Stem Structure and Function
¨ Stems have three important functions:
produce leaves, branches and flowers
hold leaves up to the sunlight
transport substances between roots and leaves
¨ Stems are
composed of three tissues:
dermal = epidermal cells and
a thick waxy coating
vascular = xylem and phloem
ground = monocots (parenchyma)
dicots (pith – inside and cortex – outside)
¨ Stems contain nodes where leaves are attached and internodes regions between the nodes.
¨ Buds are underdeveloped
tissue that produce new stems and leaves. They are found where leaves attach
to the nodes.
Monocot and Dicot Stems
¨ Monocots – vascular bundles are scattered throughout the stem
¨ Dicots – vascular bundles are arranged in rings.
¨ Epidermis encloses vascular bundles which each contain both xylem and phloem.
¨ Phloem faces the outside of the stem and xylem faces the center.
¨ Arranged in ringlike patterns
¨ The pith is parenchyma cells inside the vascular ring and outside form the cortex of the stem.
Primary Growth of Stems
¨ New cells are produced at the tips of the roots and shoots.
¨ Primary growth is growth occurring only at the ends of a plant.
¨ Primary growth is produced by cell division in the apical
meristem and takes place in all seed plants.
¨ Secondary growth increases the width of a stem.
¨ Takes place in lateral meristematic tissues called vascular cambium
and cork cambium.
¨ Vascular cambium produces vascular tissues and increases thickness of stems over time.
¨ Cork cambium produces the outer covering of stems.
Formation of the Vascular Cambium
¨ Vascular cambium first appears thin and is situated between
clusters of vascular tissue.
¨ Forms between the xylem and phloem bundles.
¨ Divisions in the vascular cambium give rise to new layers
of xylem and phloem
¨ So, the stem becomes wider and wider.
Formation of Wood
¨ “Wood” is actually layers of xylem.
¨ As woody stems grow thicker, the older xylem near the center of the
stem no longer conducts water.
¨ Heartwood is this older xylem and darkens with age.
¨ Sapwood surrounds heartwood and is active in fluid transport and lighter in color.
¨ In temperate zones, tree growth is seasonal.
¨ Early wood is the rapid growth in the spring and is light-colored. Late wood is what results
later in the season and is darker.
¨ Tree rings are produced by this alternation of light and dark wood.
¨ A ring corresponds to a year of growth.
¨ The size of the rings vary with weather conditions (wet or dry years).
¨ Thick rings indicate conditions were favorable for tree growth and thin
rings less favorable.
¨ Bark includes all of the tissues (phloem, cork cambium and cork) outside the vascular cambium.
¨ How does with form?
· As vascular cambium increases in diameter, it forces the phloem tissue
· The expansion causes the oldest tissue to split and fragment as they
are stretched by the expanding stem.
· The cork cambium surrounds the cortex and produces the thick protective
layer of cork.
· Cork is a thick wall that contains fats, oils, or waxes which prevent loss of water.
· The outermost cork cells are usually dead and as the stem increases
in size, this dead bark cracks and flakes off in strips or patches.