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RELATIONSHIPS. 



The first increase of thickness in the cycad root corresponds to that of dico- 

 tyls and gymnospenns, more especially to the fleshier forms. The structure of the 

 secondary bundle elements is fundamentally similar to and a continuation of that 

 of the stem. A periderm forms early, the primary outer cortex soon being thrown 

 off. In those stems whose first cambium ceases activity and is succeeded by a sec- 



Si-frfr- 



Fig. 1 19. — Dion edule. X 7. Showing the very robust tap-root, quite as large as the 

 trunk itself, giving rise to a secondary filamentous root system. 



ondary peripheral cambium as described, an exactly similar growth proceeds in the 

 roots as they become older. In the roots of Zamia floridana, the single xylem zone 

 is relatively far heavier than in the stem, the form and process of zonal thickening 

 possibly being similar to that seen on a far more extensive scale in such stems of 

 Cycadeoidea as that shown in transverse section on plate xiv. 



