Chapter 35
Plant Structure, Growth, and
Development
Overview: Plastic Plants?
•
To some people, the fanwort is an intrusive weed, but to others it is an
attractive aquarium plant
–
This plant exhibits developmental plasticity, the ability to alter itself (in
terms of leaf structure) in response to its environment
• Underwater leaves are feathery to protect them from damage by
flowing water
• In contrast, its surface leaves are pads that aid in flotation
–
Though both leaf types have genetically identical cells, their different
environments result in the turning on or off of different genes during leaf
development
Fig. 35-1
•
Such extreme developmental plasticity is much
more common in plants than animals
–
This may help compensate for a plant’s
inability to escape adverse conditions
by moving
•
In addition to plasticity, plant species have accumulated adaptations in
morphology, or external form
–
These characteristics vary little within the species
• Ex) Most cactus species, regardless of their local environment
have highly reduced leaves (spines)
– Reduced leaf surface area limits water loss, a morphological
adaptation enhancing the survival and reproductive success of
cacti
–
Both genetic and environmental factors influence the morphology of
plants and animals
• Because the effect of the environment is greater in plants, they
typically vary much more within a species than do animals
– Ex) All lions have the same form: 4 legs, similar body sizes at
maturity
– Ex) Ginkgo trees vary greatly in number, sizes, and positions of
their roots, branches, and leaves
Concept 35.1: The plant body has a hierarchy of organs, tissues, and cells
•
Plants, like multicellular animals, have organs composed of different tissues, which
in turn are composed of cells
–
The basic morphology of vascular plants reflects their evolution as organisms
that draw nutrients from 2 very different environments:
Fig. 35-2
–
•
Below ground (water and minerals)
Reproductive shoot (flower)
•
Above ground (sunlight and CO2)
Node
Internode
The ability to acquire these resources
arose from the evolution of 3 basic organs:
roots, stems, and leaves
•
–
These organs are organized into a
root system (roots) and a shoot
system (stems and leaves)
Almost all vascular plants rely on both of
these systems for survival
Apical bud
Apical
bud
Vegetative
shoot
Leaf
Shoot
system
Blade
Petiole
Axillary
bud
Stem
Taproot
Lateral
branch
roots
•
Nonphotosynthetic roots rely on sugars
produced by photosynthesis (photosynthates) in the shoot system
•
Shoots rely on water and minerals absorbed by the root system
Root
system
Roots
•
Roots are multicellular organs with important functions:
Fig. 35-2
•
–
Anchoring the plant
Reproductive shoot (flower)
–
Absorbing minerals and water
Node
Internode
–
Storing organic nutrients, like carbohydrates
Most eudicots and gymnosperms have a
taproot system that penetrates deeply into
the soil, consisting of:
Apical bud
Apical
bud
Vegetative
shoot
Leaf
Shoot
system
Blade
Petiole
Axillary
bud
Stem
•
One main vertical root that develops
from an embryonic root, known as the
taproot
Taproot
Lateral
branch
roots
• In many angiosperms, the taproot
stores sugars and starches that the plant will consume during
flowering and fruit production
• This is why root crops (carrots, beets) are harvested before
they flower
•
Lateral (branch) roots that arise from the taproot
Root
system
Roots
•
In seedless vascular plants and most monocots, the embryonic roots dies
rather than giving rise to a main root
•
Instead, many small roots, called adventitious roots, grow from the
stem
• Each of these small roots then forms its own lateral roots, forming a
mat of thin roots spread below the soil surface that lacks a main
root
• This type of root system is known as a fibrous root system
•
Fibrous root systems do not usually penetrate deeply into the soil
• Plants with this type of root system
are thus best adapted to shallow
soils or regions where rainfall is
light
• Ex) Most grasses have shallow
roots concentrated in the upper
few centimeters of soil (sod)
•
In most plants, absorption of water and minerals occurs near the tips of the
roots
•
Here, 1000s of tiny root hairs increase the surface area of the root
• This increased surface area greatly enhances the absorption of
water and minerals from the soil but contributes little to plant
anchorage
•
Roots hairs should not be confused with lateral roots
• Root hairs are thin tubular extensions of
