THE STEM. 



37 



two being separated by what may be called fascicular cambium, 

 since it occurs within the bundle. In a rather older stem some 

 of the cells making up the medullary rays have begun to divide 

 actively, giving rise to cambium between the primary bundles 

 or interfascicular cambium. This, together with the fascicular 

 cambium, forms a 

 cambium ring, ex- 

 tending right round 

 the stem. This and 

 even a later stage is 

 reached in the stouter 

 stems of sunflower. 

 Still older examples 

 present a compact 

 mass of vascular tis- 

 sue, most of which 

 has been formed by 

 the active division of 

 the cambium, produc- 

 ing secondary hast on 

 the outside and secon- 

 dary wood on the in- 

 side, not only in the 

 region of the primary 

 bundles, but also be- 

 tween them. The 

 elements making up 

 the new bast and 

 wood are similar to 

 those already de- 

 scribed, but spiral 

 and annular air tubes 

 are limited to the 

 protoxylem. Secon- 

 dary increase is far 

 greater in the case 

 of the xylem, giving 

 rise to the " wood " 

 of shrubs and trees, 

 while the "bark" is 

 largely composed of 

 secondary bast. The 



FIG. 8.— Secondary Thickening of Stem. [After Sachs.] A, 

 B, C. cross sections of same stem at different ages. A. 

 young bundles quite separate from one another. B. a 

 cambium ring present. C. a cylinder of secondary xylem 

 and phloem present, formed by activity of cambium ring ; 

 M. pith ; R. cortex ; medullar}' rays in A and B are the 

 broad spaces between bundles, in C are represented by 

 streaks ; p, if p. phloem ; x,fk, ifh. xylem. 



primary medullary rays are reduced 



to exceedingly narrow strips, running through from pith to 



cortex. Secondary or short medullary rays are also formed, 



taking origin in the cambium, and only extending part of 



