Development of Inverse Cambia and Structure of Secondary Xylem in Ipomoea turbinata (Convolvulaceae)

Structural transformation of mechanical tissues during the shift from a freestanding to a climbing habit is a characteristic of lianas, which are increasingly abundant in tropical forests. The modification of mechanical tissue and the evolution of a new growth pattern serve to increase stem flexibility and conductive efficiency. In Ipomoea turbinata Lag. (Convolvulaceae), the stem thickens via the formation of two distinct types of successive cambia: functionally normal successive cambia (producing xylem centripetally and phloem centrifugally), and inverse cambia (producing xylem centrifugally and phloem centripetally). The former originates from pericyclic derivatives (parenchyma cells located outside the primary phloem), while the latter originates from the conjunctive parenchyma located on the inner margin of the secondary xylem formed from vascular cambium. The secondary xylem produced by normal cambia is significantly more abundant than the xylem formed by inverse cambia. During primary growth, intraxylary primary phloem differentiates concomitantly with the protoxylem at the periphery of the pith; additional intraxylary secondary phloem is added from adjacent parenchyma cells as the plant ages. During initiation of every successive cambium, middle cells in the meristem give rise to cambium, and cells on either side of it serve as sites for initiation of future cambia. The functional role of inverse cambia remains unknown and awaits further experimental studies.