a root epidermal cell
Fig. 35-3
• Lateral roots are multicellular organs
•
Root hairs are short-lived and are thus being
constantly replaced
•
Many plants have modified roots:
–
Prop roots: support tall, top-heavy plants
•
–
Pneumatophores (air roots): project above the water’s surface, allowing the
root system to obtain the CO2 lacking in thick, waterlogged mud
•
–
The roots of mature maize plants are all adventitious after the original root
dies
Produced by trees that
inhabit tidal swamps
Fig. 35-4
Prop roots
“Strangling” aerial roots:
snake-like roots wrap around a
host tree or other objects
•
Eventually, a host tree will
of shading by this
tree’s leaves
“Strangling”
aerial roots
Storage roots
die
Buttress roots
–
Buttress roots: aerial roots
that support the tall trunks of
some tropical trees
–
Storage roots: store food and water
Pneumatophores
•
•
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
In the upper angle formed by each leaf and its stem is an axillary bud
Fig. 35-2
–
Reproductive shoot (flower)
An axillary bud is a structure that
has the potential to form a lateral
shoot, or branch
Apical bud
Node
Internode
Apical
bud
• Most axillary buds of a young
shoot are dormant (not growing)
• Elongation of the young shoot
is instead usually concentrated
near the shoot tip at the apical
(terminal) bud
– This is known as apical
dominance
Vegetative
shoot
Leaf
Shoot
system
Blade
Petiole
Axillary
bud
Stem
Taproot
Lateral
branch
roots
Root
system
Stems
• Apical dominance is an evolutionary adaptation because the
plant’s exposure to light is increased by concentrating
resources on elongation
– Axillary buds can still break dormancy if the apical bud is
damaged or shaded
Fig. 35-2
Reproductive shoot (flower)
Apical bud
• In such cases, the growing
axillary bud gives rise to a
lateral shoot, complete with
its own apical bud, leaves,
and axillary buds
Node
Internode
Apical
bud
Vegetative
shoot
Leaf
Shoot
system
Blade
Petiole
Axillary
bud
• This is why pruning trees
and shrubs will actually
make them bushier
Stem
Taproot
Lateral
branch
roots
Root
system
•
Many plants have modified stems:
–
Rhizomes: a horizontal shoot that grows just below the soil surface
• Vertical shoots emerge from axillary buds on the rhizome
Fig. 35-5
–
Bulbs: vertical underground shoots
consisting mostly of the enlarged bases
of food-storing leaves
–
Stolons: horizontal shoots that grow
along the soil surface
Rhizomes
Bulbs
Storage leaves
Stem
• These “runners” allow plants to
reproduce asexually, as plantlets
form at nodes along each runner
–
Stolons
Stolon
Tubers: enlarged ends of rhizomes or
stolons specialized for storing food
Tubers
Leaves
•
The leaf is the main photosynthetic organ of most vascular plants
–
•
However, green stems also perform photosynthesis
Leaves vary in form but generally consist of:
–
A flattened blade
–
A stalk
–
The petiole, which joins the leaf to
the stem at a node
• Grasses and many other
monocots lack petioles
– The base of these leaves
forms a sheath that
envelops the stem
•
Monocots and eudicots differ in the arrangement of veins, the vascular
tissue of leaves
Fig. 35-6
–
–
•
Most monocots have parallel veins
(a) Simple leaf
Most eudicots have branching veins
Petiole
In classifying angiosperms, taxonomists
may use leaf morphology as a criterion,
including:
–
Leaf shape (simple, compound,
doubly compound)
–
Branching pattern of veins
–
Spatial arrangement of leaves
Axillary bud
Leaflet
(b) Compound
leaf
Petiole
Axillary bud
(c) Doubly
compound
leaf
Leaflet
Petiole
Axillary bud
•
Simple leaf: has a dingle, undivided blade
•
Compound leaf: the blade consists of multiple leaflets
–
•
These can be differentiated from individual simple leaves based on the
absence of axillary buds at itsFig.base
35-6
Doubly compound leaf: each leaflet is divided (a) Simple leaf
into smaller leaflets
–
(Doubly) compound leaves are a
structural adaptation that allow these
large leaves to withstand strong wind
with less tearing
Petiole
Axillary bud
Leaflet
(b) Compound
leaf
• It may also confine pathogens that
invade a leaf to a single leaflet,
rather than allowing them to spread
to the entire leaf
Petiole
Axillary bud
(c) Doubly
compound
leaf
Leaflet
Petiole
Axillary bud
•
Though almost all leaves are specialized for photosynthesis, some plant
species have evolved modified leaves that serve various other functions:
–
Tendrils: form a “lasso” around a support and then coils to bring the
