46 



EXOGENOUS STEM. RAYS. 



tive to the changes which take place in the bark, it will be understood 

 that it is often difficult to count its annual layers, so as to estimate the 

 age of the tree by means of them. This may, however, be done in 

 some cases, as shown at fig. 99, where there are eight layers of bark, 

 e, corresponding to eight woody layers, b. 



89. Medullary Rays or Plates. While the bark and pith become 

 gradually separated by the intervention of vascular bundles, the con- 

 nection between them is kept up by means of processes called medul- 

 lary rays (figs. 94, 95 r). These form the silver grain of carpenters ; 

 they communicate with the pith and the cellular envelope of the bark, 

 and they consist of cellular tissue, which becomes compressed and flat- 

 tened so as to assume a muriform appearance (fig. 1 02 m r). At first 

 they occupy a large space (fig. 94 r) ; but as the vascular bundles in- 

 crease, they become more and more narrow, forming thin laminae or 

 plates, which separate the woody layers. On making a transverse or hori- 

 zontal section of a woody stem, the medullary rays present the aspect of 

 narrow lines running from the centre to the circumference (figs. 99, 100 

 r TO); and in making a vertical section of a similar stem through one 

 of the rays, the appearance represented in fig. 102 will be observed, 

 where a medullary ray, m r, composed of flattened muriform cells 



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ce cf ce 



passes from the pith, p, to the cellular envelope, c e, crossing the 

 tracheae of the medullary sheath, t, the ligneous tissue, , the porous 

 vessels of the wood, b, and the fibres of the liber, cf. The lamina do 

 not by any means preserve an uninterrupted course from the apex to 



Fig. 102. Vertical section of a one-year old branch of Acer campestre highly magnified, and 

 extending from the pith to the bark, parallel to the medullary rays, m r, A medullary ray or 

 plate extending from the pith, p, to the bark, c e, crossing trachea, t, woody fibres, I, porous 

 vessels, 6, and cortical fibres, cf. 



Fig. 103. Vertical section of the same branch perpendicular to medullary rays. 1 1, Woody 

 fibres which interlace, leaving spaces, mr,mr,mr, where the medullary rays pass. 



