The Plant Body - Recinto Universitario de...
Transcript of The Plant Body - Recinto Universitario de...
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33The Plant Body
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Chapter 33 Key Concepts
33.1 The Plant Body Is Organized in a
Distinctive Way
33.2 Plant Organs Are Made Up of Three
Tissue Systems
33.3 Meristems Build a Continuously
Growing Plant
33.4 Domestication Has Altered Plant
Form
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Investigating Life: Bread of the Tropics
How might plant biologists improve
the cassava plant for human use?
Roots of the cassava plant store starch,
and it is important in the diets of over
800 million people around the world.
But cassava is a poor source of protein,
and the roots contain cyanide so they
must be carefully prepared.
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Key Concept 33.1 Focus Your Learning
• Plant growth patterns reflect challenges
imposed by scarce resources and an
inability to move.
• Plant development is influenced by
apical meristems, totipotency,
vacuoles, and cell walls.
• Two basic patterns that develop early
in plant embryogenesis are apical–
basal polarity and radial symmetry.
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33.1 The Plant Body Is Organized in a Distinctive Way
Plants must harvest energy from sunlight
and collect water and mineral nutrients
from the soil.
Stems, leaves, and roots enable plants
anchored in one spot to capture scarce
resources.
Plants can grow throughout their lifetimes
and can redirect growth to respond to
environmental cues.
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33.1 The Plant Body Is Organized in a Distinctive Way
All vascular plants have essentially the
same structural organization. This
chapter describes the basic structure of
the angiosperms.
Three types of vegetative organs: roots,
stems, and leaves
• Organized into shoot systems and
root systems
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Figure 33.1 Vegetative Organs and Systems
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33.1 The Plant Body Is Organized in a Distinctive Way
Root system: Anchors plant, absorbs
water and mineral nutrients, stores
products of photosynthesis.
Extreme branching of roots provides large
surface area for absorption.
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33.1 The Plant Body Is Organized in a Distinctive Way
Shoot system: Stems, leaves, flowers.
• Leaves are the main organs of
photosynthesis.
• Stems hold and display leaves in the
sun; connect roots and leaves.
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33.1 The Plant Body Is Organized in a Distinctive Way
Shoots and roots are composed of
repeating modules called phytomers.
A shoot phytomer consists of node,
internode, and axillary buds.
A bud can develop into a leaf, a
phytomer, a flower, or a flowering stem.
The terminal bud is at the end of a stem
or branch.
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33.1 The Plant Body Is Organized in a Distinctive Way
Two major clades of angiosperms:
• Monocots—narrow-leaved plants such
as grasses, lilies, orchids, and palms.
• Eudicots—broad-leaved plants such as
soybeans, roses, sunflowers, and
maples.
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Figure 33.2 Comparing the Two Major Angiosperm Clades
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33.1 The Plant Body Is Organized in a Distinctive Way
Processes of plant development:
• Determination—commitment of cells
to their ultimate fates
• Differentiation—cell specialization
• Morphogenesis—organization of cells
into tissues and organs
• Growth—increase in body size
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33.1 The Plant Body Is Organized in a Distinctive Way
Development is influenced by 4 features:
1. Meristems—regions of
undifferentiated cells where cell
division occurs.
Apical meristems occur at tips of
shoots and roots; allow plants to
grow throughout their lives.
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33.1 The Plant Body Is Organized in a Distinctive Way
2. Totipotency
Totipotent: cells can differentiate
into any type of cell in the body.
Some differentiated plant cells can
dedifferentiate and become
totipotent.
A plant can repair damage caused
by the environment or herbivores.
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33.1 The Plant Body Is Organized in a Distinctive Way
3. Vacuoles
Mature plant cells usually have a
central vacuole containing a high
concentration of solutes.
The solutes are pumped into the
vacuole by transporter proteins in
the tonoplast, the vacuolar
membrane.
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33.1 The Plant Body Is Organized in a Distinctive Way
Active accumulation of solutes
provides osmotic force for water
uptake into the vacuole.
As the vacuole expands, it exerts
turgor pressure on the cell wall,
which keeps plants upright and is
essential for plant growth.
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33.1 The Plant Body Is Organized in a Distinctive Way
4. Cell walls
Each plant cell is surrounded by a
rigid cell wall.
Morphogenesis is controlled by the
planes of cell division, which
determine the direction in which a
piece of tissue will grow.
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Figure 33.3 Cytokinesis and Morphogenesis
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33.1 The Plant Body Is Organized in a Distinctive Way
Plants grow by cell expansion.