Fig. 35-7
plant closer to the support
Tendrils
• Some tendrils are modified stems
(grapevines)
–
–
–
–
Spines: reduced leaves that do not carry
out photosynthesis, which is instead
performed by the fleshy green stems
Storage leaves: leaves modified for
storage of nutrients, including water
Reproductive leaves: leaves that
produce adventitious plantlets that fall
off the leaf and take root in the soil
Bracts: brightly-colored leaves (often
mistaken for petals) that attract pollinators
Spines
Storage
leaves
Reproductive leaves
Bracts
Dermal, Vascular, and Ground Tissues
•
Each plant organ (root, stem, and leaf) has dermal, vascular, and ground
tissues
–
•
Each of these three categories forms a functional unit connecting all the
plant’s organs, which is known as a tissue system
The dermal tissue system is the plant’s outer protective covering
–
Fig. 35-8
Like skin, it forms the 1st line of defense
against physical damage and pathogens
• In nonwoody plants, it is usually a
single tissue composed of a layer of
tightly packed cells known as the
epidermis
– In leaves and most stems, a
waxy coating on the epidermal
surface called the cuticle helps
prevent water loss
• In woody plants, protective tissues
called periderm replace the
epidermis in older regions of stems and roots
Dermal
tissue
Ground
tissue Vascular
tissue
• The epidermis has specialized characteristics in each organ
– Ex) Root hairs are extensions of epidermal cells near the
root tip
– Ex) Hair-like outgrowths of the shoot epidermis, known as
Trichomes, reduce water loss and reflect excess light
• They can also provide defense against insects by
forming a barrier or by secreting sticky fluids and toxic
compounds
– In one experiment, scientists sought to answer the
question: Do soybean pod trichomes deter
herbivores?
•
Background: bean leaf beetles feed on developing legume pods
–
•
This causes pod scarring and decreased seed quality
Experiment: scientists investigated whether the trichomes on soybean pods
deter these beetles
–
They placed hungry beetles in bags sealed around the pods of adjacent
EXPERIMENT
plants
Fig. 35-9
• These plants had pods that
expressed different pod hairiness
(amounts of trichomes)
–
–
Results: Beetle damage to very hairy
soybean pods was much lower compared
with other pod types
Conclusion: Soybean pod trichomes protect
against beetle damage
Very hairy pod
(10 trichomes/
mm2)
Slightly hairy pod
(2 trichomes/
mm2)
Bald pod
(no trichomes)
Slightly hairy pod:
25% damage
Bald pod:
40% damage
RESULTS
Very hairy pod:
10% damage
• The vascular tissue system carries out long-distance
transport of materials between roots and shoots
– There are two types of vascular tissues:
• Xylem conveys water and dissolved minerals
upward from roots into the shoots
• Phloem transports organic nutrients (sugars) from:
– Where they are made - usually in leaves, to:
– Where they are needed - usually roots and sites
of growth
• The vascular tissue of a stem or root is collectively called the
stele
– The arrangement of the stele
varies, depending on the
species and organ
• In angiosperms the stele
of the root is a solid
central vascular
cylinder of both xylem
and phloem
• The stele of stems and
leaves is divided into
vascular bundles, made
up of separate strands
of xylem and phloem
• All other tissue that is not dermal or vascular is considered to
be part of the ground tissue system
– This system includes various cells specialized for storage,
photosynthesis, and support
• Ground tissue lying within the vascular tissue is called
pith
• Ground tissue outside
of vascular tissue is
called cortex
Common Types of Plant Cells
•
Like any multicellular organism, a plant is characterized by cellular
differentiation, the specialization of cells in structure and function
–
Differences between plant cell types can be seen in:
• The cytoplasm
• Organelles
• The cell wall
–
The major types of plant cells include:
• Parenchyma cells
• Collenchyma cells
• Sclerenchyma cells
• The water-conducting cells of the xylem
• The sugar-conducting cells of the phloem
Fig. 35-10a
•
Mature parenchyma cells
–
Have thin and flexible primary walls
–
Most lack secondary walls
–
Have a large central vacuole
–
Are the least specialized (structurally)
–
Perform the most metabolic functions,
including synthesis and storage:
–
Parenchyma cells in Elodea leaf,
with chloroplasts (LM)
60 µm
–
Photosynthesis occurs in the chloroplasts of leaf parenchyma
cells
–
