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Copyright Pearson Prentice Hall
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23–3 Stems
Slide 3 of 36
Stem Structure and Function
Stem Structure and Function
What are the three main functions of stems?
Copyright Pearson Prentice Hall
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23–3 Stems
Slide 4 of 36
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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23–3 Stems
Slide 5 of 36
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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23–3 Stems
Slide 6 of 36
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
Slide 7 of 36
Stem Structure and Function
Leaves attach to the stem at structures called nodes.
The regions of stem between the nodes are internodes.
Small buds are found where leaves attach to nodes.
Node
Internode
Bud
Node
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23–3 Stems
Slide 8 of 36
Stem Structure and Function
Buds contain undeveloped tissue that can produce new stems and leaves.
In larger plants, stems develop woody tissue that helps support leaves and flowers.
Bud
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23–3 Stems
Slide 9 of 36
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
Slide 10 of 36
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
Slide 11 of 36
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
Slide 12 of 36
Monocot and Dicot Stems
Vascular bundles are scattered throughout the ground tissue.
Ground tissue consists mainly of parenchyma cells.
Monocot
Vascular bundles
Epidermis
Ground tissue
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23–3 Stems
Slide 13 of 36
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.
Dicot
Cortex
Pith
Vascular bundles Epidermis
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23–3 Stems
Slide 14 of 36
Monocot and Dicot Stems
The parenchyma cells outside of the vascular tissue form the cortex of the stem.
Dicot
Cortex
Pith
Vascular bundles Epidermis
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23–3 Stems
Slide 15 of 36
Primary Growth in Stems
How do primary growth and secondary growth occur in stems?
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23–3 Stems
Slide 16 of 36
Primary Growth of Stems
Primary Growth of Stems
All seed plants undergo primary growth, which is an increase in length.
For the entire life of the plant, new cells are produced at the tips of roots and shoots.
Year 1Year 2Year 3
Primary growth
Apical meristem
Primary growth
Leaf scar
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23–3 Stems
Slide 17 of 36
Primary Growth of Stems
Primary growth of stems is produced by cell divisions in the apical meristem. It takes place in all seed plants.
Apical meristem
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23–3 Stems
Slide 18 of 36
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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23–3 Stems
Slide 19 of 36
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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23–3 Stems
Slide 20 of 36
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
Slide 21 of 36
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 xylem
Secondary phloem
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23–3 Stems
Slide 22 of 36
Secondary Growth of Stems
These different tissues form the bark and wood of a mature stem.
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23–3 Stems
Slide 23 of 36
Secondary Growth of Stems
Formation of Wood
Wood is actually layers of xylem. These cells build up year after year.
Wood Bark
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23–3 Stems
Slide 24 of 36
Secondary Growth of Stems
As woody stems grow thicker, older xylem cells near the center of the stem no longer conduct water.
This is called heartwood. Heartwood supports the tree.
Xylem: Heartwood
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23–3 Stems
Slide 25 of 36
Secondary Growth of Stems
Heartwood is surrounded by sapwood.
Sapwood is active in water and mineral transport.
Xylem: Sapwood
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23–3 Stems
Slide 26 of 36
Secondary Growth of Stems
Formation of Bark
On most trees, bark includes all of the tissues outside the vascular cambium — phloem, the cork cambium and cork.
Bark
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23–3 Stems
Slide 27 of 36
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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23–3 Stems
Slide 28 of 36
Secondary Growth of Stems
The phloem transports sugars produced by photosynthesis.
Phloem
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23–3 Stems
Slide 29 of 36
Secondary Growth of Stems
The cork cambium produces a protective layer of cork.
Cork cambium
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23–3 Stems
Slide 30 of 36
Secondary Growth of Stems
The cork contains old, nonfunctioning phloem that protects the tree.
Cork
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Slide 32 of 36
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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Slide 33 of 36
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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Slide 34 of 36
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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Slide 35 of 36
23–3
Xylem and phloem are contained in
a. the epidermis.
b. vascular bundles.
c. the pith.
d. cork cambium.
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Slide 36 of 36
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.