Cambium Formation in Wounded Solanaceous Stems

When stems of certain Solanaceous species are wounded the internalphloem strands that lie near to the damaged surface become enlargedthrough cell division, and some of them develop a vascular cambium.This ‘internal cambium’ forms phloem inwards andxylem outwards; it is thus in inverted orientation. Such a structuredoes not occur in the normal stem of any Solanaceous plant.Further, during the regeneration of a cambial cylinder afterwounding, the regenerating cambium may turn inward to meet woundedinternal phloem strands. Where this happens, subsequent secondarythickening leads to the formation of thin tubes of cambium runningradially through the xylem and forming a limited amount of enclosedphloem which connects the internal and external phloem systemsof the stem. The jpositions in which these cambia arise are explicable bythe ‘gradient induction’ hypothesis of cambial regeneration,but not by the ‘free surfacfe’ and ‘cambialring’ theories.