Some parenchyma cells in stems and roots have colorless
plastids that store starch
Retain the ability to divide and differentiate into other types of plant
cells under particular conditions (ex: during wound repair)
–
Scientists can even grown entire plants from a single
parenchyma cell
BioFlix: Tour of a Plant Cell
•
Collenchyma cells are grouped in strands and help
support young parts of the plant shoot
•
They have unevenly thickened primary walls as
compared to parenchyma cells and lack
secondary walls
•
Because the the hardening agent lignin is absent
from their primary walls, they provide flexible
support without restraining growth
•
They are found just below the epidermis in young
stems and petioles (the “strings” of celery, which
5 µm
is a petiole)
Fig. 35-10b
• These cells elongate
with the stems and
leaves they support
Collenchyma cells (in Helianthus stem) (LM)
Fig. 35-10c
•
5 µm
Sclerenchyma cells also function as
supprting elements in the plant
Sclereid cells in pear (LM)
•
•
In contrast to collenchyma cells,
these cells are rigid because of thick
secondary walls containing lignin
They are dead at functional maturity,
losing the ability to elongate (grow)
25 µm
Cell wall
• Rather, their rigid secondary
walls (produced while still alive)
remain as skeletons that support the plant
•
There are two types of sclerenchyma cells:
–
Sclereids are short and irregular in shape,
with thick lignified secondary walls
–
Fibers are long, slender, and tapered and
usually arranged in threads
• Some are used commercially
– Ex) Hemp fibers are used to make
rope
Fiber cells (cross section from ash tree) (LM)
•
The two types of water-conducting cells are both tubular, elongated, and
dead at functional maturity
•
Tracheids: long thin cells with tapered ends and secondary walls
hardened with lignin
• They also function in support, preventing collapse under the
tension of water support
Fig. 35-10d
• They are found in the xylem of
almost all vascular plants
•
Vessel
Tracheids
100 µm
Vessel elements: cells that are
wider, shorter, thinner-walled and
less tapered than tracheids
• Aligned end-to-end, these cells
form long micropipes called
vessels
• They are found in most
angiosperms, and in a few
gymnosperms and seedless
vascular plants
Pits
Tracheids and vessels
(colorized SEM)
Perforation
plate
Vessel
element
Vessel elements, with
perforated end walls
Tracheids
• Both of these cells form nonliving conduits through which water
can flow
•
Their secondary walls are interrupted by thinner regions,
known as pits, where only primary walls are present
Fig. 35-10d
• Water can migrate
laterally between
neighboring cells
through these pits
•
In addition, the end walls of
vessel elements have
perforation plates that allow
water to flow freely through
their vessels
Vessel
Tracheids
100 µm
Pits
Tracheids and vessels
(colorized SEM)
Perforation
plate
Vessel
element
Vessel elements, with
perforated end walls
Tracheids
•
The sugar-conducting cells of the xylem are alive at functional maturity
•
•
In seedless vascular plants and gymnosperms, sugars are transported
through long, narrow sieve cells
In the phloem of angiosperms, sugars are transported through sieve tubes
composed of chains of cells called sieve-tube elements
•
Though they are alive at Fig. 35-10e
maturity, these elements lack
a nucleus, ribosomes, a
distinct vacuole, and
cytoskeletal elements
• This reduction in cell
content allows nutrients
to pass more easily
through the cell
•
The end walls between these
elements are called sieve
plates and contain pores that
help move fluid from one cell
to the next along the sieve
tube
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
10 µm
Nucleus of
companion
cells
Sieve-tube elements:
longitudinal view
Sieve plate with pores (SEM)
• Each sieve-tube element has a companion cell that connects
with it via plasmodesmata and numerous channels
•
The nucleus and ribosomes of these companion cell serve
both cells
Sieve-tube elements:
Fig. 35-10e
3 µm
•
In addition, companion
cells in leaves
sometimes help load
sugars into the sievetube elements,
readying them for
transport to other
parts of the plant
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
10 µm
Nucleus of
companion
cells
Sieve-tube elements:
longitudinal view
Sieve plate with pores (SEM)
Concept Check 35.1
•
1) How does the vascular tissue system enable leaves and roots to function
together in supporting growth and development of the whole plant?
