Download vascular cambium

CAMPBELL BIOLOGY IN FOCUS
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
28
Plant Structure
and Growth
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
© 2014 Pearson Education, Inc.
Overview: Are Plants Computers?
 Romanesco grows according to a repetitive program
 The development of plants depends on the
environment and is highly adaptive
© 2014 Pearson Education, Inc.
Figure 28.1
© 2014 Pearson Education, Inc.
 Angiosperms are primary producers and are
important in agriculture
 Taxonomists split the angiosperms into two major
clades: monocots and eudicots
© 2014 Pearson Education, Inc.
Figure 28.2
Monocots
Eudicots
One cotyledon
Two cotyledons
Veins usually parallel
Veins
usually netlike
Vascular tissue
scattered
Vascular tissue
usually arranged in ring
Root system usually
fibrous (no main root)
Taproot (main root)
usually present
Pollen grain with
one opening
Pollen grain with
three openings
Floral organs usually
in multiples of three
Floral organs usually in
multiples of four or five
Embryos
Leaf
venation
Stems
Roots
Pollen
Flowers
© 2014 Pearson Education, Inc.
Figure 28.2a
Monocots
Eudicots
One cotyledon
Two cotyledons
Veins usually parallel
Veins
usually netlike
Embryos
Leaf
venation
© 2014 Pearson Education, Inc.
Figure 28.2b
Monocots
Eudicots
Vascular tissue
scattered
Vascular tissue
usually arranged in ring
Root system usually
fibrous (no main root)
Taproot (main root)
usually present
Stems
Roots
© 2014 Pearson Education, Inc.
Figure 28.2c
Monocots
Eudicots
Pollen grain with
one opening
Pollen grain with
three openings
Floral organs usually
in multiples of three
Floral organs usually in
multiples of four or five
Pollen
Flowers
© 2014 Pearson Education, Inc.
Concept 28.1: 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
 The 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
© 2014 Pearson Education, Inc.
 Three basic organs evolved to acquire these
resources: roots, stems, and leaves
 The root system includes all of the plant’s roots
 The shoot system includes the stems, leaves, and
(in angiosperms) flowers
© 2014 Pearson Education, Inc.
Figure 28.3
Reproductive shoot (flower)
Apical bud
Node
Internode
Apical bud
Vegetative shoot
Shoot
system
Blade
Petiole
Axillary bud
Leaf
Stem
Taproot
Lateral
(branch)
roots
© 2014 Pearson Education, Inc.
Root
system
 Roots rely on sugar produced by photosynthesis in
the shoot system
 Shoots rely on water and minerals absorbed by the
root system
© 2014 Pearson Education, Inc.
Roots
 A root is an organ with important functions
 Anchoring the plant
 Absorbing minerals and water
 Storing carbohydrates
© 2014 Pearson Education, Inc.
 Tall, erect plants with large shoot masses have a
taproot system, which consists of
 A taproot, the main vertical root
 Lateral roots branching off the taproot
 Small or trailing plants have a fibrous root system,
which consists of
 Adventitious roots that arise from stems
 Lateral roots that arise from the adventitious roots and
form their own lateral roots
© 2014 Pearson Education, Inc.
 In most plants, absorption of water and minerals
occurs through the root hairs, thin extensions of
epidermal cells that increase surface area
© 2014 Pearson Education, Inc.
Figure 28.4
© 2014 Pearson Education, Inc.
 Most terrestrial plants form mycorrhizal associations,
which increase mineral absorption
 Many plants have root adaptations with specialized
functions
© 2014 Pearson Education, Inc.
Figure 28.5
Storage roots
Pneumatophores
“Strangling” aerial roots
© 2014 Pearson Education, Inc.
Figure 28.5a
Pneumatophores
© 2014 Pearson Education, Inc.
Figure 28.5b
Storage roots
© 2014 Pearson Education, Inc.
Figure 28.5c
“Strangling” aerial roots
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
 An apical bud, or terminal bud, is located near the
shoot tip and causes elongation of a young shoot
 An axillary bud is located in the upper angle formed
by the leaf and the stem and has the potential to
form a lateral branch, thorn, or flower
 Axillary buds are generally dormant
© 2014 Pearson Education, Inc.
