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CAMPBELL
BIOLOGY
TENTH
EDITION
Reece • Urry • Cain • Wasserman •
Minorsky • Jackson
35
Plant Structure
and Growth
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
© 2014 Pearson Education, Inc.
Are Plants Computers?
 Romanesco grows according to a repetitive
program
 The development of plants depends on the
environment and is highly adaptive
 Monocots and eudicots are the two major groups
of angiosperms
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Figure 35.1
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Figure 35.1a
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Concept 35.1: Plants have a hierarchical
organization consisting of organs, tissues, and
cells
 Plants have organs composed of different tissues,
which in turn are composed of different cell types
 An organ consists of several types of tissues that
together carry out particular functions
 A tissue is a group of cells consisting of one or
more cell types that together perform a specialized
function
© 2014 Pearson Education, Inc.
The Three Basic Plant Organs: Roots, Stems,
and Leaves
 Basic morphology of vascular plants reflects their
evolution as organisms that draw nutrients from
below ground and above ground
 Plants take up water and minerals from below
ground
 Plants take up CO2 and light from above ground
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 Three basic organs evolved: roots, stems, and
leaves
 They are organized into a root system and a
shoot system
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Figure 35.2
Reproductive shoot (flower)
Apical bud
Node
Internode
Apical bud
Vegetative
shoot
Shoot
system
Blade
Petiole
Axillary bud
Leaf
Stem
Taproot
Lateral
(branch)
roots
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Root
system
 Roots rely on sugar produced by photosynthesis in
the shoot system
 Shoots rely on water and minerals absorbed by
the root system
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Roots
 A root is an organ with important functions
 Anchoring the plant
 Absorbing minerals and water
 Storing carbohydrates
 The primary root is the first root to emerge
 The primary root branches to form lateral roots,
which improve anchorage and water absorption
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 Tall plants with large shoot masses have a taproot
system
 The taproot develops from the primary root and
prevents the plant from toppling; lateral roots are
responsible for absorption
 Small or trailing plants generally have a fibrous
root system, which consists of
 Adventitious roots that arise from stems or leaves
 Lateral roots that arise from the adventitious roots
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 In most plants, absorption of water and minerals
occurs near the tips of roots
 Root hairs near the root tip increase the surface
area of the root
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Figure 35.3
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 Many plants have root adaptations with
specialized functions
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Figure 35.4
Prop roots
Storage
roots
Pneumatophores
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“Strangling” aerial
roots
Buttress roots
Figure 35.4a
Prop roots
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Figure 35.4b
Storage roots
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Figure 35.4c
Pneumatophores
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Figure 35.4d
“Strangling” aerial roots
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Figure 35.4e
Buttress roots
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Stems
 A stem is an organ consisting of
 An alternating system of nodes, the points at which
leaves are attached
 Internodes, the stem segments between nodes
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 The growing shoot tip, or apical bud, causes
elongation of a young shoot
 An axillary bud is a structure that has the
potential to form a lateral branch, thorn, or flower
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 Many plants have modified stems (e.g., rhizomes,
stolons, tubers)
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Figure 35.5
Stolon
Rhizome
Root
Rhizomes
Stolons
Tubers
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Figure 35.5a
Rhizome
Root
Rhizomes
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Figure 35.5b
Stolon
Stolons
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Figure 35.5c
Tubers
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Leaves
 The leaf is the main photosynthetic organ of most
vascular plants
 Leaves intercept light, exchange gases, dissipate
heat, and defend the plant from herbivores and
pathogens
 Leaves generally consist of a flattened blade and
a stalk called the petiole, which joins the leaf to a
node of the stem
© 2014 Pearson Education, Inc.
 Monocots and eudicots differ in the arrangement
of veins, the vascular tissue of leaves
 Most monocots have parallel veins
 Most eudicots have branching veins
 In classifying angiosperms, taxonomists may use
leaf morphology as a criterion
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Figure 35.6
Simple leaf
Axillary
bud
Petiole
Compound leaf
Leaflet
Axillary
bud
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Petiole
 Some plant species have evolved modified leaves
that serve various functions
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Figure 35.7
Spines
Tendrils
Storage leaves
Stem
Reproductive leaves
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Storage leaves
Figure 35.7a
Spines
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Figure 35.7b
Tendrils
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Figure 35.7c
Reproductive leaves
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Figure 35.7d
Storage leaves
Stem
Storage leaves
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Dermal, Vascular, and Ground Tissue Systems
 Each plant organ has dermal, vascular, and
ground tissues
 Each of these three categories forms a tissue
system
 Each tissue system is continuous throughout the
plant
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Figure 35.8
Dermal
tissue
Ground
tissue
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Vascular
tissue
 In nonwoody plants, the dermal tissue system
consists of the epidermis
 A waxy coating called the cuticle helps prevent
water loss from the epidermis
 In woody plants, protective tissues called
periderm replace the epidermis in older regions of
stems and roots
 Trichomes are outgrowths of the shoot epidermis
and can help with reducing water loss and insect
defense
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Trichomes
© 2014 Pearson Education, Inc.