•
2) When you eat the following, what plant structures are you consuming? (a)
brussels sprouts (b) celery sticks (c) onions (d) carrot sticks
•
3) Characterize the role of each of the 3 tissue systems in a leaf.
•
4) Describe at least 3 specializations in plant organs and plant cells that are
adaptations to life on land.
•
5) If humans were photoautotrophs, making food by capturing light energy
from photosynthesis, how might our anatomy be different?
Concept 35.2: Meristems generate cells for new organs
•
A plant can grow throughout its life, a process called indeterminate growth
–
At any given time, a plant consists of embryonic, developing, and
mature organs
• Some plant organs cease to grow at a certain size, known as
determinate growth
– Ex) Most leaves, thorns, and flowers
•
Plants can be categorized based on the length of their life cycles:
–
Annuals complete their life cycle in a year or less
• Ex) Many wildflowers and most staple food crops (legumes, wheat,
rice)
–
Biennials require two growing seasons to complete their life cycle,
flowering and fruiting only in their second year
• Ex) Radishes and carrots
–
Perennials live for many years
• Ex) Trees, shrubs, some grasses
•
Plants are capable of indeterminate growth because they have perpetually
embryonic tissues called meristems
–
•
There are 2 main types of meristems: apical and lateral meristems
Apical meristems: located at the tips of roots and shoots and at axillary
buds of shoots
–
Apical meristems elongate shoots and roots, a process called primary
growth
• In herbaceous plants, primary growth produces all of the plant body
–
Woody plants also grow in girth in the parts of stems and roots that no
longer grow in length
• This growth in thickness is called secondary growth
– It is causes by the activity of lateral meristems called vascular
cambium and cork cambium
•
•
Both types of lateral meristems consists of cylinders of dividing cells that
extend along the lengths of roots and stems
–
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
Meristems produce 2 types of cells:
–
Fig. 35-11
Cells that remain in the meristem as sources of new cells are called
initials
Primary growth in stems
Epidermis
–
Cells that are displaced
from the meristem and
differentiate to become
part of different tissues
and organs within the
plant are called derivatives
Cortex
Shoot tip (shoot
apical meristem
and young leaves)
Primary phloem
Primary xylem
Pith
Lateral meristems:
Vascular cambium
Cork cambium
Secondary growth in stems
Periderm
Axillary bud
meristem
Cork
cambium
Cortex
Root apical
meristems
Primary
phloem
Pith
Primary
xylem
Secondary
xylem
Secondary
phloem
Vascular cambium
Concept Check 35.2
•
1) Distinguish between primary and secondary growth.
•
2) Cells in lower layers of your skin divide and replace dead cells sloughed
from the surface. Why is it inaccurate to compare such regions of cell
division to a plant meristem?
•
3) Roots and stems grow indeterminately, but leaves do not. How might this
benefit the plant?
•
4) Suppose a gardener picks some radishes and finds they are too small.
