THE WALLFLOWER 93 



must become stretched. This stretching is partly pro- 

 vided for by the tangential expansion, followed by 

 radial division, of the cells forming the phloem-rays, 

 so that the phloem itself does not get distorted. 



The cells of the secondary wood and bast are essen- 



FIG. 35. Diagrammatic transverse section of part of an old 

 stem of Wallflower, including three of the original bundles. 

 e, epidermis ; co, cortex ; pd, periderm ; pr.ph, primary 

 phloem ; ph, phloem ; c, cambium ; sec, secondary tissues ; 

 pr.x, primary xylem ; p.m.r, a principal medullary ray ; 

 s.m.r, a secondary medullary ray ; p, pith. Magnified 

 about 20. (R. S.) 



tially like those of the primary tissues. The chief 

 forms of xylem -elements are shown in Fig. 36. 



The cambial cells are elongated prisms with inclined 

 ends. Where a medullary ray is being formed, how- 