300 μm
Figure 35.9
 The vascular tissue system facilitates the
transport of materials through the plant and
provides mechanical support
 The two vascular tissues are xylem and phloem
 Xylem conducts water and dissolved minerals
upward from roots into the shoots
 Phloem transports sugars from where they are
made to where they are needed
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 The vascular tissue of a root or stem is collectively
called the stele
 In angiosperms the stele of the root is a solid
central vascular cylinder
 The stele of stems and leaves is divided into
vascular bundles, strands of xylem and phloem
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 Tissues that are neither dermal nor vascular are
the ground tissue system
 Ground tissue internal to the vascular tissue is
pith; ground tissue external to the vascular tissue
is cortex
 Ground tissue includes cells specialized for
storage, photosynthesis, support, and transport
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Common Types of Plant Cells
 As in any multicellular organism, a plant is
characterized by cellular differentiation, the
specialization of cells in structure and function
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 The major types of plant cells are
 Parenchyma
 Collenchyma
 Sclerenchyma
 Water-conducting cells of the xylem
 Sugar-conducting cells of the phloem
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Parenchyma Cells
 Mature parenchyma cells
 Have thin and flexible primary walls
 Lack secondary walls
 Are the least specialized
 Perform the most metabolic functions
 Retain the ability to divide and differentiate
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Figure 35.10a
Parenchyma cells in a privet
(Ligustrum) leaf (LM)
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25 μm
Collenchyma Cells
 Collenchyma cells are grouped in strands and
help support young parts of the plant shoot
 They have thicker and uneven cell walls
 These cells provide flexible support without
restraining growth
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Figure 35.10b
Collenchyma cells
(in Helianthus stem) (LM)
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5 μm
Sclerenchyma Cells
 Sclerenchyma cells are rigid because of thick
secondary walls strengthened with lignin, an
indigestible strengthening polymer
 They are dead at functional maturity
 There are two types
 Sclereids are short and irregular in shape and have
thick lignified secondary walls
 Fibers are long and slender and arranged in
threads
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Figure 35.10c
5 μm
Sclereid cells in pear (LM)
25 μm
Cell wall
Fiber cells (cross section from ash tree) (LM)
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Figure 35.10ca
5 μm
Sclereid cells
in pear (LM)
Cell wall
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Figure 35.10cb
25 μm
Fiber cells (cross section from ash tree) (LM)
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Water-Conducting Cells of the Xylem
 The two types of water-conducting cells,
tracheids and vessel elements, are dead at
maturity
 Tracheids are found in the xylem of all vascular
plants
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Figure 35.10d
Vessel Tracheids 100 μm
Pits
Tracheids and vessels
(colorized SEM)
Perforation
plate
Vessel
element
Vessel elements, with
perforated end walls
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Tracheids
Figure 35.10da
Vessel Tracheids 100 μm
Tracheids and vessels
(colorized SEM)
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 Vessel elements are common to most
angiosperms and a few gymnosperms
 Vessel elements align end to end to form long
micropipes called vessels
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Sugar-Conducting Cells of the Phloem
 Sieve-tube elements are alive at functional
maturity, though they lack organelles
 Sieve plates are the porous end walls that allow
fluid to flow between cells along the sieve tube
 Each sieve-tube element has a companion cell
whose nucleus and ribosomes serve both cells
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Figure 35.10e
3 μm
Sieve-tube elements:
longitudinal view (LM)
Sieve plate
Sieve-tube element (left)
and companion cell:
cross section (TEM)
Companion
cells
Sieve-tube
elements
Plasmodesma
Sieve
plate
30 μm
Nucleus of
companion
cell
15 μm
Sieve-tube elements:
longitudinal view
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Sieve plate with pores (LM)
Figure 35.10ea
3 μm
Sieve-tube element (left)
and companion cell:
cross section (TEM)
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Figure 35.10eb
Sieve-tube elements:
longitudinal view (LM)
Sieve plate
Companion
cells
Sieve-tube
elements
30 μm
© 2014 Pearson Education, Inc.