Since radishes are biennials, the gardener leaves the remaining plants in the
ground, thinking that they will grow larger during their second year. Is this a
good idea? Explain.
Concept 35.3: Primary growth lengthens roots and
shoots
• Primary growth produces the primary plant
body, the parts of the root and shoot systems
produced by apical meristems
– In herbaceous plants, it is usually the entire
plant
– In woody plants, it consists only of the
youngest parts that are not yet woody
•
During primary growth, a root tip is covered by a root cap that protects the
apical meristem as the root pushes through soil
–
•
This root cap also secretes a polysaccharide slime that lubricates the
soil around the tip of the root
Growth occurs just behind the root tip, in three zones of cells:
–
Zone of cell division: includes the
root apical meristem
Fig. 35-13
Cortex
• New root cells are produced
in this region, including the
root cap
–
Epidermis
Key
to labels
Dermal
Root hair
Zone of
differentiation
Ground
Zone of elongation: region about
1 mm behind the root tip where
root cells elongate, sometimes to
more than 10X their original length
Vascular
Zone of
elongation
• Cell elongation pushes the
root tip farther into the soil
–
Vascular cylinder
Zone of maturation: region where
cells complete their differentiation
and become distinct cell types
Apical
meristem
Root cap
100 µm
Zone of cell
division
•
The primary growth of roots produces the epidermis, ground tissue, and vascular
tissue
–
Water and minerals absorbed from the soil must enter through the root’s
epidermis
•
Root hairs enhance this process by greatly increasing the surface area of
Fig. 35-14
the epidermal cells
Epidermis
•
Cortex
In most roots, the stele is a
vascular cylinder containing a
solid core of xylem and phloem
–
–
In most eudicot roots, the
xylem has a star-like
appearance and the phloem
occupies the indentations
between the arms of the
xylem “star”
Endodermis
Vascular
cylinder
Pericycle
Core of
parenchyma
cells
Xylem
100 µm
100 µm
(b) Root with parenchyma in the center (typical of
monocots)
Endodermis
Pericycle
In many monocot roots, the
vascular tissue consists of a
central core of parenchyma cells
surrounded by a ring of xylem
and a ring of phloem
Key
to labels
Dermal
Ground
Vascular
Xylem
Phloem
50 µm
•
Phloem
(a) Root with xylem and phloem in the center
(typical of eudicots)
The central region is often
called pith (not ground tissue like stem pith)
• The ground tissue of roots consists mostly of parenchyma cells
and fills the cortex, the region between the vascular cylinder
and epidermis
– Cells within the ground tissue store carbohydrates, and
their plasma membranes absorb water and minerals from
the soil
– The innermost layer of the cortex is called the endodermis
• The endodermis is a cylinder one cell thick and forms
the boundary with the vascular cylinder
• Lateral roots arise from the outermost cell layer in the vascular
cylinder, known as the pericycle
– The pericycle is adjacent to and just inside the endodermis
– A lateral root pushes through the cortex and epidermis until
it emerges from the established root
Fig. 35-15-3
• A lateral root cannot originate near the root’s surface
because its vascular system must be continuous with
the vascular cylinder at the center of the established
root
100 µm
Epidermis
Emerging
lateral
root
Lateral root
Cortex
1
Vascular
cylinder
2
3
•
A shoot apical meristem is a dome-shaped mass of dividing cells at the
shoot tip
–
Leaves develop from finger-like projections called leaf primordia
along the sides of the apical meristem
–
Axillary buds develop from meristematic cells left at the bases of leaf
primordia
• These buds can form lateral shoots at some later time during plant
growth
Fig. 35-16
Shoot apical meristem
–
Leaf primordia
In some plants (grasses), a few
leaf cells are produced by other
areas of meristemic tissue, known
as intercalary meristems, that are
