Development of successive cambia and structure of secondary xylem in the stems and roots of Distimake tuberosus (Convolvulaceae)

Abstract

In the present study, development of trilobed stem is investigated in Distimake tuberosus (Convolvulaceae) by histological methods. Increase in stem thickness and formation of trilobed stem was achieved by forming crescent shaped successive cambial segments from the pericyclic parenchyma at three equidistant sites. In contrast, the alternate segments of the vascular cambium were functionally less active, dividing leisurely (referred henceforth as functionally less active cambium) and did not form the successive cambium. The newly formed cambial segments divide bi-directionally and produced its derivatives, while functionally less active segment lagged behind. The secondary xylem formed from the functionally less active cambial segments have unique pattern of vessels distribution i.e., wide vessels rare or completely absent. Parenchyma (both, axial and ray) proliferation, formation of ray cambium, functionally inverse cambia, tyloses in fibriform vessels and the occurrence of bordered pits in xylem ray cells are observed in all samples. Root diameter increased by developing concentric rings of successive cambia and maintained a circular outline. Formation of successive cambia on three sides and alteration in stem outline may be associated with protection of conducting elements against stem torsion and internal injury. Formation of conducting elements from the ray cambium and internal cambium suggest rapid and safe conduction of photosynthate, water and mineral elements.