24
The Plant Body
Chapter 24 The Plant Body
Key Concepts
•  24.1 The Plant Body Is Organized and
Constructed in a Distinctive Way
Chapter 24 Opening Question
What are the properties of the kenaf plant
that make it suitable for papermaking?
•  24.2 Meristems Build Roots, Stems, and
Leaves
•  24.3 Domestication Has Altered Plant Form
Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
Plants must harvest energy from sunlight and
mineral nutrients from the soil.
Body plans and physiology enable plants to do
these things.
They also grow throughout their lifetime; they
can redirect growth to respond to
environmental opportunities.
Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
Figure 24.1 Vegetative Plant Organs and Systems
Two systems of plant vegetative organs:
Root system—anchors plant, absorbs water
and minerals, stores products of
photosynthesis. Branching increases surface
area.
Shoot system
•  Leaves—main photosynthetic organs
•  Stems—hold leaves up in the sunlight; connect
roots and leaves
Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
Plant development is influenced by three unique
properties:
•  Apical meristems
•  Cell walls
•  Totipotency of most cells.
Apical meristems are always embryonic,
producing new tissues throughout the plant’s
life.
Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
Figure 24.2 Cytokinesis and Morphogenesis
Cell walls are a rigid extracellular matrix.
Plant morphogenesis occurs through changes in
the plane of cell division at cytokinesis.
This changes the direction of tissue growth.
Cytokinesis can be uneven; location of the cell
plate is determined by differentiation signals
early in mitosis.
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Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
Most plant cells are totipotent (can differentiate
into any kind of cell).
Two growth patterns are established in the
embryo:
Plants can readily repair damage caused by the
environment or herbivores.
•  Apical–basal axis: arrangement of cells and
tissues along the main axis
Figure 24.3 Two Patterns for Plant Morphogenesis
•  Radial axis: concentric arrangement of the
tissue systems
Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
Figure 24.4 Plant Embryogenesis
Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
First division of zygote is uneven; sets up apicalbasal axis and polarity.
In eudicots, the cotyledons begin to grow, and a
shoot apical meristem forms between them.
•  Smaller cell becomes the embryo
At the other end of the embryo, a root apical
meristem forms.
•  Larger cell becomes a supporting structure
(suspensor)
Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
Three tissue systems are established during
embryogensis:
Aboveground epidermal cells secrete a waxy
cuticle.
1. Dermal—forms epidermis, usually one cell
layer.
Limits water loss, reflects damaging solar
radiation, barrier against pathogens.
Figure 24.5 Three Tissue Systems Extend throughout the Plant Body
Some cells differentiate:
•  Stomata—pores for gas exchange
•  Trichomes—leaf hairs, protect from
herbivores and damaging solar radiation
•  Root hairs—increase root surface area
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Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
2. Ground tissue—between dermal and vascular
tissue;
Three cell types:
•  Parenchyma cells
•  Collenchyma cells
•  Sclerenchyma cells
Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
In-Text Art, Ch. 24, p. 510 (1)
Parenchyma cells
•  most abundant
•  large vacuoles and thin cell walls
•  do photosynthesis
•  store protein and starch
In-Text Art, Ch. 24, p. 510 (2)
Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
Collenchyma cells
Sclerenchyma cells
•  elongated
•  very thick walls reinforced with lignin
•  thick cell walls
•  undergo programmed cell death
•  provide support
•  cell walls remain to provide support
In-Text Art, Ch. 24, p. 510 (4)
Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
In-Text Art, Ch. 24, p. 510 (5)
3. Vascular tissue—the transport system
Xylem carries water and minerals from roots to
rest of plant.
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Concept 24.1 The Plant Body Is Organized
and Constructed in a Distinctive Way
In-Text Art, Ch. 24, p. 511 (1)
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Phloem
Primary growth—lengthening of shoots and
roots; branching.
•  living cells
Results in nonwoody tissues—herbaceous
•  moves carbohydrates from production sites to
sites where they are used or stored
Secondary growth—increase in thickness
Woody plants have a secondary plant body
consisting of wood and bark.