Figure 35.10ec
Sieve
plate
15 μm
Sieve plate with pores (LM)
© 2014 Pearson Education, Inc.
BioFlix: Tour of a Plant Cell
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Concept 35.2: Different meristems generate new
cells for primary and secondary growth
 A plant can grow throughout its life; this is called
indeterminate growth
 Meristems are perpetually embryonic tissue and
allow for indeterminate growth
 Some plant organs cease to grow at a certain size;
this is called determinate growth
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 There are two main types of meristems: apical
meristems and lateral meristems
 Apical meristems are located at the tips of roots
and shoots
 Apical meristems elongate shoots and roots, a
process called primary growth
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 Lateral meristems add thickness to woody plants,
a process called secondary growth
 There are two lateral meristems: the vascular
cambium and the cork cambium
 The vascular cambium adds layers of vascular
tissue called secondary xylem (wood) and
secondary phloem
 The cork cambium replaces the epidermis with
periderm, which is thicker and tougher
© 2014 Pearson Education, Inc.
Figure 35.11
Primary growth in stems
Epidermis
Cortex
Shoot tip
(shoot apical
meristem and
young leaves)
Axillary bud
meristem
Primary phloem
Primary xylem
Pith
Vascular
cambium
Cork cambium
Lateral
Secondary growth in stems
meristems
Cork cambium
Periderm
Pith
Root apical
meristems
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Primary
xylem
Secondary
xylem
Cortex
Primary
phloem
Secondary
phloem
Vascular
cambium
Figure 35.11a
Shoot tip
(shoot apical
meristem and
young leaves)
Axillary bud
meristem
Root apical
meristems
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Vascular cambium
Cork cambium
Lateral
meristems
Figure 35.11b
Primary growth in stems
Epidermis
Cortex
Primary phloem
Primary xylem
Pith
Secondary growth in stems
Periderm
Pith
Primary
xylem
Secondary
xylem
© 2014 Pearson Education, Inc.
Cortex
Primary
phloem
Secondary
phloem
Vascular
cambium
 Meristems give rise to
 Initials, also called stem cells, which remain in the
meristem
 Derivatives, which become specialized in mature
tissues
 In woody plants, primary growth and secondary
growth occur simultaneously but in different
locations
© 2014 Pearson Education, Inc.
Figure 35.12
Apical bud
Bud scale
Axillary buds
This year’s growth
(one year old)
Leaf
scar
Bud
scar
Node
Internode
Last year’s growth
(two years old)
Leaf scar
Stem
Bud scar
Growth of two
years ago
(three years old)
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Leaf scar
One-year-old
branch formed
from axillary bud
near shoot tip
 Flowering plants can be categorized based on the
length of their life cycle
 Annuals complete their life cycle in a year or less
 Biennials require two growing seasons
 Perennials live for many years
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Concept 35.3: Primary growth lengthens roots
and shoots
 Primary growth arises from cells produced by
apical meristems and elongates parts of the root
and shoot systems
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Primary Growth of Roots
 The root tip is covered by a root cap, which
protects the apical meristem as the root pushes
through soil
 Growth occurs just behind the root tip, in three
zones of cells
 Zone of cell division
 Zone of elongation
 Zone of differentiation, or maturation
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Figure 35.13
Cortex
Vascular cylinder
Epidermis
Root hair
Dermal
Ground
Vascular
Zone of
differentiation
Zone of
elongation
Zone of cell
division
(including
apical
meristem)
Root cap
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Mitotic
cells
100 μm
Figure 35.13a
Mitotic
cells
100 μm
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Video: Root Growth in a Radish Seed (Time
Lapse)
© 2014 Pearson Education, Inc.
 The primary growth of roots produces the
epidermis, ground tissue, and vascular tissue
 In angiosperm roots, the stele is a vascular
cylinder
 In most eudicots, the xylem is starlike in
appearance with phloem between the “arms”
 In many monocots, a core of parenchyma cells is
surrounded by rings of xylem then phloem
© 2014 Pearson Education, Inc.
