336 



THE WOODY STEM 



De.con.dary xylem 



Primary xyle 



FIG. 55. Diagrams of secondary thickening in the stem. A, cross- 

 section of stem with bundles separated by broad primary rays 

 in the first year of thickening ; secondary xylem and phloem 

 are formed all round the cylinder, bridging the broad primary 

 rays. B, ditto, with bundles separated by narrow primary rays ; 

 the cambium continues the primary rays as secondary (principal) 

 rays; s.t., secondary tissue. C, section of stem after six years 

 of thickening ; the primary rays are continued as principal rays, 

 and new narrower subordinate rays are formed in the wedges of 

 secondary tissue formed opposite the primary bundles. 



