Plant stem

A stem is the above ground axis of a vascular plant. The young stem develops in the germinating seedling from embryonic tissue known as the hypocotyl. Lengthening of the hypocotyl lifts the cotyledon(s), plumule (leaf precursor tissues), and terminal or apical meristem above the ground. The surface cells differentiate and mature into a protective epidermal layer. A few cells interior differentiate as chollenchyma, providing support to the young stem. Clusters of elongated cells appear — these are the provascular strands. The remainder of the stem consists of parenchyma cells: those lying between the epidermis and the provascular strands forming a cortex and those interior to the provascular strands forming a pith. Parenchyma radiating from the pith between the precursors of the vascular bundles are called pith rays.

Dicot Stems

Dicot stems have a pith in the center with vascular tissues in a distinct ring visible in a cross section. They have secondary growth originating from their lateral or secondary meristems: the vascular cambium and the cork cambium. The vascular cambium grows cells that differentiate into secondary xylem and secondary phloem. Secondary xylem is commercially important as wood. The seasonal variation in growth from the vascular cambium is what creates yearly tree rings in most climates.

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