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Chapter 25
Plant Structure and Function
Table of Contents
Section 1 The Vascular Plant Body
Section 2 Transport in Plants
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Chapter 25
Section 1 The Vascular Plant Body
Objectives
• Identify the three kinds of tissues in a vascular
plant’s body, and state the function of each.
• Compare the structures of different types of roots,
stems, and leaves.
• Relate the structures of roots, stems, and leaves to
their functions.
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Chapter 25
Section 1 The Vascular Plant Body
Tissues
• In vascular plants, there are three types of tissues—
dermal tissue, ground tissue, and vascular tissue.
• Vascular tissue forms strands that conduct water
and nutrients throughout a vascular plant.
• Dermal tissue forms the protective outer layer of a
plant.
• Ground tissue makes up much of the inside of the
nonwoody parts of a plant, including roots, stems,
and leaves.
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Chapter 25
Section 1 The Vascular Plant Body
Tissues, continued
• The leaves,
stems, and roots
of a vascular
plant contain all
three kinds of
plant tissues.
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Chapter 25
Section 1 The Vascular Plant Body
Tissues, continued
Dermal Tissue System
• In the nonwoody parts of a plant, dermal tissue forms
a “skin” called the epidermis.
• The epidermis of most plants is made up of a single
layer of flat cells.
• The dermal tissue on woody stems and roots
consists of several layers of dead cells that are
referred to as cork.
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Chapter 25
Section 1 The Vascular Plant Body
Dermal Tissue Systems in Plants
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Chapter 25
Section 1 The Vascular Plant Body
Tissues, continued
Ground Tissue System
• Most ground tissue consists of thin-walled cells that
remain alive and keep their nucleus after they
mature.
• The ground tissue in leaves, which is packed with
chloroplasts, is specialized for photosynthesis.
• The ground tissue in stems and roots functions
mainly in the storage of water, sugar, and starch.
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Chapter 25
Section 1 The Vascular Plant Body
Ground Tissue Systems in Plants
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Chapter 25
Section 1 The Vascular Plant Body
Vascular Tissue Systems
Xylem
• Xylem has thick-walled cells that conduct water and
mineral nutrients from a plant’s roots through its
stems to its leaves.
• The conducting cells in xylem must lose their cell
membrane, nucleus, and cytoplasm before they can
conduct water.
• Gnetophytes and flowering plants also have a second
type of xylem cell, which makes up conducting
strands called vessels.
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Chapter 25
Section 1 The Vascular Plant Body
Structure of Xylem
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Chapter 25
Section 1 The Vascular Plant Body
Vascular Tissue Systems, continued
Phloem
• Phloem contains cells that conduct sugars and other
nutrients throughout a plant’s body.
• The conducting cells of phloem have a cell wall, a cell
membrane, and cytoplasm.
• The conducting strands in phloem are called sieve
tubes. Pores in the walls between neighboring sievetube cells connect the cytoplasms and allow
substances to pass freely from cell to cell.
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Chapter 25
Section 1 The Vascular Plant Body
Structure of Phloem
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Chapter 25
Section 1 The Vascular Plant Body
Roots
• Most plants are anchored to the spot where they
grow by roots, which also absorb water and
mineral nutrients.
• Many dicots, such as carrots and radishes, have a
large central root from which much smaller roots
branch. This type of root system is called a taproot
system.
• In contrast, most monocots, such as grasses, have
a highly branched, fibrous root system.
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Chapter 25
Section 1 The Vascular Plant Body
Types of Roots
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Chapter 25
Section 1 The Vascular Plant Body
Roots, continued
• The ground tissue surrounding the vascular tissue is
called the cortex.
• An epidermis covers the end of a root, or root tip. The
epidermal cells just behind a root tip often produce
root hairs, which are slender projections of the cell
membrane.
• A mass of cells called the root cap covers and
protects the actively growing root tip.
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Chapter 25
Section 1 The Vascular Plant Body
Radish Root Structure
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Chapter 25
Section 1 The Vascular Plant Body
Dicot and Monocot Root Structures
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Chapter 25
Section 1 The Vascular Plant Body
Stems
Nonwoody Stems
• A plant with stems that are flexible and usually
green is called a herbaceous plant.