separate from the apical meristem
Young
leaf
Developing
vascular
strand
• This help grasses tolerate
grazing, since the elevated part
of the leaf blade can be removed
without stopping growth
Axillary bud
meristems
0.25 mm
Tissue Organization of Stems
•
The epidermis covers stems as part of the continuous dermal tissue system
•
Vascular tissue also runs the length of the stem, organized into vascular
bundles, and lateral shoots develop from axillary buds on the stem’s surface
–
In most eudicots, the vascular tissue consists of vascular bundles that
are arranged in a ring
Fig. 35-17a
Phloem
• The xylem in each
bundle is located
adjacent to the pith
Xylem
Sclerenchyma
(fiber cells)
Ground tissue
connecting
pith to cortex
• The phloem in each
bundle is located
adjacent to the cortex
Pith
Key
to labels
Cortex
Epidermis
Vascular
bundle
Dermal
Ground
1 mm
(a) Cross section of stem with vascular bundles forming
a ring (typical of eudicots)
Vascular
Tissue Organization of Stems
•
In most monocot stems, the vascular bundles are scattered throughout the
ground tissue instead of forming a ring
•
In the stems of both monocots and eudicots, the ground tissue consists
mostly parenchyma cells
–
Collenchyma cells are also present just beneath the epidermis in the
stems of many plants,
Ground
tissue
helping to strengthen those
stems
Fig. 35-17b
–
Sclerenchyma cells also
provide support in those
parts of the stems that
are no longer elongating
Epidermis
Key
to labels
Dermal
Vascular
bundles
Ground
Vascular
1 mm
(b) Cross section of stem with scattered vascular bundles
(typical of monocots)
Tissue Organization of Leaves
•
The epidermis in leaves is interrupted by pores called stomata that allow
CO2 exchange between the air and the photosynthetic cells in a leaf
•
Stomata are also major avenues for the evaporative loss of water
•
Each stomatal pore is flanked by two guard cells that regulate its
opening and closing
Fig. 35-18a
•
The ground tissue in a leaf,
called mesophyll, is
sandwiched between the
upper and lower epidermis
•
Mesophyll consists mainly
of parenchyma cells
specialized for
photosynthesis
Key
to labels
Dermal
Ground
Vascular
Cuticle
Sclerenchyma
fibers
Stoma
Upper
epidermis
Palisade
mesophyll
Spongy
mesophyll
Bundlesheath
cell
Lower
epidermis
Cuticle
Xylem
Vein
Phloem
(a) Cutaway drawing of leaf tissues
Guard
cells
•
The leaves of many eudicots have 2 distinct areas of mesophyll:
–
Palisade mesophyll: consists of one or more layers of elongated
parenchyma cells on the upper part of the leaf
–
Spongy mesophyll: lower layer consisting of more loosely arranged
parenchyma cells, creating air spaces through which CO2 and O2 can
circulate around the cells and up to the palisade region
Fig. 35-18a
• These air spaces are
particularly large near
stomata, where gas
exchange with the
outside air occurs
Key
to labels
Dermal
Ground
Vascular
Cuticle
Sclerenchyma
fibers
Stoma
Upper
epidermis
Palisade
mesophyll
Spongy
mesophyll
Bundlesheath
cell
Lower
epidermis
Cuticle
Xylem
Vein
Phloem
(a) Cutaway drawing of leaf tissues
Guard
cells
•
The vascular tissue of each leaf is continuous with the vascular tissue of the
stem
–
Connections from vascular bundles in the stem called leaf traces pass
through petioles and into leaves
–
Veins are the leaf’s vascular bundles and function as the leaf’s
skeleton, reinforcing the shape of the leaf
Fig. 35-18a
• Each vein in a leaf is
enclosed by a
protective bundle
sheath consisting
of one or more
layers of
(parenchyma) cells
Key
to labels
Dermal
Ground
Vascular
Cuticle
Sclerenchyma
fibers
Stoma
Upper
epidermis
Palisade
mesophyll
Spongy
mesophyll
Bundlesheath
cell
Lower
epidermis
Cuticle
Xylem
Vein
Phloem
(a) Cutaway drawing of leaf tissues
Guard
cells
Concept Check 35.3
• 1) Describe how roots and shoots differ in branching.
• 2) Contrast primary growth in roots and shoots.
• 3) When grazing animals are removed from grasslands,
eudicots often replace grasses. Suggest a reason why.