Figure 35.14
Epidermis
Cortex
Endodermis
Vascular cylinder
Pericycle
Core of
parenchyma
cells
100 μm
(a) Root with xylem and phloem in
the center (typical of eudicots)
Xylem
Phloem
(b) Root with parenchyma in the
center (typical of monocots)
Endodermis
Pericycle
Xylem
Phloem
70 μm
© 2014 Pearson Education, Inc.
100 μm
Dermal
Ground
Vascular
Figure 35.14a
Epidermis
Cortex
Endodermis
Vascular cylinder
Pericycle
Xylem
Phloem
100 μm
(a) Root with xylem and
phloem in the center
(typical of eudicots)
© 2014 Pearson Education, Inc.
Dermal
Ground
Vascular
Figure 35.14b
Epidermis
Cortex
Endodermis
Vascular cylinder
Pericycle
Core of
parenchyma
cells
Xylem
Phloem
Dermal
Ground
Vascular
© 2014 Pearson Education, Inc.
100 μm
(b) Root with parenchyma in the
center (typical of monocots)
Figure 35.14c
Endodermis
Pericycle
Xylem
Phloem
70 μm
© 2014 Pearson Education, Inc.
Ground
Vascular
 The ground tissue, mostly parenchyma cells, fills
the cortex, the region between the vascular
cylinder and epidermis
 The innermost layer of the cortex is called the
endodermis
 The endodermis regulates passage of substances
from the soil into the vascular cylinder
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 Lateral roots arise from within the pericycle, the
outermost cell layer in the vascular cylinder
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Figure 35.15-1
Emerging
lateral
root
100 μm
Cortex
Vascular
cylinder
1
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Pericycle
Figure 35.15-2
Emerging
lateral
root
100 μm
Epidermis
Lateral root
Cortex
Vascular
cylinder
1
© 2014 Pearson Education, Inc.
Pericycle
2
Figure 35.15-3
Emerging
lateral
root
100 μm
Epidermis
Lateral root
Cortex
Vascular
cylinder
1
© 2014 Pearson Education, Inc.
Pericycle
2
3
Figure 35.15a
Emerging
lateral
root
100 μm
Cortex
Vascular
cylinder
© 2014 Pearson Education, Inc.
Pericycle
Figure 35.15b
100 μm
© 2014 Pearson Education, Inc.
Epidermis
Lateral root
Figure 35.15c
Epidermis
Lateral root
© 2014 Pearson Education, Inc.
100 μm
Primary Growth of Shoots
 A shoot apical meristem is a dome-shaped mass
of dividing cells at the shoot tip
 Leaves develop from leaf primordia along the
sides of the apical meristem
 Axillary buds develop from meristematic cells left
at the bases of leaf primordia
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Figure 35.16
Leaf primordia
Young leaf
Shoot apical
meristem
Developing
vascular
strand
Axillary bud
meristems
0.25 mm
© 2014 Pearson Education, Inc.
 The closer an axillary bud is to the active apical
bud, the more inhibited it is
 Axillary buds are released from this apical
dominance if the shoot tip is removed or shaded
 In some monocots, meristematic activity occurs at
the bases of stems and leaves
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Tissue Organization of Stems
 Lateral shoots develop from axillary buds on the
stem’s surface
 In most eudicots, the vascular tissue consists of
vascular bundles arranged in a ring
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Figure 35.17
Sclerenchyma
(fiber cells)
Phloem
Xylem
Ground tissue
connecting
pith to cortex
Ground
tissue
Pith
Epidermis
Cortex
Epidermis
Vascular
bundle
1 mm
(a) Cross section of stem with
vascular bundles forming a
ring (typical of eudicots) (LM)
© 2014 Pearson Education, Inc.
Vascular
bundles
1 mm
Dermal
Ground
Vascular
(b) Cross section of stem with
scattered vascular bundles
(typical of monocots) (LM)
Figure 35.17a
Sclerenchyma
(fiber cells)
Phloem
Xylem
Ground tissue
connecting
pith to cortex
Pith
Cortex
Epidermis
Vascular
bundle
1 mm
(a) Cross section of stem with vascular bundle
forming a ring (typical of eudicots) (LM)
© 2014 Pearson Education, Inc.
