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Biology 2014-2015

Stems

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SWS Life Science

Section 22-3

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.