Proteins called expansins in the cell wall
help loosen it by disrupting noncovalent
bonds between cellulose microfibrils and
other polysaccharides.
This is followed by assembly of new
polysaccharides and microfibrils,
allowing the cell wall to grow.
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33.1 The Plant Body Is Organized in a Distinctive Way
Primary cell wall—wall of a growing cell.
When cell expansion stops, some plants
deposit more cellulose layers to form a
rigid secondary cell wall.
Secondary walls cannot expand. They
contain lignin, a complex polymer that is
a major component of wood.
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In-Text Art, Ch. 33, p. 718 (1)
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In-Text Art, Ch. 33, p. 718 (2)
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33.1 The Plant Body Is Organized in a Distinctive Way
Two basic patterns are established in the
plant embryo:
• Apical–basal axis: Arrangement of cells
and tissues along the main axis from
root to shoot.
• Radial axis: Concentric arrangement of
the tissue systems.
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Figure 33.4 Two Patterns for Plant Morphogenesis
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33.1 The Plant Body Is Organized in a Distinctive Way
The first division of a zygote results in
uneven distribution of the cytoplasm,
which establishes polarity.
One cell produces the embryo, the other
produces a supporting structure, the
suspensor.
In eudicots, the cotyledons begin to
develop in the heart stage. Elongation
results in the torpedo stage.
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Figure 33.5 Plant Embryogenesis
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33.1 The Plant Body Is Organized in a Distinctive Way
The shoot apical meristem develops
between the cotyledons.
At the other end of the axis, the root
apical meristem forms.
By the end of embryogenesis, radial
symmetry has been established; the 3
tissue systems are arranged
concentrically.
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Key Concept 33.1 Learning Outcomes
• Identify ways in which plants have
overcome the problems of scarce
resources and an inability to move.
• Analyze major differences in plant and
animal development.
• Examine the process by which a
zygote develops into an embryo with an
apical−basal axis.
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Key Concept 33.2 Focus Your Learning
• Ground tissue forms most of the plant
body and includes parenchyma,
collenchyma, and sclerenchyma. The
dermal and vascular systems have
parenchyma and sclerenchyma.
• Vascular tissue consists of xylem and
phloem, which are the plant’s transport
system.
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33.2 Plant Organs Are Made Up of Three Tissue Systems
Plant tissues are grouped into 3 tissue
systems: dermal, ground, and vascular.
These ultimately extend throughout the
plant body in a concentric arrangement.
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Figure 33.6 Three Tissue Systems Extend throughout the Plant Body
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33.2 Plant Organs Are Made Up of Three Tissue Systems
Dermal tissue system:
• Forms the epidermis, or outer covering
• Usually a single layer of cells
• Stems and roots of woody plants have
a dermal tissue called periderm
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33.2 Plant Organs Are Made Up of Three Tissue Systems
Epidermal cells can differentiate to form:
• Stomatal guard cells—form stomata
(pores) for gas exchange
• Trichomes (leaf hairs)—protection
against insects and damaging solar
radiation
• Root hairs—increase root surface area
for uptake of water and mineral
nutrients
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33.2 Plant Organs Are Made Up of Three Tissue Systems
Above ground epidermis secretes a waxy
extracellular cuticle.
Made up of cutin, a complex mixture of
waxes and cell wall polysaccharides.
It limits water loss, protects against
damaging solar radiation, and is a
barrier to pathogens.
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33.2 Plant Organs Are Made Up of Three Tissue Systems
Ground tissue system:
• Makes up most of the plant body
• Functions in storage, support, and
photosynthesis
• Has three cell types:
Collenchyma
Parenchyma
Schlerenchyma
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33.2 Plant Organs Are Made Up of Three Tissue Systems
Parenchyma cells:
• Thin primary walls, large central
vacuoles
• Middle lamella—layer of pectin that
cements adjacent cells together
• Sites of photosynthesis and storage
(e.g., starch in roots)
• Many can divide and can give rise to
new cells (e.g., to heal a wound)
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Figure 33.7 Ground Tissue Cell Types (Part 1)
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33.2 Plant Organs Are Made Up of Three Tissue Systems
Collenchyma cells:
• Primary walls thickened by pectins;
usually elongate
• Provide support in leaf petioles,
nonwoody stems, and growing organs
• Tissue is flexible; can bend without
snapping
• Celery “strings” are collenchyma cells
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Figure 33.7 Ground Tissue Cell Types (Part 2)
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33.2 Plant Organs Are Made Up of Three Tissue Systems
Sclerenchyma cells:
• Thickened secondary walls; many
undergo apoptosis after secondary wall
is laid down.
