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.
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.