• The stems of herbaceous plants contain bundles
of xylem and phloem called vascular bundles.
• The ground tissue outside the ring of vascular
bundles is called the cortex. The ground tissue
inside the ring is called the pith.
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Chapter 25
Section 1 The Vascular Plant Body
Stem
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Chapter 25
Section 1 The Vascular Plant Body
Dicot and Monocot Stem Structure
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Chapter 25
Section 1 The Vascular Plant Body
Stems, continued
Woody Stems
• Trees and shrubs, such as pines, oaks, roses, and
hollies, have woody stems. Woody stems are stiff
and nongreen.
• The wood in the center of a mature stem or tree trunk
is called heartwood. The xylem in heartwood, which
can no longer conduct water, provides support.
• Sapwood, which lies outside the heartwood, contains
vessel cells that can conduct water.
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Chapter 25
Section 1 The Vascular Plant Body
Structure of Stems
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Chapter 25
Section 1 The Vascular Plant Body
Leaves
• Most leaves have a flattened portion, called the
blade, that is often attached to a stem by a stalk
called the petiole.
• A leaf blade may be divided into two or more sections
called leaflets.
• Leaves with an undivided blade are called simple
leaves. Leaves with two or more leaflets are called
compound leaves.
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Chapter 25
Section 1 The Vascular Plant Body
Leaf
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Chapter 25
Section 1 The Vascular Plant Body
Simple and Compound Leaves
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Chapter 25
Section 1 The Vascular Plant Body
Leaves, continued
• Both xylem and phloem are found in the veins of a
leaf. Veins are extensions of vascular bundles that
run from the tips of roots to the edges of leaves.
• In leaves, the ground tissue is called mesophyll.
Mesophyll cells are packed with chloroplasts, where
photosynthesis occurs.
• Most plants have leaves with two layers of mesophyll.
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Chapter 25
Section 1 The Vascular Plant Body
Structure of a Leaf
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Chapter 25
Section 2 Transport in Plants
Objectives
• Relate transpiration to the movement of water up a
plant.
• Describe how guard cells regulate the rate of
transpiration.
• Recognize several distinguishing features of sugar
maple trees.
• Describe the process of translocation in a plant.
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Chapter 25
Section 2 Transport in Plants
Movement of Water
• Transpiration drives the movement of water through a
plant:
Step 1 When the stomata are open, water vapor
diffuses out of a leaf. This loss of water vapor from a
plant is called transpiration.
Step 2 The loss of water creates a pull that draws
water up through the xylem.
Step 3 Water drawn into the roots from the soil by
osmosis moves up the stem.
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Chapter 25
Section 2 Transport in Plants
Water Movement in Plants
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Chapter 25
Section 2 Transport in Plants
Movement of Water, continued
Guard Cells and Transpiration
• A stoma is surrounded by a pair of guard cells that
are shaped like two cupped hands.
• Guard cells that take in water bend away from each
other, opening the stoma and allowing transpiration
to proceed.
• When water leaves the guard cells, they shorten and
move closer to each other, closing the stoma and
stopping transpiration.
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Chapter 25
Section 2 Transport in Plants
Control of Stomatal Opening
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Chapter 25
Section 2 Transport in Plants
Movement of Organic Compounds
• Botanists use the term source to refer to a part of a
plant that provides organic compounds for other parts
of the plant.
• Botanists use the term sink to refer to a part of a
plant that organic compounds are delivered to.
• The movement of organic compounds within a plant
from a source to a sink is called translocation.
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Chapter 25
Section 2 Transport in Plants
Translocation
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Chapter 25
Section 2 Transport in Plants
Movement of Organic Compounds, continued
• The German botanist Ernst Münch proposed a model of
translocation called the pressure-flow model:
Step 1 Sugar from a source enters phloem cells by active
transport.
Step 2 When the sugar concentration in the phloem increases,
water enters the sieve tubes in phloem from xylem by osmosis.
Step 3 Pressure builds up inside the sieve-tube cells and
pushes sugar through the sieve tubes.
Step 4 Sugar moves from phloem cells into a sink by active
transport.
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Chapter 25
Section 2 Transport in Plants
The PressureFlow Model
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