Figure 24.6 Apical and Lateral Meristems (Part 1)
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Many vegetative organs have evolved novel
roles, such as roots or stems that are used to
store water.
When cells divide in meristem tissue, one
daughter cell can differentiate, the other
remains undifferentiated.
These are examples of natural selection working
with what is already present and the interaction
between evolution and development.
Apical meristems result in primary growth; cell
division followed by cell elongation
Figure 24.6 Apical and Lateral Meristems (Part 2)
Lateral meristems result in secondary growth
Figure 24.6 Apical and Lateral Meristems (Part 3)
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Apical meristems can divide indefinitely, so
growth of roots and shoots is indeterminate.
Apical meristems produce primary meristems.
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Figure 24.7 Tissues and Regions of the Root Tip (Part 1)
Figure 24.7 Tissues and Regions of the Root Tip (Part 2)
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Root apical meristems
Daughter cells on the root tip form the root cap
—protects root as it pushes through soil.
Root cap cells detect gravity and control
downward growth of the root.
Above the root cap, three zones result as cells
divide and mature.
Figure 24.8 Products of the Root’s Primary Meristems (Part 1)
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Root primary meristems give rise to root tissues:
•  Protoderm produces the epidermis; many
epidermal cells have root hairs.
•  Ground meristem produces the cortex,
consisting of parenchyma cells and the
endodermis.
Endodermal cells have waterproof suberin in
the cell walls and can control movement of
water and mineral ions into the vascular
system.
Figure 24.8 Products of the Root’s Primary Meristems (Part 2)
Concept 24.2 Meristems Build Roots, Stems, and Leaves
•  Procambium produces the vascular cylinder
(stele), made up of pericycle, xylem, phloem.
Pericycle has 3 functions:
• Tissue within which lateral roots arise.
• Contributes to secondary growth by giving rise
to lateral meristems.
• Membrane transport proteins export nutrient
ions into the xylem.
Figure 24.8 Products of the Root’s Primary Meristems (Part 3)
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Angiosperm roots begin to grow as a radicle,
which develops into the primary root (taproot)
in eudicots.
Taproots often store nutrients (e.g., carrots,
beets, sweet potato).
Monocots form a fibrous root system; roots are
equal in diameter (e.g., grasses, leeks). Also
called adventitious roots.
Some monocots have prop roots to support the
shoot (e.g., corn, banyan trees).
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Figure 24.9 Root Systems of Eudicots and Monocots (Part 1)
Figure 24.9 Root Systems of Eudicots and Monocots (Part 2)
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Figure 24.9 Root Systems of Eudicots and Monocots (Part 3)
Figure 24.10 Vascular Bundles in Stems (Part 1)
Shoots are composed of repeating modules
(phytomers).
Each has a node with attached leaves,
internode (stem section), and one or more
axillary buds.
Shoots grow by adding more phytomers.
Shoot apical meristem also produces three
primary meristems, which give rise to shoot
tissue systems.
Stems have vascular bundles with xylem,
phloem, and fibers.
The bundles have different arrangements in
eudicots and monocots.
Figure 24.10 Vascular Bundles in Stems (Part 2)
Figure 24.11 Modified Stems (Part 1)
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Stem modifications
Potato tubers are underground stems; the
“eyes” are axillary buds.
Many desert plants have enlarged stems that
store water.
Strawberry plant runners are horizontal stems
from which roots grow. If the runners break,
new plants develop on either side (asexual
reproduction).
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Figure 24.11 Modified Stems (Part 2)
Figure 24.11 Modified Stems (Part 3)
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Growth of leaves is determinate: they stop
growing once they reach a predetermined
mature size.
Leaves consist of a blade, attached to the plant
stem by a petiole.
Leaves are often oriented perpendicular to the
sun’s rays, to maximize the amount of light for
photosynthesis.
Figure 24.12 Eudicot Leaf Anatomy
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Leaf anatomy is well adapted to:
•  Carry out photosynthesis
•  Exchange O2 and CO2 with the environment
•  Limit evaporative water loss
•  Export products of photosynthesis to the rest of
the plant
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Leaf surfaces are covered with
nonphotosynthetic epidermal cells.