 Some plants have modified stems with additional
functions
© 2014 Pearson Education, Inc.
Figure 28.6
Stolon
Rhizome
Root
Rhizomes
Stolons
Tubers
© 2014 Pearson Education, Inc.
Figure 28.6a
Rhizome
Root
Rhizomes
© 2014 Pearson Education, Inc.
Figure 28.6b
Stolon
Stolons
© 2014 Pearson Education, Inc.
Figure 28.6c
Tubers
© 2014 Pearson Education, Inc.
Leaves
 The leaf is the main photosynthetic organ of most
vascular plants
 Leaves have other functions including gas exchange,
dissipation of heat, and defense
 Leaves generally consist of a flattened blade and a
stalk called the petiole, which joins the leaf to 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
© 2014 Pearson Education, Inc.
 Some plant species have evolved modified leaves
that serve various functions
© 2014 Pearson Education, Inc.
Figure 28.7
Spines
Tendrils
Storage leaves
Stem
Reproductive leaves
© 2014 Pearson Education, Inc.
Storage leaves
Figure 28.7a
Tendrils
© 2014 Pearson Education, Inc.
Figure 28.7b
Spines
© 2014 Pearson Education, Inc.
Figure 28.7c
Storage leaves
Stem
Storage leaves
© 2014 Pearson Education, Inc.
Figure 28.7d
Reproductive leaves
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Figure 28.8
Dermal
tissue
Ground
tissue
© 2014 Pearson Education, Inc.
Vascular
tissue
 The dermal tissue system is the outer protective
covering
 In nonwoody plants, the dermal tissue system
consists of the epidermis, a layer of densely
packed cells
 In leaves and most stems, a waxy coating called the
cuticle helps prevent water loss from the epidermis
© 2014 Pearson Education, Inc.
 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 insect defense
© 2014 Pearson Education, Inc.
 The vascular tissue system facilitates transport of
materials through the plant and provides support
 The two vascular tissues are xylem and phloem
 Xylem conducts water and dissolved minerals
upward from roots into the shoots
 Phloem transports organic nutrients from where
they are made to where they are needed
© 2014 Pearson Education, Inc.
 The vascular tissue of a root or stem is collectively
called the stele
 In angiosperms, the root stele is a solid central
vascular cylinder
 The stele of stems and leaves is divided into
vascular bundles, strands of xylem and phloem
© 2014 Pearson Education, Inc.
 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
photosynthesis, short-distance transport, storage,
or support
© 2014 Pearson Education, Inc.
Common Types of Plant Cells
 Plant cells have structural adaptations that make
their specific functions possible
 The major types of plant cells are
 Parenchyma
 Collenchyma
 Sclerenchyma
 Water-conducting cells of the xylem
 Sugar-conducting cells of the phloem
© 2014 Pearson Education, Inc.
Animation: Tour of a Plant Cell
 Parenchyma cells
 Have thin and flexible primary walls
 Lack secondary walls
 Have a large central vacuole
 Perform the most metabolic functions
 Retain the ability to divide and differentiate
© 2014 Pearson Education, Inc.
Figure 28.9a
Parenchyma cells with
chloroplasts (in Elodea leaf)
(LM)
© 2014 Pearson Education, Inc.
60 m
 Collenchyma cells
 Are grouped in strands
 Help support young parts of the plant shoot
 Have thicker and uneven cell walls
 Lack secondary walls
 Provide flexible support without restraining growth
© 2014 Pearson Education, Inc.
Figure 28.9b
Collenchyma cells
(in Helianthus stem) (LM)
© 2014 Pearson Education, Inc.
5 m
 Sclerenchyma cells are rigid due to thick
secondary walls containing lignin, a 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 grouped in strands
© 2014 Pearson Education, Inc.
Figure 28.9c
5 m
Sclereid cells (in pear) (LM)
25 m
Cell wall
Fiber cells (cross section from ash tree) (LM)
© 2014 Pearson Education, Inc.
Figure 28.9ca
5 m
Sclereid cells
(in pear) (LM)
Cell wall
© 2014 Pearson Education, Inc.
Figure 28.9cb
25 m
Fiber cells (cross section from ash tree) (LM)
© 2014 Pearson Education, Inc.
