Stem Structure and Function

¨       Stems have three important functions:

1.          produce leaves, branches and flowers

2.         hold leaves up to the sunlight

3.         transport substances between roots and leaves

¨        Stems are composed of three tissues:

1.          dermal = epidermal cells and a thick waxy coating

2.         vascular = xylem and phloem

3.         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.

Monocot Stems

¨       Epidermis encloses vascular bundles which each contain both xylem and phloem.

¨       Phloem faces the outside of the stem and xylem faces the center.

Dicot Stems

¨       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

¨       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.

Formation of Bark

¨       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 outward.

·         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.