23–3 Stems
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23–3 Stems
Stem Structure and Function
Stem Structure and Function
Stems have three important functions:
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they produce leaves, branches and flowers
they hold leaves up to the sunlight
they transport substances between roots and
leaves
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23–3 Stems
Stem Structure and Function
Stems make up an essential part of the water and
mineral transport systems of the plant.
Xylem and phloem form continuous tubes from the
roots through the stems to the leaves.
This allows water and nutrients to be carried
throughout the plant.
In many plants, stems also function as storage
systems and aid in the process of photosynthesis.
Stems are surrounded by epidermal cells that have
thick cell walls and a waxy protective coating.
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23–3 Stems
Stem Structure and Function
Leaves attach to the stem
at structures called nodes.
Bud
The regions of stem
between the nodes are
internodes.
Small buds are found
where leaves attach to
nodes.
Node
Internode
Node
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23–3 Stems
Stem Structure and Function
Buds contain undeveloped
tissue that can produce
new stems and leaves.
Bud
In larger plants, stems
develop woody tissue that
helps support leaves and
flowers.
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23–3 Stems
Monocot and Dicot Stems
Monocot and Dicot Stems
The arrangement of tissues in a stem differs among
seed plants.
In monocots, vascular bundles are scattered
throughout the stem.
In dicots and most gymnosperms, vascular bundles
are arranged in a ringlike pattern.
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23–3 Stems
Monocot and Dicot Stems
Monocot Stems
Monocot stems have a distinct epidermis, which
encloses vascular bundles.
Each vascular bundle contains xylem and phloem
tissue.
Phloem faces the outside of the stem, and xylem
faces the center.
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23–3 Stems
Monocot and Dicot Stems
Epidermis
Vascular
bundles
Vascular bundles are
scattered throughout the
ground tissue.
Ground tissue consists
mainly of parenchyma
cells.
Ground
tissue
Monocot
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23–3 Stems
Monocot and Dicot Stems
Dicot Stems
Dicot stems have
vascular bundles
arranged in a ringlike
pattern.
Vascular
bundles
Epidermis
The parenchyma cells
inside the vascular tissue
are known as pith.
The parenchyma cells
outside of the vascular
tissue form the cortex of
the stem.
Cortex
Pith
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Dicot
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23–3 Stems
Primary Growth of Stems
Primary Growth of Stems
All seed plants undergo Primary
growth
Apical meristem
primary growth, which is
an increase in length at the
ends of the plant.
New cells are produced at
the tips of roots and shoots
throughout the plant’s life.
Primary
growth
Primary growth of stems is
produced by cell divisions in
the apical meristem. It takes
place in all seed plants.
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Leaf
scar
Year
Year 3
21
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23–3 Stems
Secondary Growth of Stems
Secondary Growth of Stems
The method of growth in which stems increase in
width is called secondary growth.
In conifers and dicots, secondary growth takes place
in the lateral meristematic tissues called the
vascular cambium and cork cambium.
Vascular cambium produces vascular tissues and
increases the thickness of stems over time.
Cork cambium produces the outer covering of stems.
The addition of new tissue in these cambium layers
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increases the thickness of the stem.
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23–3 Stems
Secondary Growth of Stems
Formation of the Vascular Cambium
Once secondary growth begins, the vascular
cambium appears as a thin layer between the xylem
and phloem of each vascular bundle.
Vascular cambium
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23–3 Stems
Secondary Growth of Stems
Vascular cambium divides producing xylem toward
the center of the stem and phloem toward the outside.
These different tissues form the bark and wood of a
mature stem.
Secondary phloem
Secondary xylem
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23–3 Stems
Secondary Growth of Stems
Formation of Wood
Wood
Bark
Wood is actually layers of
xylem. These cells build
up year after year.
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23–3 Stems
Secondary Growth of Stems
As woody stems grow
thicker, older xylem cells
near the center of the stem
no longer conduct water.
Xylem: Heartwood
This is called heartwood.
Heartwood usually darkens
with age because it
accumulates impurities that
cannot be removed and
supports the tree.
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23–3 Stems
Secondary Growth of Stems
Xylem: Sapwood
Heartwood is surrounded
by sapwood.
Sapwood is active in
water and mineral
transport and therefore
lighter in color.
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23–3 Stems
Secondary Growth of Stems
In most of the temperate zone, tree growth is
seasonal.
Growth begins in the spring and the vascular
cambium begins to grow rapidly, producing large lightcolored xylem cells with thin cell walls.
The result is a light-colored layer of wood called early
wood.
As the growing season continues, the cells become
smaller with thicker cell walls, forming a layer of
darker wood called late wood.
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23–3 Stems
Secondary Growth of Stems
This alternation of dark and light wood produces what
we normally call tree rings. Each ring represents a
year of growth.
The size of the rings may even provide information
about weather conditions, such as wet or dry years.
Thick rings indicate that weather conditions were
favorable for the tree growth and visa versa.
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23–3 Stems
Secondary Growth of Stems
Formation of Bark
Vascular
cambium
Bark
On most trees, bark
includes all of the tissues
outside the vascular
cambium — phloem, the
cork cambium and cork.
The vascular cambium
produces new xylem and
phloem, which increase
the width of the stem.
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23–3 Stems
Secondary Growth of Stems
The phloem transports sugars
produced by photosynthesis.
Phloem
As the vascular cambium
increases in diameter, it
forces the phloem tissue
outward.
This causes the oldest
tissues to split and fragment
as they are stretched by the
expanding stem.
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23–3 Stems
Secondary Growth of Stems
The cork cambium
produces a thick protective
layer of cork which
surrounds the cortex.
Cork cambium
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23–3 Stems
Secondary Growth of Stems
The cork contains cells that
have thick cell walls and
usually contain fats, oils, or
waxes.
Cork
These waterproof substances
help to prevent loss of water.
The outermost cork cells are
usually old, dead,
nonfunctioning phloem that
protects the tree.
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