Formation of tri-lobed stem and development of successive cambia in the stems of Argyreia hookeri C. B. Clarke (Convolvulaceae)

Abstract

Structural transformation of the xylem is a fundamental feature for the successful adaptation of climbing habit in plants. Therefore, their stems at juvenile stage are inflexible before they start climbing but their pattern of secondary growth and type of xylem cell production typically changes as they start climbing. Consequently, stem become flexible and most often show development of successive cambia in many species. In several climbers, successive cambia show unequal cambial activity and deposition of secondary xylem. Subsequently, stems with different shapes can be observed in trans-sections. In the present study light microscopic analysis revealed that stems of Argyreia hookeri C. B. Clarke (Convolvulaceae) increased in thickness by forming crescent-shaped successive segments of cambia at three sides of the stem. Development of such cambia resulted in tri-lobed (“Y” shaped) outline of the stem in transverse view. Initially the stem was circular in outline but with the advancement of secondary growth, alternate segments of cambium become functionally slow on three alternate places while rest of the alternate segments showed normal activity, thus the stem become triangular. After a short period of secondary growth, new sectors of crescent-shaped cambia originated from the pericyclic parenchyma opposite to functional segments. In contrast, alternate segment (functionally slow) of the first cambium did not form the cambium on its outside. Development of subsequent crescent-shaped cambia consequently forms tri-lobed stems. Development of successive cambia on three sides of A. hookeri stems appear to be associated with the decrease in flexural rigidity and tri-lobed outline of stem may be helpful to limit physical disruption of vascular tissues during twisting/hanging/bending, and slipping or falling from supporting host. Structure and development of secondary xylem is described in detail and correlated with the climbing habit.