

 which means, however large the space between the pith and the 

 bark ultimately becomes, the two are always kept in connexion 

 by their means. Besides the medullary rays which thus extend 

 throughout the entire thickness of the wood, others are also com- 

 monly developed between them in each succeeding year, which 

 extend from the zones of those years respectively to the bark. 

 These are called secondary medullary rays. In the Cork-oak both 

 kinds may be well seen in a transverse section {fig. 173, 1, 2, 

 3, 4). The medullary rays are composed of flattened six-sided 

 -p. ,..„ cells, which are placed one 



^' ' * above the other in one or more 



rows, like the bricks in a 

 wall {fig. 171, B, I, i ; and^^. 

 52), hence the tissue which 

 they form is commonly termed 

 muriform parenchyma. It is 

 a variety of tabular parenchyma 

 as already noticed (p. 28). The 

 tissue formed by the medullary 

 rays is rarely continuous from 

 one end of tiie stem to the other, 



Fig.173. Transverse eection of a portion of \)^i ^Jig j-^VS are e;enerally morC 

 the stem of the Cork-oak, four. years , . •' r ^ y j.{' 



old. m. Pith. 1, 2, 3.4. Medullary or Icss mtcrrupted by the pas- 

 rays of successive years, pe. Liber gj^ge between them of the fibro- 



and mesophloeum. s. Corky layers. =■ , , ,, ^i. ^ ^i 



vascular bundles, so that tney 

 split up vertically into a number of distinct ])ortions. This 

 arrangement may be observed by examining the surface of a 



Fig. 174. 



Fig. 175. 



Fig. 174. Surface of the stem 

 of a Dicotyledonous tree 

 from whicl\ the bark lias 



been removed. Fig. 17.5. 



Vertical section of a branch 

 of the common Maple, per- 

 pendiculartothe medullary 

 rays. Jl. Fibro-vaxcular 



■ bundles, rm. Medullary 

 rays. 



Stem from which the bark lias been removed {fig. 174), or 

 still better by making thin sections of the wood perpendicular 



