Cambial anatomy and absence of rays in the stem of Boerhaavia species (Nyctaginaceae)

Structure of vascular cambium and its derivatives in Boerhaavia diffusa L., B. verticillata Poir. and B. rependa Willd. (Nyctaginaceae) was studied by anatomical methods.The stem revealed anomalous secondary growth characterised by the development of successive rings of xylem and phloem. The cambium was exclusively composed of fusiform initials giving rise to rayless secondary vascular tissues. While undergoing periclinal divisions cambium appeared nonstoried but remained storied when the divisions ceased. Each successive ring of cambium was originated from the outermost phloem parenchyma cells. The cambial ring was functionally segmented into fascicular and interfascicular regions, the former mainly producing conducting elements of xylem and phloem and the latter giving rise to parenchyma cells. The parenchyma cells on the xylem side developed into conjunctive tissue following thickening and lignification of cell walls. However, in B. verticillata and B. rependa the parenchyma cells on the pholem side also became lignified. In B. diffusa parenchyma cells did not undergo lignification. As a result, alternate bands of lignified and parenchyma bands became distinct in the stem. Vessel elements were short with simple perforation plate on slightly oblique to transverse endwall. Sieve tube members were slightly shorter as compared to the fusiform cambial cells.