

99 





FIG. 8. Diagrams illustrating secondary growth in thickness in a typica 

 Dicotyledon ; the din grams are based on drawings of transverse sections of 

 the hypocotyl of Ricinus. A, B, C represent the condition of the stern at 

 different stages of development : A, before the origin of the interfascicular 

 cambium ; B, after the interfascicular cambium has been formed ; C, after the 

 cambium has been active for some time, producing internally a broad ring of 

 secondary xylem, externally a narrower ring of secondary phloem. R = primary 

 cortex : M = pith ; p = phloem ; x = xylem ; b, b, b = three groups of bast fibres 

 at the periphery of the phloem ;fc = fascicular cambium ; ic = interfascicular 

 cambium ;/ h = wood developed from the fascicular cambium; if ft = wood 

 developed from the interfascicular cambium : ifp = secondary bast developed 

 from the interfascicular cambium. By the int- rcalation of the secondary xylem 

 and secondary phloem, the primary groups of bast fibres, &, ft, b, are removed a 

 considerable distance from the primary xylern, x, x, though in the young stem 

 A these are in close proximity to one another. Note also in C the primary 

 medullary rays, which extend the whole distance from the periphery of the ring 

 to the pith, while the secondary medullary rays only extend through part of 

 that distance. (After Sachs.) 



H 2 



