Stems in Sesuvium portulacastrum L. (Aizoaceae) increase in thickness by forming successive rings of cambia that formed concentric rings of xylem alternating with phloem. The cambium is semi-storied and exclusively composed of vertically elongated fusiform initials while cambial rays were absent in the early part of the secondary growth. In the older stems some of the fusiform cambial cells undergo further division and develop into vertically upright rays. The first ring of cambium formed thick walled lignified elements centripetally and phloem centrifugally for a definite period. A second ring of cambium is developed from axial parenchyma cells located outside the phloem produced by the previous cambium. Prior to the complete differentiation of new cambial ring, these parenchyma cells divide bi-directionally to form additional parenchyma cells (referred here as secondary cortex) to the outer and inner side. Cells formed outside to the new ring formed secondary cortex, which served as a site for the future cambium. Inner cells differentiated later on into conjunctive tissue between two successive rings. In the newly formed cambium, alternate small segments of the cambium gave rise to conducting elements of xylem towards the center and phloem towards the periphery while the other segments formed thin walled parenchyma on both the sides. Secondary xylem was composed of vessel elements, fiber tracheids and libriform fibers with nuclei whereas secondary phloem consisted of sieve tube elements, companion cells and parenchyma with no rays in the young stem.
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Vol. 135 • No. 4