• Fibers: Elongated cells provide rigid
support; often in bundles.
• Sclereids may be densely packed as
in nut shells, or in clumps as in stone
cells in pears.
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Figure 33.7 Ground Tissue Cell Types (Part 3)
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33.2 Plant Organs Are Made Up of Three Tissue Systems
Vascular tissue system:
• Xylem distributes water and minerals
taken up by roots to all parts of the
plant.
• Phloem transports carbohydrates from
site of production (sources) to sites of
utilization or storage (sinks).
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33.2 Plant Organs Are Made Up of Three Tissue Systems
Xylem: Mature cells are dead.
Two types of tracheary elements:
1. Gymnosperms have tracheids with
pits in the secondary walls that allow
materials to move freely; major cell
type in gymnosperm wood.
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Figure 33.8 Vascular Tissue Cell Types (Part 1)
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33.2 Plant Organs Are Made Up of Three Tissue Systems
2. Flowering plants have vessels made
of vessel element cells end-to-end,
also with pits.
Pits larger diameter than tracheids.
End walls break down before death,
forming hollow tubes.
Xylem of many angiosperms also
contains tracheids.
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Figure 33.8 Vascular Tissue Cell Types (Part 2)
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33.2 Plant Organs Are Made Up of Three Tissue Systems
Phloem: Mature cells are living.
• Sieve tube elements: Cells meet end-
to-end; plasmodesmata in the end
walls enlarge to form pores—the sieve
plate.
• Some cell components break down, but
companion cells retain all organelles
and act as “life support” for sieve tube
elements.
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Figure 33.8 Vascular Tissue Cell Types (Part 3)
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Key Concept 33.2 Learning Outcomes
• List and discuss the importance of the
various functions of parenchyma.
• Define cell characteristics that make
collenchyma useful as plant support
structures.
• Compare collenchyma and
sclerenchyma in terms of their ability to
provide support to plants.
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Key Concept 33.2 Learning Outcomes
• Describe the water-conducting
elements in plants, and compare these
elements in gymnosperms and
angiosperms.
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Key Concept 33.3 Focus Your Learning
• Growth in plants can be either
determinate or indeterminate,
depending on the organ structure.
• Growth in terms of cell numbers occurs
at meristems.
• Different apical meristems are where
growth in cell numbers occurs and
gives rise to leaves, stems, flowers,
and roots.
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Key Concept 33.3 Focus Your Learning
• At the root meristem, zones of cell
division, elongation, and maturation
(differentiation) form the tissues of the
root and root cap.
• The root consists of several tissue
layers outside of the inner vascular
tissues. These tissues have different
arrangements in eudicot and monocot
roots.
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Key Concept 33.3 Focus Your Learning
• Secondary growth in eudicots causes
increase in diameter and forms wood
and bark.
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33.3 Meristems Build a Continuously Growing Plant
Plants grow toward sunlight, and toward
water and dissolved minerals in the soil.
In most animals, growth is determinate—
growth of the individual and all its parts
stops in adult stage.
Shoots and roots have indeterminate
growth—continuous throughout life.
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33.3 Meristems Build a Continuously Growing Plant
Primary growth: Cell division followed by
cell enlargement; lengthens shoots and
roots.
• Results in the primary plant body: All
non-woody parts of the plant.
Many herbaceous plants consist entirely
of primary plant body.
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33.3 Meristems Build a Continuously Growing Plant
Secondary growth: Increases plant
thickness.
Trees and shrubs have a secondary plant
body consisting of wood and bark.
Stems and roots thicken as tissues are
laid down.
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33.3 Meristems Build a Continuously Growing Plant
Meristems: Localized regions of
undifferentiated cells; source of all new
growth in adult plants.
Cells that perpetuate the meristem are
called initials (comparable to animal
stem cells).
When initials divide, some daughter cells
become specialized, others develop into
new initials.
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33.3 Meristems Build a Continuously Growing Plant
Types of meristems:
• Apical meristems result in primary
growth; give rise to every cell in the
primary body.
• Primary meristems develop from
initials; give rise to the 3 tissue
systems.
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33.3 Meristems Build a Continuously Growing Plant
• Lateral meristems orchestrate
secondary growth.
Vascular cambium and cork cambium
contribute to the secondary plant
body.