Dermal
Ground
Vascular
Figure 35.17b
Ground
tissue
Epidermis
Vascular
bundles
Dermal
Ground
Vascular
1 mm
(b) Cross section of stem with scattered vascular
bundles (typical of monocots) (LM)
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 In most monocot stems, the vascular bundles are
scattered throughout the ground tissue, rather
than forming a ring
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Tissue Organization of Leaves
 The epidermis in leaves is interrupted by stomata,
pores that allow CO2 and O2 exchange between
the air and the photosynthetic cells in a leaf
 Stomata are also major avenues for evaporative
loss of water
 Each stomatal pore is flanked by two guard cells,
which regulate its opening and closing
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 The ground tissue in a leaf, called mesophyll, is
sandwiched between the upper and lower
epidermis
 The mesophyll of eudicots has two layers
 The palisade mesophyll in the upper part of the leaf
 The spongy mesophyll in the lower part of the leaf;
the loose arrangement allows for gas exchange
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 The vascular tissue of each leaf is continuous with
the vascular tissue of the stem
 Veins are the leaf’s vascular bundles and function
as the leaf’s skeleton
 Each vein in a leaf is enclosed by a protective
bundle sheath
© 2014 Pearson Education, Inc.
Figure 35.18
Cuticle
Sclerenchyma
fibers
Stoma
50 μm
Guard
cells
Stomatal
pore
Epidermal
cell
(b) Surface view of a spiderwort
(Tradescantia) leaf (LM)
Upper
epidermis
Palisade
mesophyll
Lower
epidermis
Bundlesheath
cell
Vein
Xylem
Phloem
(a) Cutaway drawing of leaf tissues
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Guard
cells
100 μm
Spongy
mesophyll
Cuticle
Dermal
Ground
Vascular
Vein Air spaces
Guard cells
(c) Cross section of a lilac
(Syringa) leaf (LM)
Figure 35.18a
Cuticle
Sclerenchyma
fibers
Stoma
Upper
epidermis
Palisade
mesophyll
Spongy
mesophyll
Lower
epidermis
Bundlesheath
cell
Xylem
Vein
Phloem
Guard
cells
(a) Cutaway drawing of leaf tissues
© 2014 Pearson Education, Inc.
Cuticle
Dermal
Ground
Vascular
Figure 35.18b
Guard
cells
50 μm
Stomatal
pore
Epidermal
cell
Dermal
© 2014 Pearson Education, Inc.
(b) Surface view of a spiderwort
(Tradescantia) leaf (LM)
Figure 35.18c
Upper
epidermis
Palisade
mesophyll
100 μm
Spongy
mesophyll
Lower
epidermis
Dermal
Ground
© 2014 Pearson Education, Inc.
Vein
Air spaces
Guard cells
(c) Cross section of a lilac
(Syringa) leaf (LM)
Concept 35.4: Secondary growth increases the
diameter of stems and roots in woody plants
 Many land plants display secondary growth, the
growth in thickness produced by lateral meristems
 Secondary growth is characteristic of
gymnosperms and many eudicots, but not
monocots
© 2014 Pearson Education, Inc.
 Secondary growth occurs in stems and roots of
woody plants but rarely in leaves
 Secondary growth consists of the tissues
produced by the vascular cambium and cork
cambium
 Primary growth and secondary growth occur
simultaneously
© 2014 Pearson Education, Inc.
Figure 35.19
(a) Primary and secondary growth
in a two-year-old woody stem
Pith
Primary xylem
Vascular cambium
Primary phloem
Epidermis
Cortex
Cortex
Epidermis
Primary
phloem
Vascular
cambium
Vascular ray
Primary
xylem
Secondary xylem
Pith
Secondary phloem
First cork cambium
Cork
Most recent
cork cambium
Periderm
(mainly
cork
cambia
and cork)
Secondary phloem
Vascular cambium
Bark
Cork
Late wood
Early wood
Layers of
periderm
Cork
cambium Periderm
Cork
1 mm
Secondary
phloem
Secondary
xylem
Bark
Vascular ray Growth ring
Secondary
xylem
© 2014 Pearson Education, Inc.
1.4 mm
(b) Cross section of a three-yearold Tilia (linden) stem (LM)
Figure 35.19a-1
(a) Primary and secondary growth
in a two-year-old woody stem
Epidermis
Cortex
Primary phloem
Vascular cambium
Primary xylem
Pith
Periderm (mainly
cork cambia
and cork)
Secondary
phloem
Secondary
xylem
© 2014 Pearson Education, Inc.
Pith
Primary xylem
Vascular cambium
Primary phloem
Cortex
Epidermis
Figure 35.19a-2
(a) Primary and secondary growth
in a two-year-old woody stem
Epidermis
Cortex
Primary phloem
Vascular cambium
Primary xylem
Pith
Pith
Primary xylem
Vascular cambium
Primary phloem
Cortex
Epidermis
Vascular ray
Secondary xylem
Secondary phloem
First cork cambium
Cork
Periderm (mainly
cork cambia
and cork)
Secondary
phloem
Secondary
xylem
© 2014 Pearson Education, Inc.