 Water-conducting cells of the xylem are dead at
functional maturity
 There are two types of water-conducting cells
 Tracheids are long, thin cells with tapered ends that
move water through pits
 Vessel elements align end to end to form long
micropipes called vessels
© 2014 Pearson Education, Inc.
 Tracheids occur in the xylem of all vascular plants
 Vessel elements are common to most angiosperms,
a few gymnosperms, and a few seedless vascular
plants
© 2014 Pearson Education, Inc.
Figure 28.9d
Vessel Tracheids
100 m
Pits
Tracheids and vessels
(colorized SEM)
Perforation
plate
Vessel element
Vessel elements, with
perforated end walls
© 2014 Pearson Education, Inc.
Tracheids
Figure 28.9da
Vessel Tracheids 100 m
Tracheids and vessels
(colorized SEM)
© 2014 Pearson Education, Inc.
 Sugar-conducting cells of the phloem are alive at
functional maturity
 In seedless vascular plants and gymnosperms,
sugars are transported through sieve cells
© 2014 Pearson Education, Inc.
 In angiosperms, sugars are transported in sieve
tubes, chains of cells called sieve-tube elements
 Sieve plates are the porous end walls that allow
fluid to flow between cells along the sieve tube
 Sieve-tube elements lack organelles, but each has a
companion cell whose nucleus and ribosomes
serve both cells
© 2014 Pearson Education, Inc.
Figure 28.9e
3 m
Sieve-tube elements:
longitudinal view (LM)
Sieve plate
Sieve-tube element (left)
Companion
and companion cell:
cells
cross section (TEM)
Sieve-tube
elements
Plasmodesma
Sieve
plate
30 m
Nucleus of
companion
cell
15 m
Sieve-tube elements:
longitudinal view
© 2014 Pearson Education, Inc.
Sieve plate with pores (LM)
Figure 28.9ea
3 m
Sieve-tube element (left)
and companion cell:
cross section (TEM)
© 2014 Pearson Education, Inc.
Figure 28.9eb
Sieve-tube elements:
longitudinal view (LM)
Sieve plate
Companion
cells
Sieve-tube
elements
30 m
© 2014 Pearson Education, Inc.
Figure 28.9ec
Sieve
plate
15 m
Sieve plate with pores (LM)
© 2014 Pearson Education, Inc.
Concept 28.2: Meristems generate new cells for
growth and control the developmental phases and
life spans of plants
 A plant can grow throughout its life; this is called
indeterminate growth
 Indeterminate growth is enabled by meristems,
which are perpetually undifferentiated tissues
 Some plant organs cease to grow at a certain size;
this is called determinate growth
© 2014 Pearson Education, Inc.
Different Meristems Produce Primary and
Secondary Growth
 There are two main types of meristems
 Apical meristems
 Lateral meristems
© 2014 Pearson Education, Inc.
Figure 28.10
Primary growth in stems
Epidermis
Cortex
Primary
phloem
Primary
xylem
Shoot tip
(shoot apical
meristem and
young leaves)
Pith
Vascular cambium
Cork cambium
Axillary bud
meristem
Secondary growth in stems
Lateral
meristems
Cork cambium
Periderm
Pith
Cortex
Primary
phloem
Root apical
meristems
© 2014 Pearson Education, Inc.
Primary
xylem
Secondary
xylem
Secondary
phloem
Vascular
cambium
Figure 28.10a
Shoot tip
(shoot apical
meristem and
young leaves)
Axillary bud
meristem
Root apical
meristems
© 2014 Pearson Education, Inc.
Vascular cambium
Cork
cambium
Lateral
meristems
Figure 28.10b
Primary growth in stems
Epidermis
Cortex
Primary
phloem
Primary
xylem
Pith
© 2014 Pearson Education, Inc.
Figure 28.10c
Secondary growth in stems
Cork cambium
Periderm
Pith
Cortex
Primary
phloem
Primary
xylem
Secondary
xylem
© 2014 Pearson Education, Inc.
Secondary
phloem
Vascular
cambium
 Apical meristems are located at the tips of roots
and shoots and at the axillary buds of shoots
 Apical meristems elongate shoots and roots, a
process called primary growth
© 2014 Pearson Education, Inc.
 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.
 In woody plants, primary growth extends the shoots
and secondary growth increases the diameter of
previously formed parts
© 2014 Pearson Education, Inc.
