382 



Morphogenetic Factors 



Cells which 

 normally ' 

 form fibres 



Phloem 



Primary , 

 xylem 



Secondary 

 xylem 



Primary 

 xylem 



Fig. 18-6. Stimulation of cambial activity by auxin. Above, two bundles from control 

 plant, decapitated but no auxin applied. Below, a similar region from a plant in which 

 auxin had been applied to the decapitated tip. Cambial growth is markedly stimu- 

 lated by the auxin. ( From Snow. ) 