Figure 35.19a-3
Pith
Primary xylem
Vascular cambium
Primary phloem
Cortex
Epidermis
(a) Primary and secondary growth
in a two-year-old woody stem
Epidermis
Cortex
Primary phloem
Vascular cambium
Vascular ray
Primary xylem
Secondary xylem
Secondary phloem
Pith
First cork cambium
Cork
Periderm (mainly
cork cambia
and cork)
Most recent
cork cambium
Bark
Secondary
phloem
Secondary
xylem
© 2014 Pearson Education, Inc.
Cork
Layers of
periderm
Figure 35.19b
Secondary phloem
Vascular cambium
Secondary
xylem
Bark
Cork
cambium Periderm
Late wood
Early wood
1 mm
Cork
Vascular ray
1.4 mm
© 2014 Pearson Education, Inc.
Growth ring
(b) Cross section of a three-yearold Tilia (linden) stem (LM)
The Vascular Cambium and Secondary Vascular
Tissue
 The vascular cambium, a cylinder of meristematic
cells one cell layer thick, is wholly responsible for
the production of secondary vascular tissue
 It develops from undifferentiated parenchyma cells
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 In a typical woody stem, the vascular cambium is
located outside the pith and primary xylem and to
the inside of the primary phloem and the cortex
 In a typical woody root, the vascular cambium
forms exterior to the primary xylem and interior to
the primary phloem and pericycle
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 In cross section, the vascular cambium appears as
a ring of meristematic cells
 Division of these cells increases the vascular
cambium’s circumference and adds secondary
xylem to the inside and secondary phloem to the
outside
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Figure 35.20
Vascular
cambium
Growth
X X C P P
X X C P
Secondary
xylem
Vascular
cambium
Secondary
phloem
X C P
X C
C
After one year
of growth
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After two years
of growth
 Elongated initials produce tracheids, vessel
elements, fibers of xylem, sieve-tube elements,
companion cells, axially oriented parenchyma, and
fibers of the phloem
 Shorter initials produce vascular rays, radial files
of parenchyma cells that connect secondary xylem
and phloem
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 Secondary xylem accumulates as wood and
consists of tracheids, vessel elements (only in
angiosperms), and fibers
 Early wood, formed in the spring, has thin cell
walls to maximize water delivery
 Late wood, formed in late summer, has thickwalled cells and contributes more to stem support
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 Tree rings are visible where late and early wood
meet, and can be used to estimate a tree’s age
 Dendrochronology is the analysis of tree ring
growth patterns and can be used to study past
climate change
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Ring-width
indexes
Figure 35.21
2
1.5
1
0.5
0
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1600
1700
1800
Year
1900
2000
 As a tree or woody shrub ages, the older layers of
secondary xylem, the heartwood, no longer
transport water and minerals
 The outer layers, known as sapwood, still
transport materials through the xylem
 Older secondary phloem sloughs off and does not
accumulate
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Figure 35.22
Growth
ring
Vascular
ray
Heartwood
Secondary
xylem
Sapwood
Vascular cambium
Secondary phloem
Bark
Layers of periderm
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Figure 35.23
© 2014 Pearson Education, Inc.
The Cork Cambium and the Production of
Periderm
 Cork cambium gives rise to cork cells that
accumulate to the exterior of the cork cambium
 Cork cells deposit waxy suberin in their walls, then
die
 The cork cambium and the tissues it produces
compose a layer of periderm
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 Lenticels in the periderm allow for gas exchange
between living stem or root cells and the outside
air
 Bark consists of all the tissues external to the
vascular cambium, including secondary phloem
and periderm
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Evolution of Secondary Growth
 In the herbaceous plant Arabidopsis, the addition
of weights to the plants triggered secondary
growth
 This suggests that stem weight is the cue for wood
formation
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Concept 35.5: Growth, morphogenesis, and cell
differentiation produce the plant body
 Cells form specialized tissues, organs, and
organisms through the process of development
 Developmental plasticity describes the effect of
environment on development
 For example, the aquatic plant fanwort forms
different leaves depending on whether or not the
apical meristem is submerged
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Figure 35.24
© 2014 Pearson Education, Inc.
