Comparative study on the secondary xylem and phloem formation in three varieties of Mucuna pruriens (L.) DC (Fabaceae)

The structure and composition of the main stem and the increase in stem diameter were compared histologically in Mucuna pruriens (L.) DC. var. pruriens, M. pruriens var. hirsuta (Wight & Arn.) Wilmot-Dear and M. pruriens var. utilis (Wall. ex Wight) Baker ex Burck. Initially, all three varieties showed regular secondary growth like most eudicots, the vascular cambium divided bi-facially and deposited secondary xylem centripetally and phloem centrifugally. The secondary xylem showed typical anatomical features characteristic of the lianescent habit, such as the co-occurrence of dimorphic (narrow and wide) vessels and an abundance of thin-walled, nonlignified parenchyma cells. After a certain period of secondary growth, earlier formed axial parenchyma began to dedifferentiate into interxylary phloem in all three varieties investigated. The number of sieve elements increased gradually with the increase in age. However, the thick stems of M. pruriens var. hirsuta showed simultaneous differentiation of phloem on both centripetal and a centrifugal direction, which was exclusively observed in this variety. In thick stems, the formation of vascular cylinders external to the regular phloem (neoformed vascular cylinders) was observed in M. pruriens var. hirsuta (8 mm) and M. pruriens var. pruriens (14 mm). At the same time, they were found absent in M. pruriens var. utilis (10 mm). Ontogenetically, the initiation of these vascular cylinders was unique and it was initiated from the dilating phloem ray cells and pericyclic parenchyma cells. The vascular cylinders developed from the dilating ray cells were irregularly (tangentially or diagonally) oriented while cylinders originating from pericyclic parenchyma had regular polarity (phloem externally and xylem internally). The initiation of the interxylary phloem and the structure of the secondary xylem of both regular and neoformed external vascular cylinders are elucidated in detail.