FIG. 6. Diagrams illustrating secondary growth in thickness in a typical Dicotyledon; 

 the diagrams are based on drawings of transverse sections of the hypocotyl of 

 Ricinus. A, B, C represent the condition of the stem at different stages of 

 development : A, before the origin of the interfascicular cambium ; /?, 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 narrow ring of secondary phloem. ^=primary cortex : ^/=pith ; ^=phloem ; 

 _r = xylem ; A, b, b = three groups of bast fibres at the periphery of the phloem ; 

 J c = fascicular cambium : / c = interfascicular cambium ; f h = wood developed 

 from the fascicular cambium : /_/" = wood developed from the interfascicular 

 cambium: ifp == secondary bast developed from the interfascicular cambium. 

 By the intercalation of the secondary xylem and secondary phloem, the primary 

 groups of bast fibres, b, b, b, are removed a considerable distance from the primary 

 xylem, 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 second medullary 

 rays only extend through part of that distance. (After Sachs. ) 



