23.3_Stems

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1 Review What are three important functions of
stems
Explain How does the arrangement of vascular
bundles in monocot stems differ from dicot stems
Apply Concepts How do the functions of a stem
relate to the functions of the roots and leaves of a
plant
2 Review Define primary and secondary growth
Predict Describe what would happen over time to a
tree sapling that could grow only taller, not wider
CH 23 PLANT STRUCTURE AND FUNCTION
23.3 Stems
Stem Functions



Stems produce leaves, branches, and flowers
Stems hold leaves up to the sun
Stems transport substances throughout the plant.
Anatomy of a Stem

Epidermal cells have thick cell walls and a waxy
protective coating.
Anatomy of a Stem

Nodes
 Where

leaves are attached
Buds
 Contain
apical meristems that
can produce new stems and
leaves

Stems produce woody tissue
in larger plants.
Monocot Stems


Clusters of xylem and
phloem tissue are
scattered throughout the
stem
Ground tissue is fairly
uniform consisting mainly
of parenchyma cells.
Dicot Stems


Vascular bundles are
arranged in a cylinder, or
ring
Pith
 Parenchyma
cells inside the
ring of vascular tissue

Cortex
 Parenchyma
cells outside
the ring of vascular tissue.
Primary Growth

Growth that occurs at the apical meristems.
Secondary Growth

Stems increase in thickness to support larger plant.
Secondary Growth




Very common among dicots and non-flowering
seed plants such as pines
Rare in monocots
Dicots have meristems within stems and roots that
produce true secondary growth
Dicots can grow much larger.
Conifers and Dicots


Secondary growth takes place in meristems called:
Vascular cambium
 Produces
vascular tissues and increases the thickness
of stems over time

Cork cambium
 Produces
the outer covering of stems.

Vascular cambium appears as a thin, cylindrical
layer of cells between the xylem and phloem of
each vascular bundle.


Divisions in the vascular cambium give rise to new
layers of xylem and phloem
Cambium continues to produce new layers of
vascular tissue each year, causing the stem to
become thicker.
Formation of Wood


Most of “wood” is actually layers of secondary
xylem
Heartwood
 Older
xylem near the center of the stem no longer
conducts water.

Sapwood
 Active
in fluid transport and is usually lighter in color.
Tree Rings

Spring growth
 Vascular
cambium begins to grow rapidly, producing
large, light-colored xylem cells, resulting in a lightcolored layer of early wood

As growing season continues
 Cells
grow less and have thicker cell walls, resulting in
a layer of darker wood

Thick rings good growing conditions and thing rings
poor growing conditions.
Formation of Bark

Bark
 All
tissues found outside the vascular cambium
 Tissues include phloem, the cork cambium, and cork.

Cork cambium produces a thick, protective layer of
waterproof cork that prevents water loss.
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