35] CHAPTER II. THE TISSUES. 145 



among the longitudinally arranged tissues of the wood and of the 

 bast (Fig. 112). Its size varies, even in the same member, both 

 as regards its vertical (height) and its lateral (breadth) dimen- 

 sions. With regard to the former, the ray may consist of only a 

 single row of cells (as in Abietinese, Quercus, Fagus) ; the limits 

 may be generally stated at 112 rows of cells, though in some 

 cases they are considerably larger than this when they include 

 resin-ducts (e.g. Abietinese) or other forms of secretory tissue. In 

 any case, the secondary medullary rays, unlike the primary, do not 

 extend throughout the whole length of an internode. The breadth 

 of the secondary medullary rays is never nearly so great as their 

 height : as seen in tangential longitudinal section, they are narrow 

 above and below and broader in the middle ; it is only in the 

 middle that they ever consist of more 

 than one row of cells in breadth, the 

 upper and lower margins consisting 

 of a single row only. With regard 

 to their radial extent, it is only the 

 primary medullary rays which extend 



from pith to pericyde; the Subse- FjG . ns :.D iagrammatic repre- 



quently formed rays (secondary, ter- sentation of the course of the 

 tiary, etc.) extend between the wood medulla 7 y s in a segment cut 



, . out of the wood of a tree-trunk: 



and the bast of the year in which they Q horizontal surface; R radial 



were formed. surface; T tangential (external) 



surface of the wood; the shaded 

 AS instances of especially large portions m are the medullary rays. 



secondary medullary rays should be 



mentioned those formed in roots (see Pig. 110, p. 139) where the 

 cambium forms only conjunctive tissue opposite the primary 

 xylem-bundles. Again, in some few stems (see p. 139) the forma- 

 tion of secondary vascular tissue^ is confined to the f ascicular cam- 

 bium > the interfascicular cambium in the primary medullary rays 

 giving rise only to conjunctive tissue; hence the primary medul- 

 lary rays persist as broad bands of conjunctive tissue between the 

 bundles, and are not broken up, as is usually the case, by the 

 formation of secondary bundles from the interfascicular cambium. 



The Differentiation of the Secondary Tissues. The cells, 

 formed as the result of division in the cambium, which are to 

 become transformed into secondary permanent tissue have, to begin 

 with, the same form and structure as the corresponding cambium- 

 cells, but they gradually under- go^changes in both respects, as they 

 become transformed into permanent tissue. 