 Development consists of growth, morphogenesis,
and cell differentiation
 Growth is an irreversible increase in size
 Morphogenesis is the development of body form
and organization
 Cell differentiation is the process by which cells
with the same genes become different from each
other
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Model Organisms: Revolutionizing the Study of
Plants
 New techniques and model organisms are
catalyzing explosive progress in our understanding
of plants
 Arabidopsis is a model organism and the first plant
to have its entire genome sequenced
 Arabidopsis has 27,000 genes divided among
five pairs of chromosomes
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Figure 35.UN02
© 2014 Pearson Education, Inc.
 Arabidopsis is easily transformed by introducing
foreign DNA via genetically altered bacteria
 Studying the genes and biochemical pathways of
Arabidopsis will provide insights into plant
development, a major goal of systems biology
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Figure 35.25-1
Agrobacterium tumefaciens
1
Ti
plasmid
Site where
restriction
enzyme cuts
T DNA
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Figure 35.25-2
Agrobacterium tumefaciens
1
Ti
plasmid
Site where
restriction
enzyme cuts
T DNA
2
DNA with
the gene
of interest
Recombinant
Ti plasmid
© 2014 Pearson Education, Inc.
Figure 35.25-3
Agrobacterium tumefaciens
3
1
Ti
plasmid
Site where
restriction
enzyme cuts
T DNA
2
DNA with
the gene
of interest
Recombinant
Ti plasmid
© 2014 Pearson Education, Inc.
Figure 35.25-4
Agrobacterium tumefaciens
3
1
Ti
plasmid
Site where
restriction
enzyme cuts
T DNA
2
DNA with
the gene
of interest
Recombinant
Ti plasmid
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Plant with
new trait
Growth: Cell Division and Cell Expansion
 By increasing cell number, cell division in
meristems increases the potential for growth
 Cell enlargement accounts for the actual increase
in plant size
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The Plane and Symmetry of Cell Division
 New cell walls form in a plane (direction)
perpendicular to the main axis of cell expansion
 The plane in which a cell divides is determined
during late interphase
 Microtubules become concentrated into a ring
called the preprophase band that predicts the
future plane of cell division
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Figure 35.26
Preprophase band
7 μm
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 Leaf growth results from a combination of
transverse and longitudinal cell divisions
 It was previously thought that the plane of cell
division determines leaf form
 A mutation in the tangled-1 gene that affects
longitudinal divisions does not affect leaf shape
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30 μm
Figure 35.27
Leaf epidermal cells of
wild-type maize
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Leaf epidermal cells of
tangled-1 maize mutant
30 μm
Figure 35.27a
Leaf epidermal cells of
wild-type maize
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30 μm
Figure 35.27b
Leaf epidermal cells of
tangled-1 maize mutant
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 The symmetry of cell division, the distribution of
cytoplasm between daughter cells, determines cell
fate
 Asymmetrical cell division signals a key event in
development
 For example, the formation of guard cells involves
asymmetrical cell division and a change in the
plane of cell division
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Figure 35.28
Asymmetrical
cell division
Unspecialized
epidermal cell
Guard cell
“mother cell”
Developing
guard cells
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 Polarity is the condition of having structural or
chemical differences at opposite ends of an
organism
 For example, plants have a root end and a shoot
end
 Asymmetrical cell divisions play a role in
establishing polarity
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 The first division of a plant zygote is normally
asymmetrical and initiates polarization into the
shoot and root
 The gnom mutant of Arabidopsis results from a
symmetrical first division
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Figure 35.29
© 2014 Pearson Education, Inc.
Orientation of Cell Expansion
 Plant cells grow rapidly and “cheaply” by intake
and storage of water in vacuoles
 Plant cells expand primarily along the plant’s main
axis
 Cellulose microfibrils in the cell wall restrict the
direction of cell elongation
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Figure 35.30
Expansion
Cellulose
microfibrils
Nucleus
Vacuoles
5 μm
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Figure 35.30a
© 2014 Pearson Education, Inc.
Figure 35.30b
5 μm
© 2014 Pearson Education, Inc.
Morphogenesis and Pattern Formation
 Pattern formation is the development of specific
structures in specific locations
 Two types of hypotheses explain the fate of plant
cells
 Lineage-based mechanisms propose that cell fate
is determined early in development and passed on
to daughter cells
 Position-based mechanisms propose that cell fate
is determined by final position
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 Experiments suggest that plant cell fate is
established late in development and depends on
cell position
 In contrast, cell fate in animals is largely lineage
dependent
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 Hox genes in animals affect the number and
placement of appendages in embryos
 A plant homolog of Hox genes called KNOTTED-1
does not affect the number or placement of plant
organs
 KNOTTED-1 is important in the development of
leaf morphology
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Figure 35.31
© 2014 Pearson Education, Inc.
