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.
These tissues are the early precursors of the mature stem tissues, and define the basic functions of the stem:
- Structural support;
- Growth through increase in diameter (girth) and elongation;
- Transport of fluids between the roots and the leaves.