Figure 28.11
Apical bud
Bud scale
Axillary buds
This year’s growth
(one year old)
Leaf
scar
Bud
scar
Last year’s growth
(two years old)
Node
One-year-old
branch formed
Internode from axillary bud
near shoot tip
Leaf scar
Stem
Bud scar
Growth of two
years ago
(three years old)
© 2014 Pearson Education, Inc.
Leaf scar
 Meristems give rise to
 Initials, also called stem cells, which remain in the
meristem
 Derivatives, which become specialized in mature
tissues
© 2014 Pearson Education, Inc.
Gene Expression and 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
© 2014 Pearson Education, Inc.
 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
© 2014 Pearson Education, Inc.
Figure 28.12
Cortical
cells
GLABRA-2 is expressed,
and the cell remains hairless.
20 m
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.
© 2014 Pearson Education, Inc.
Meristematic Control of the Transition to
Flowering and the Life Spans of Plants
 Flower formation involves a transition from
vegetative growth to reproductive growth
 It is triggered by a combination of environmental
cues and internal signals
 Reproductive growth is determinate; the production
of a flower stops primary growth of that shoot
© 2014 Pearson Education, Inc.
 Flowering plants can be categorized based on the
timing and completeness of the switch from
vegetative to reproductive growth
 Annuals complete their life cycle in a year or less
 Biennials require two growing seasons
 Perennials live for many years
© 2014 Pearson Education, Inc.
Concept 28.3: Primary growth lengthens roots
and shoots
 Primary growth arises from the apical meristems
and produces parts of the root and shoot systems
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Video: Root Time Lapse
Figure 28.13
Cortex
Vascular cylinder
Epidermis
Root hair
Dermal
Ground
Vascular
Zone of
differentiation
Zone of
elongation
Zone of cell
division
(including
root apical
meristem)
Root cap
© 2014 Pearson Education, Inc.
Mitotic
cells
100 m
Figure 28.13a
Mitotic
cells
100 m
© 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 28.14
Epidermis
Cortex
Endodermis
Vascular cylinder
Pericycle
Core of
parenchyma cells
100 m
(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
Dermal
Ground
Vascular
70 m
© 2014 Pearson Education, Inc.
100 m
Figure 28.14a
Epidermis
Cortex
Endodermis
Vascular cylinder
Pericycle
Xylem
Phloem
100 m
(a) Root with xylem and phloem in
the center (typical of eudicots)
Dermal
Ground
Vascular
© 2014 Pearson Education, Inc.
Figure 28.14b
Epidermis
Cortex
Endodermis
Vascular cylinder
Pericycle
Core of
parenchyma cells
Xylem
Phloem
100 m
Dermal
Ground
© 2014 Pearson Education, Inc.
Vascular
(b) Root with parenchyma in the
center (typical of monocots)
Figure 28.14c
Endodermis
Pericycle
Xylem
Phloem
Dermal
Ground
Vascular
70 m
© 2014 Pearson Education, Inc.
 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
© 2014 Pearson Education, Inc.
 Lateral roots arise from within the pericycle, the
outermost cell layer in the vascular cylinder
© 2014 Pearson Education, Inc.
Figure 28.15-1
Emerging
lateral
root
100 m
Cortex
Vascular
cylinder
© 2014 Pearson Education, Inc.
Pericycle
Figure 28.15-2
Emerging
lateral
root
Epidermis
100 m
Cortex
Vascular
cylinder
© 2014 Pearson Education, Inc.
Pericycle
Lateral root
Figure 28.15-3
Emerging
lateral
root
Epidermis
100 m
Cortex
Vascular
cylinder
© 2014 Pearson Education, Inc.
Pericycle
Lateral root
Figure 28.15a
Emerging
lateral
root
100 m
Cortex
Vascular
cylinder
© 2014 Pearson Education, Inc.
Pericycle
Figure 28.15b
Epidermis
Lateral
root
© 2014 Pearson Education, Inc.
Figure 28.15c
Epidermis
Lateral root
© 2014 Pearson Education, Inc.
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, projections
along the sides of the apical meristem
 Shoot elongation results from lengthening of
internode cells below the shoot tip
© 2014 Pearson Education, Inc.