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Figure 33.9 Apical and Lateral Meristems (Part 1)
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Figure 33.9 Apical and Lateral Meristems (Part 2)
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Figure 33.9 Apical and Lateral Meristems (Part 3)
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33.3 Meristems Build a Continuously Growing Plant
Types of apical meristems:
• Vegetative meristems give rise to
stems, leaves, and roots.
• Inflorescence meristems arise from
shoot apical meristems, and in turn
develop floral meristems.
• Root apical meristems extend roots.
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33.3 Meristems Build a Continuously Growing Plant
Apical meristems give rise to primary
meristems: protoderm, ground
meristem, procambium.
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In-Text Art, Ch. 33, p. 724
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33.3 Meristems Build a Continuously Growing Plant
Root apical meristem:
• Some daughter cells become the root
cap—protects root tip as it grows
through the soil.
• The cap secretes a muco-
polysaccharide (slime) as a lubricant.
• The root cap detects gravity and
controls downward growth of roots.
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33.3 Meristems Build a Continuously Growing Plant
A quiescent center in the apical meristem
can become active if needed.
Zone of cell division: Apical and primary
meristems.
Zone of cell elongation: Newly formed
cells elongate, pushing the root farther
into the soil.
Zone of maturation: Cells begin to
differentiate.
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Figure 33.10 Tissues and Regions of the Root Tip (Part 1)
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Figure 33.10 Tissues and Regions of the Root Tip (Part 2)
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33.3 Meristems Build a Continuously Growing Plant
Root tissues:
• Arrangement of tissues is different in
monocots and eudicots.
• Protoderm gives rise to the
epidermis—protection and absorption.
Many epidermal cells produce root
hairs, which increase the root surface
area.
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Figure 33.11 Products of the Root’s Primary Meristems (Part 1)
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Figure 33.11 Products of the Root’s Primary Meristems (Part 2)
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Figure 33.11 Products of the Root’s Primary Meristems (Part 3)
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Figure 33.11 Products of the Root’s Primary Meristems (Part 4)
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33.3 Meristems Build a Continuously Growing Plant
Root tissues:
Ground meristem gives rise to the
cortex and endodermis.
Endodermal cell walls have suberin, a
waterproof substance.
Placement of suberin in certain parts
of the cell wall allows control of water
movement and mineral ions into the
vascular tissue system.
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33.3 Meristems Build a Continuously Growing Plant
Procambium produces the vascular
cylinder or stele.
Pericycle—undifferentiated cells:
• Gives rise to lateral roots
• Gives rise to lateral meristems that
thicken the root
• Membrane transport proteins move
nutrient ions into the xylem
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Figure 33.12 Lateral Root Anatomy
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33.3 Meristems Build a Continuously Growing Plant
In eudicot roots, xylem is at the center,
often in a star shape in cross section;
between the points are bundles of
phloem.
Monocot roots have pith (parenchyma
cells) at the center, which stores
carbohydrates.
Pith is also found in stems of both
monocots and eudicots.
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33.3 Meristems Build a Continuously Growing Plant
Papyrus is made from strips of pith from
stems of the papyrus plant Cyperus
papyrus.
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In-Text Art, Ch. 33, p. 725
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33.3 Meristems Build a Continuously Growing Plant
Water and minerals enter through the root
system in most plants. The root system is
often larger than the shoot system.
The embryonic root is called the radicle.
In most eudicots the radicle develops into a
primary root or taproot with outgrowth of
lateral roots, forming a taproot system.
Taproots often function as food storage.
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Figure 33.13 Root Systems of Eudicots and Monocots (Part 1)
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Figure 33.13 Root Systems of Eudicots and Monocots (Part 2)
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33.3 Meristems Build a Continuously Growing Plant
Typical monocot roots arise from the stem
near ground level and are called
adventitious roots.
They form a fibrous root system: many
thin roots of equal diameter originate
from the stem at ground level or below.
• Large surface area; cling to soil well
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33.3 Meristems Build a Continuously Growing Plant
Prop roots are adventitious roots that
help support the stem in some monocots
(corn, banyan trees, some palms).
These species cannot support
aboveground growth by the thickening of
their stems.
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Figure 33.13 Root Systems of Eudicots and Monocots (Part 3)
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33.3 Meristems Build a Continuously Growing Plant
Stem tissues
Shoots are composed of repeating
modules called phytomers; shoots grow
by adding new phytomers.
New phytomers originate from cells in
shoot apical meristems at stem tips and
axillary buds.