They secrete the waterproof cuticle.
Water and gases are exchanged through pores
called stomata.
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Leaf mesophyll has two zones of photosynthetic
parenchyma tissue.
A network of air spaces allows CO2 to diffuse to
photosynthetic cells.
Vascular bundles form veins that extend to within
a few diameters of all cells—to ensure transport
of water and minerals in and carbohydrates out.
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Leaves can also be modified for other functions:
•  Nutrient storage (e.g., onion bulbs)
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Many eudicot stems and roots have secondary
growth:
•  Water storage (e.g., in succulent plants)
Wood and bark are derived by secondary growth
from the two lateral meristems:
•  Protection (e.g., cacti have spines that are
modified leaves)
• Vascular cambium produces secondary xylem
(wood) and secondary phloem (inner bark).
•  Tendrils that wrap around structures to support
climbing plants (e.g., peas)
• Cork cambium produces waxy-walled
protective cells; some become the outer bark.
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Figure 24.13 A Woody Twig Has Both Primary and Secondary Tissues (Part 1)
Figure 24.13 A Woody Twig Has Both Primary and Secondary Tissues (Part 2)
Figure 24.13 A Woody Twig Has Both Primary and Secondary Tissues (Part 3)
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Concept 24.2 Meristems Build Roots, Stems, and Leaves
A stem or root increases in diameter when cells
of the vascular cambium divide, producing
secondary xylem cells toward the inside and
secondary phloem cells toward the outside.
Some cells of the secondary phloem divide and
form a cork cambium, which produces layers of
protective cork.
Cork cambium produces cells to the inside,
forming the phelloderm.
In temperate zones, annual rings form in the
wood.
Periderm—secondary dermal tissue composed
of cork cambium, cork, and phelloderm
In spring, tracheids or vessel elements tend to
be large in diameter and thin-walled.
Bark—periderm plus secondary phloem
In summer, thick-walled, narrow cells are
produced.
The cork soon becomes the outermost tissue of
the stem or root.
Figure 24.14 Annual Rings
Concept 24.2 Meristems Build Roots, Stems, and Leaves
Monocots do not have secondary growth.
A few have thickened stems (e.g., palms).
Palms have a very wide apical meristem that
produces a wide stem, and dead leaf bases
add to the diameter of the stem.
Concept 24.3 Domestication Has Altered Plant Form
A simple body plan underlies the diversity of
flowering plant forms.
Plant body form is subject to natural selection.
Example: some plants have become vines; the
climbing phenotype gives them access to light
in crowded conditions.
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Figure 24.15 Corn Was Domesticated from the Wild Grass Teosinte
Concept 24.3 Domestication Has Altered Plant Form
Concept 24.3 Domestication Has Altered Plant Form
Humans domesticate crop plants by artificial
selection for phenotypes best suited for
agriculture.
Brassica oleracea (wild mustard) is the ancestor
of several morphologically diverse crops: kale,
broccoli, brussels sprouts, cabbage.
Corn was domesticated from the wild grass
teosinte, which still grows in Mexico.
Starting with diverse populations of wild mustard,
humans selected and planted seeds from
variants with traits they found desirable.
Teosinte is highly branched; corn has a single
shoot. Branching is controlled by a single gene
that regulates axillary buds.
Figure 15.4 Many Vegetables from One Species
Answer to Opening Question
Answer to Opening Question
•  Kenaf grows rapidly, reaching 4–5 meters in 5
months.
Kenaf phloem fibers are longer than wood fibers,
which makes stronger products.
•  There are over 500 known genetic strains.
Cells walls don’t have lignin, making it easier to
pulp.
•  Since its domestication, it has been selected to
grow taller and branch less.
•  The adventitious roots have become longer (2
meters!), and more numerous, promoting
growth in dense stands.
A hectare of kenaf produces 3 times more fiber
than a hectare of southern pine (which takes
20 years to grow).
Figure 24.16 Kenaf Stems
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