Figure 28.16
Shoot apical meristem
Leaf primordia
Young
leaf
Developing
vascular
strand
Axillary bud
meristems
0.25 mm
© 2014 Pearson Education, Inc.
 Axillary buds are dormant due to apical
dominance, inhibition from an active apical bud
 When released from dormancy, axillary buds give
rise to lateral shoots (branches)
© 2014 Pearson Education, Inc.
Tissue Organization of Leaves
 The epidermis in leaves is interrupted by stomata,
which allow CO2 and O2 exchange between the air
and the photosynthetic cells in a leaf
 Each stomatal pore is flanked by two guard cells,
which regulate its opening and closing
 The ground tissue in a leaf, called mesophyll, is
composed of parenchyma cells sandwiched
between the upper and lower epidermis
© 2014 Pearson Education, Inc.
Figure 28.17
Guard
cells
Dermal
Ground
Vascular
Cuticle
Sclerenchyma
fibers
Stoma
50 m
Stomatal
pore
Epidermal
cell
(b) Surface view of a spiderwort
(Tradescantia) leaf (LM)
Upper
epidermis
Palisade
mesophyll
Bundlesheath
cell
100 m
Spongy
mesophyll
Lower
epidermis
Cuticle
Xylem
Phloem
(a) Cutaway drawing of leaf tissues
© 2014 Pearson Education, Inc.
Vein
Guard
cells
Vein
Air spaces
Guard cells
(c) Cross section of a lilac
(Syringa) leaf (LM)
Figure 28.17a
Dermal
Ground
Vascular
Cuticle
Sclerenchyma
fibers
Stoma
Upper
epidermis
Palisade
mesophyll
Spongy
mesophyll
Bundlesheath
cell
Lower
epidermis
Cuticle
Xylem
Phloem
(a) Cutaway drawing of leaf tissues
© 2014 Pearson Education, Inc.
Vein
Guard
cells
Figure 28.17b
Guard
cells
50 m
Stomatal
pore
Epidermal
cell
Dermal
© 2014 Pearson Education, Inc.
(b) Surface view of a spiderwort
(Tradescantia) leaf (LM)
Figure 28.17c
Upper
epidermis
Palisade
mesophyll
100 m
Spongy
mesophyll
Lower
epidermis
Dermal
Ground
© 2014 Pearson Education, Inc.
Vein
Air spaces
Guard cells
(c) Cross section of a lilac
(Syringa) leaf (LM)
 The mesophyll of eudicots has two layers
 The palisade mesophyll in the upper part of the leaf
consists of elongated cells for photosynthesis
 The spongy mesophyll in the lower part of the leaf
consists of loosely arranged cells for gas exchange
© 2014 Pearson Education, Inc.
 The vascular tissue of each leaf is continuous with
the vascular tissue of the stem
 Veins are the leaf’s vascular bundles, which function
in fluid transport and provide structure to the leaf
 Each vein in a leaf is enclosed by a protective
bundle sheath
© 2014 Pearson Education, Inc.
Tissue Organization of Stems
 Stems are covered in epidermis and contain
vascular bundles that run the length of the stem
 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
© 2014 Pearson Education, Inc.
Figure 28.18
Sclerenchyma
(fiber cells)
Phloem
Xylem
Ground tissue
connecting
pith to cortex
Pith
Ground
tissue
Epidermis
Dermal
Cortex
Epidermis
Vascular
bundle
Vascular
1 mm
(a) Cross section of stem with vascular
bundles forming a ring (typical of eudicots)
© 2014 Pearson Education, Inc.
Ground
Vascular
bundles
1 mm
(b) Cross section of stem with scattered
vascular bundles (typical of monocots)
Figure 28.18a
Sclerenchyma
(fiber cells)
Phloem
Xylem
Ground tissue
connecting
pith to cortex
Pith
Dermal
Cortex
Epidermis
Vascular
bundle
Vascular
1 mm
(a) Cross section of stem with vascular
bundles forming a ring (typical of eudicots)
© 2014 Pearson Education, Inc.
Ground
Figure 28.18b
Ground
tissue
Epidermis
Dermal
Ground
Vascular
Vascular
bundles
1 mm
(b) Cross section of stem with scattered
vascular bundles (typical of monocots)
© 2014 Pearson Education, Inc.