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33.3 Meristems Build a Continuously Growing Plant
Shoot apical meristem forms 3 primary
meristems that give rise to the 3 tissue
systems.
Leaf primordia develop on the sides of
the shoot apical meristem at regular
intervals—these sites become the nodes.
Bud primordia form at the bases of the
leaf primordia. They can become apical
meristems of new shoots.
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33.3 Meristems Build a Continuously Growing Plant
In young stems, vascular tissue is
arranged in vascular bundles of both
xylem and phloem.
• Eudicots: Vascular bundles form a
cylinder
• Monocots: Bundles are scattered
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Figure 33.14 Vascular Bundles in Stems (Part 1)
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Figure 33.14 Vascular Bundles in Stems (Part 2)
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33.3 Meristems Build a Continuously Growing Plant
In eudicots, pith is in the center and
extends between the vascular bundles,
forming pith rays.
The cortex can contain supportive
collenchyma cells with thickened walls.
Pith and cortex constitute the ground
tissue system.
The outermost cell layer is the epidermis.
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33.3 Meristems Build a Continuously Growing Plant
Stems elevate and support flowers and
leaves. There are many modifications:
• Potato tubers are underground stems;
the “eyes” are axillary buds.
• Many desert plants have enlarged
stems that store water.
• Runners are horizontal stems; roots
grow at intervals and independent
plants can arise from them.
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Figure 33.15 Modified Stems (Part 1)
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Figure 33.15 Modified Stems (Part 2)
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Figure 33.15 Modified Stems (Part 3)
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33.3 Meristems Build a Continuously Growing Plant
Leaves are produced from apical
meristems called vegetative meristems.
Growth of a leaf is determinate.
Leaf anatomy is adapted to carry out
photosynthesis and exchange of O2 and
CO2 with the environment, while limiting
water losses.
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Figure 33.16 The Eudicot Leaf (Part 1)
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33.3 Meristems Build a Continuously Growing Plant
Two zones of photosynthetic parenchyma
cells make up the mesophyll:
• Palisade mesophyll
• Spongy mesophyll—includes air space
for diffusion of gases
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33.3 Meristems Build a Continuously Growing Plant
Vascular tissue forms a network of veins
in leaves.
Veins extend to within a few cell
diameters of all the cells, so mesophyll
cells are well supplied with water and
minerals, and the products of
photosynthesis can be conducted to the
phloem.
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Figure 33.16 The Eudicot Leaf (Part 2)
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Investigating Life: Understanding the Synthesis and Transport of Cyanogenic
Glycosides
Leaves may also produce defensive
chemicals, such as cyanide in the
cassava plant.
Hypothesis: Molecules that produce
cyanide in the cassava are made in the
leaves and transported to the
underground root.
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Investigating Life: Understanding the Synthesis and Transport of Cyanogenic Glycosides,
Experiment
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Investigating Life: Understanding the Synthesis and Transport of
Cyanogenic Glycosides
Conclusion:
Cyanogenic glycosides are made in
leaves and are transported through
petioles to the stem, from which they are
transported to the root.
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33.3 Meristems Build a Continuously Growing Plant
Epidermal cells are nonphotosynthetic,
and have a waxy cuticle that is
impermeable to water.
The cuticle prevents water loss, but also
prevents diffusion of gases.
Pores called stomata allow gas
exchange. They are opened and closed
by guard cells.
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Figure 33.16 The Eudicot Leaf (Part 3)
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33.3 Meristems Build a Continuously Growing Plant
Secondary growth (wood and bark) arises
from two lateral meristems in eudicots:
• Vascular cambium: Elongated cells
that divide often; supplies cells of
secondary xylem and secondary
phloem.
• Cork cambium produces waxy-walled
protective cells; some become part of
the bark.
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33.3 Meristems Build a Continuously Growing Plant
Woody twigs have both primary and
secondary growth.
Apical meristems are enclosed in buds
protected by bud scales.
Only the buds consist entirely of primary
tissues.
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Figure 33.17 A Woody Twig Has Both Primary and Secondary Growth (Part 1)
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Figure 33.17 A Woody Twig Has Both Primary and Secondary Growth (Part 2)
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Figure 33.17 A Woody Twig Has Both Primary and Secondary Growth (Part 3)
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33.3 Meristems Build a Continuously Growing Plant
Vascular cambium is initially a single layer
of cells between primary xylem and
phloem.