Figure 35.31a
© 2014 Pearson Education, Inc.
Figure 35.31b
© 2014 Pearson Education, Inc.
Gene Expression and the Control of Cell
Differentiation
 Cells of a developing organism synthesize
different proteins and diverge in structure and
function even though they have a common
genome
 Cellular differentiation depends on gene
expression, but is determined by position
 Positional information is communicated through
cell interactions
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 Gene activation or inactivation depends on cell-tocell communication
 For example, Arabidopsis root epidermis forms root
hairs or hairless cells depending on the number of
cortical cells it is touching
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Figure 35.32
Cortical
cells
GLABRA-2 is
expressed, and the
cell remains hairless.
20 μm
GLABRA-2 is
not expressed,
and the cell will
develop a root
hair.
The root cap cells will be sloughed off before
root hairs emerge.
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Shifts in Development: Phase Changes
 Plants pass through developmental phases, called
phase changes, developing from a juvenile phase
to an adult phase
 Phase changes occur within the shoot apical
meristem
 The most obvious morphological changes typically
occur in leaf size and shape
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Figure 35.33
Leaves produced
by adult phase
of apical meristem
Leaves produced
by juvenile phase
of apical meristem
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Genetic Control of Flowering
 Flower formation involves a phase change from
vegetative growth to reproductive growth
 It is triggered by a combination of environmental
cues and internal signals
 Transition from vegetative growth to flowering is
associated with the switching on of floral
meristem identity genes
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 In a developing flower, the order of each
primordium’s emergence determines its fate: sepal,
petal, stamen, or carpel
 Plant biologists have identified several organ
identity genes (plant homeotic genes) that
regulate the development of floral pattern
 These are MADS-box genes
 A mutation in a plant organ identity gene can
cause abnormal floral development
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Figure 35.34
Ca
St
Pe
Se
Pe
Se
Normal Arabidopsis flower
Pe
Pe
Se
Abnormal
Arabidopsis flower
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Figure 35.34a
Ca
St
Pe
Se
Normal Arabidopsis flower
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Figure 35.34b
Pe
Se
Pe
Pe
Se
Abnormal
Arabidopsis flower
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 Researchers have identified three classes of floral
organ identity genes
 The ABC hypothesis of flower formation identifies
how floral organ identity genes direct the formation
of the four types of floral organs
 An understanding of mutants of the organ identity
genes depicts how this model accounts for floral
phenotypes
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Figure 35.35
(a) A schematic diagram of the
ABC hypothesis
Sepals
Petals
Stamens
A
B
Carpels
C
C gene
activity
AB
gene
activity
BC
gene
activity
Petal
Stamen
A gene
activity
Active
genes:
Carpel
Sepal
B B
BB
A AC CCC AA
BB
BB
CCCCCC C C
A A C CC C A A
AA
AA
AB BA AB BA
Mutant lacking A
Mutant lacking B
Mutant lacking C
Whorls:
Stamen Carpel
Petal
Sepal
Wild type
(b) Side view of flowers with organ identity mutations
© 2014 Pearson Education, Inc.
Figure 35.35a
(a) A schematic diagram of the
ABC hypothesis
Sepals
Petals
Stamens
A
B
Carpels
C
C gene
activity
AB
gene
activity
A gene
activity
© 2014 Pearson Education, Inc.
BC
gene
activity
Carpel
Petal
Stamen
Sepal
Figure 35.35b
Active
genes:
B B
BB
AA CC CC AA
B B
BB
CC CC C C CC
A A C CC C AA
A A
AA
AB BA A B BA
Mutant lacking A
Mutant lacking B
Mutant lacking C
Whorls:
Stamen Carpel
Petal
Sepal
Wild type
(b) Side view of flowers with organ identity mutations
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Figure 35.UN03
Shoot tip
(shoot apical
meristem and
young leaves)
Axillary bud
meristem
Root apical
meristems
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Vascular
cambium
Cork
cambium
Lateral
meristems
Figure 35.UN04
© 2014 Pearson Education, Inc.
Figure 35.UN05
© 2014 Pearson Education, Inc.