 In most monocot stems, the vascular bundles are
scattered throughout the ground tissue, rather than
forming a ring
© 2014 Pearson Education, Inc.
Concept 28.4: Secondary growth increases the
diameter of stems and roots in woody plants
 Secondary growth is characteristic of gymnosperms
and many eudicots, but not monocots
 Secondary growth occurs in stems and roots of
woody plants but rarely in leaves
 The secondary plant body consists of the tissues
produced by the vascular cambium and cork
cambium
© 2014 Pearson Education, Inc.
 In woody plants, primary growth and secondary
growth occur simultaneously but in different
locations
© 2014 Pearson Education, Inc.
Figure 28.19
(a) Primary and secondary growth
in a two-year-old woody stem
Pith
Primary xylem
Vascular cambium
Primary phloem
Cortex
Epidermis
Epidermis
Cortex
Primary
phloem
Vascular
cambium
Vascular
ray
Periderm
Secondary
Cork
phloem
cambium
Vascular
Cork
cambium
Bark
Secondary Late wood
xylem
Early wood
Primary
xylem
Pith
Cork
1 mm
Secondary
xylem
Secondary
phloem
First cork cambium
Periderm
(mainly
cork
cambia
and cork)
Vascular Growth
ray
ring
1.4 mm
Secondary
phloem
Most recent
cork cambium
Secondary
xylem
© 2014 Pearson Education, Inc.
Cork
Layers of
periderm
Bark
(b) Cross section of a three-yearold Tilia (linden) stem (LM)
Figure 28.19a-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 28.19a-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 28.19a-3
(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.
Most recent cork cambium
Cork
Bark
Layers of
periderm
Figure 28.19b
Vascular
cambium
Secondary
xylem
Periderm
Cork
cambium
Secondary
phloem
Bark
Cork
Late wood
1 mm
Early wood
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 is a cylinder of meristematic
cells one cell layer thick
 In cross section, the vascular cambium appears as
a ring of initials (stem cells)
 The initials increase the vascular cambium’s
circumference and add secondary xylem to the
inside and secondary phloem to the outside
© 2014 Pearson Education, Inc.
Figure 28.20
Vascular
cambium
Growth
Vascular
cambium
Secondary
xylem
After one year
of growth
© 2014 Pearson Education, Inc.
Secondary
phloem
After two years
of growth
 Elongated, parallel-oriented initials produce
tracheids, vessel elements, fibers of xylem, sievetube elements, companion cells, axially oriented
parenchyma, and fibers of the phloem
 Shorter, perpendicularly oriented initials produce
vascular rays, radial files of parenchyma cells that
connect secondary xylem and phloem
© 2014 Pearson Education, Inc.
 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 thick-walled
cells and contributes more to stem support
 In temperate regions, the vascular cambium of
perennials is inactive through the winter
© 2014 Pearson Education, Inc.
 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
© 2014 Pearson Education, Inc.
 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
 Only young secondary phloem transports sugar;
older secondary phloem sloughs off and does not
accumulate
© 2014 Pearson Education, Inc.
Figure 28.21
Growth
ring
Vascular
ray
Heartwood
Secondary
xylem
Sapwood
Vascular cambium
Secondary phloem
Bark
Layers of periderm
© 2014 Pearson Education, Inc.
The Cork Cambium and the Production of
Periderm
 The cork cambium gives rise to cork cells that
accumulate to the exterior of the cork cambium
 Cork cells deposit waxy suberin in their walls and
then die
 Periderm consists of the cork cambium and the cork
cells it produces
 Periderm functions as a protective barrier,
impermeable to water and gasses
© 2014 Pearson Education, Inc.
 Bark consists of all the tissues external to the
vascular cambium, including secondary phloem and
periderm
 Lenticels in the periderm allow for gas exchange
between living stem or root cells and the outside air
© 2014 Pearson Education, Inc.
Figure 28.UN01
© 2014 Pearson Education, Inc.
Figure 28.UN02
Shoot tip
(shoot apical
meristem and
young leaves)
Axillary bud
meristems
Root apical
meristems
© 2014 Pearson Education, Inc.
Vascular
cambium
Cork
cambium
Lateral
meristems
Figure 28.UN03
© 2014 Pearson Education, Inc.