Division of these cells produces
secondary phloem cells toward the
outside, and secondary xylem cells
toward the inside.
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33.3 Meristems Build a Continuously Growing Plant
A continuous cylinder of vascular
cambium runs the length of the stem and
gives rise to complete cylinders of
secondary xylem (wood) and secondary
phloem, which contributes to the bark.
It also produces vascular rays for lateral
transport.
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33.3 Meristems Build a Continuously Growing Plant
As secondary growth continues, the
epidermis and outer cortex are stretched
and flake away.
Cells near the surface of the secondary
phloem begin to divide, forming a cork
cambium.
Cork has thick-walled cells, waterproofed
with suberin. Cork becomes the
outermost tissue of the stem or root.
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33.3 Meristems Build a Continuously Growing Plant
Cork cambium sometimes produces cells
toward the inside, which forms the
phelloderm.
The cork, cork cambium, and phelloderm
form a tissue called periderm.
The periderm and secondary phloem— all
the tissues external to the vascular
cambium—constitute the bark.
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33.3 Meristems Build a Continuously Growing Plant
Lenticels are spongy regions in the
periderm that allow gas exchange for
underlying tissues.
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Figure 33.18 Lenticels Allow Gas Exchange through the Periderm
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33.3 Meristems Build a Continuously Growing Plant
Tree trunks from temperate regions have
annual rings that result from seasonal
conditions.
• Spring—water is plentiful, tracheids or
vessel elements produced have large
diameters.
• Summer—less water, smaller diameter
cells with thicker walls are produced.
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Figure 33.19 Annual Rings
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33.3 Meristems Build a Continuously Growing Plant
Some monocots, such as palms, have
thickened stems, but they do not have
vascular or cork cambiums.
Palms have a wide apical meristem that
produces a wide stem. Dead leaf bases
also contribute to the stem diameter.
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Key Concept 33.3 Learning Outcomes
• Describe characteristics of determinate
and indeterminate growth in plants.
• Compare meristematic with non-
meristematic cells, and explain the
function of meristematic cells in plant
growth.
• Analyze how apical meristems are able
to produce different organs, including
leaves, stems, flowers, and roots.
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Key Concept 33.3 Learning Outcomes
• Describe or illustrate a longitudinal
section of the developing root, and
explain the functions of special zones
of cells from the tip upward.
• Explain how different types of meristem
result in the various layers of the
mature root.
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Key Concept 33.4 Focus Your Learning
• Genetic variation in the structure of
plants is a valuable natural resource for
crop plant evolution.
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33.4 Domestication Has Altered Plant Form
Members of the same plant species can
be remarkably diverse in form.
This suggests that minor differences in
genes or gene regulation can underlie
dramatic differences in plant form.
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33.4 Domestication Has Altered Plant Form
Modern corn was domesticated from the
grass teosinte, which still grows in
Mexico.
Teosinte is highly branched, while
domesticated corn has a single shoot.
This is due to a single gene called
teosinte branched 1 (tb1). The protein
product regulates growth of axillary
buds.
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Figure 33.20 Modern Corn Was Domesticated from the Wild Grass Teosinte
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33.4 Domestication Has Altered Plant Form
A single species, Brassica oleracea (wild
mustard), is the ancestor of many
morphologically diverse crops: kale,
broccoli, Brussels sprouts, cabbage.
Humans selected seed from
morphological variants in the wild
population with the trait they found
desirable.
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Figure 20.4 Many Vegetables from One Species
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33.4 Domestication Has Altered Plant Form
The genomes of plants are still priceless
resources today.
Genetic variation in crop plants and their
wild relatives can be used to improve our
crop plants or adapt them to changing
conditions.
This is especially important as human
activities change the planet and lead to
extinction of plant species.
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33.4 Domestication Has Altered Plant Form
Various organizations around the world
have developed seed banks, where
seeds of diverse species and variants
are stored.
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Key Concept 33.4 Learning Outcomes
• Discuss reasons why wild plant
genomes should be preserved.
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Investigating Life: Bread of the Tropics
Many people depend on cassava for food,
but the roots must be processed to
remove the sources of cyanide.
RNA interference has been used to block
cassava leaves from making the cyanide
precursor.
How might plant biologists improve
the cassava plant for human use?
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Investigating Life: Bread of the Tropics
BioCassava Plus is a consortium of
scientists working to improve nutritional
quality and drought resistance.
Cassava has been crossed with a treelike
relative; the resulting plants have edible
roots that grow deep, where they can tap
into deep water supplies.