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23–3 Stems
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23–3 Stems
Stem Structure and Function
Stem Structure and Function
What are the three main functions of
stems?
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23–3 Stems
Stem Structure and Function
Stems have three important functions:
• they produce leaves, branches and
flowers
• they hold leaves up to the sunlight
• they transport substances between roots
and leaves
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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.
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Stem Structure and Function
Stems are surrounded by a layer of 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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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 arrangemnet of tissues in a stem differs
among seed plants.
How do monocot and dicot stems differ?
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23–3 Stems
Monocot and Dicot Stems
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.
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23–3 Stems
Monocot and Dicot Stems
Vascular bundles are
scattered throughout the
ground tissue.
Epidermis
Vascular
bundles
Ground tissue consists
mainly of parenchyma
cells.
Ground
tissue
Monocot
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Monocot and Dicot Stems
Dicot Stems
Dicot stems have
vascular bundles
arranged in a ringlike
pattern.
The parenchyma cells
inside the vascular
tissue are known as
pith.
Vascular
bundles
Epidermis
Cortex
Pith
Dicot
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23–3 Stems
Monocot and Dicot Stems
The parenchyma
cells outside of the
vascular tissue
form the cortex of
the stem.
Vascular
bundles
Epidermis
Cortex
Pith
Dicot
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23–3 Stems
Primary Growth in Stems
How do primary growth and secondary
growth occur in stems?
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Primary Growth of Stems
Primary Growth of
Stems
All seed plants
undergo primary
growth, which is an
increase in length.
Primary
growth
Apical meristem
Primary
growth
For the entire life of
the plant, new cells are
produced at the tips of
roots and shoots.
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Leaf
scar
Year
Year 3
21
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Primary Growth of Stems
Apical meristem
Primary growth
of stems is
produced by cell
divisions in the
apical meristem.
It takes place in
all seed plants.
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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 vascular cambium and
cork cambium.
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Secondary Growth of Stems
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
increases the thickness of the stem.
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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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Secondary Growth of Stems
The vascular cambium divides to produce xylem cells
toward the center of the stem and phloem cells
toward the outside.
Secondary phloem
Secondary xylem
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Secondary Growth of Stems
These different tissues form the bark and wood of a
mature stem.
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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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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
supports the tree.
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Secondary Growth of Stems
Heartwood is surrounded
by sapwood.
Xylem: Sapwood
Sapwood is active in water
and mineral transport.
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Secondary Growth of Stems
Formation of Bark
Bark
On most trees, bark
includes all of the
tissues outside the
vascular cambium —
phloem, the cork
cambium and cork.
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Secondary Growth of Stems
The vascular cambium
produces new xylem and
phloem, which increase
the width of the stem.
Vascular
cambium
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Secondary Growth of Stems
The phloem transports
sugars produced by
photosynthesis.
Phloem
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Secondary Growth of Stems
The cork cambium
produces a protective
layer of cork.
Cork cambium
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Secondary Growth of Stems
The cork contains old,
nonfunctioning phloem
that protects the tree.
Cork
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23–3
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23–3
Structures on a stem that can produce new
stems and leaves are called
a. nodes.
b. internodes.
c. buds.
d. branches.
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23–3
The vascular bundles in a monocot stem
a. form a cylinder, or ring.
b. are scattered throughout the stem.
c. form concentric rings.
d. separate into xylem bundles and phloem
bundles.
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23–3
The outermost layer of a tree that contains old,
nonfunctioning phloem is
a. bark.
b. cork.
c. pith.
d. apical meristem.
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23–3
Xylem and phloem are contained in
a. the epidermis.
b. vascular bundles.
c. the pith.
d. cork cambium.
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23–3
In stems, secondary growth results in
a. growth at the tips of roots.
b. growth at the tips of shoots.
c. an increase in the width of stems.
d. an increase in the length of stems.
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