• 4) If a leaf is vertically oriented, would you expect its mesophyll
to be divided into spongy and palisade layers? Explain.
Concept 35.4: Secondary growth adds girth to stems and roots in
woody plants
•
•
Secondary growth is growth in thickness produced by lateral meristems
–
It 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
•
The vascular cambium adds secondary xylem (wood) and secondary
phloem, increasing vascular flow and support for the shoot system
•
The cork cambium produces a tough, thick covering consisting mainly of
wax-filled cells that protect the stem from water loss and invasion by other
organisms
Secondary growth is characteristic of gymnosperms and many eudicots, but not
monocots
–
Primary and secondary growth occur simultaneously
•
Primary growth adds leaves and lengthens stems and roots in younger
regions of a plant
•
Secondary growth thickens stems and roots in older regions where primary
growth has ended
•
Step 1: Primary growth from the activity of apical meristems is nearing
completion
–
•
The vascular cambium has just formed
Step 2: Secondary growth thickens the stem as vascular cambium forms
secondary xylem to the inside and secondary phloem to the outside
Fig. 35-19a2
•
Step 3: Some initials of the
vascular cambium give rise
to vascular rays
Pith
Primary xylem
Vascular cambium
Primary phloem
(a) Primary and secondary growth
in a two-year-old stem
Epidermis
Cortex
Cortex
Epidermis
Primary phloem
Vascular cambium
Primary xylem
Pith
w th
G ro
Vascular ray
Secondary xylem
Secondary phloem
First cork cambium
Cork
Periderm (mainly
cork cambia
and cork)
Secondary phloem
Secondary
xylem
•
Step 4: As the vascular cambium’s diameter increases, the secondary
phloem and other tissues outside of the cambium can’t keep up since their
cells no longer divide
–
These tissues, including the epidermis, thus eventually rupture
Fig. 35-19a2
–
A second lateral
meristem known as
the cork cambium
develops from
parenchyma cells
in the cortex
• The cork
cambium
produces cork
cells that
replace the
epidermis
Pith
Primary xylem
Vascular cambium
Primary phloem
(a) Primary and secondary growth
in a two-year-old stem
Epidermis
Cortex
Cortex
Epidermis
Primary phloem
Vascular cambium
Primary xylem
Pith
w th
G ro
Vascular ray
Secondary xylem
Secondary phloem
First cork cambium
Cork
Periderm (mainly
cork cambia
and cork)
Secondary phloem
Secondary
xylem
•
Step 5: In year 2 of secondary growth, the vascular cambium produces
Fig. 35-19a3
more secondary
xylem and phloem
Pith
(a) Primary and secondary growth
–
•
The cork cambium also produces more cork
Epidermis
Step 6: As theCortex
stem’s diameter increases, the outermost Cortex
tissues outside of
Primary phloem
Epidermis
the cork rupture and are sloughed off
Vascular cambium
•
Primary xylem
Vascular cambium
Primary phloem
in a two-year-old stem
w th
G ro
Vascular ray
Primary
xylemthe cork cambium reforms deeper in the cortex
Step 7: In many
cases,
Secondary xylem
Pith
–
Secondary phloem
When none of the cortex is left, the cambium develops from
phloem
First cork cambium
parenchyma cells
Cork
Periderm (mainly
cork cambia
and cork)
Most recent cork
cambium
Secondary phloem
Bark
Secondary
xylem
Cork
Layers of
periderm
Fig. 35-19a3
•
Step 8: Each cork
cambium and the
tissues it produces
form a layer of
periderm
Pith
Primary xylem
Vascular cambium
Primary phloem
(a) Primary and secondary growth
in a two-year-old stem
Epidermis
Cortex
Cortex
Epidermis
Primary phloem
Vascular cambium
w th
G ro
Primary xylem
Vascular ray
Secondary xylem
Pith
Secondary phloem
Step 9: Bark consists
of all tissues exterior
to the vascular
cambium
First cork cambium
Cork
Periderm (mainly
cork cambia
and cork)
Most recent cork
cambium
Secondary phloem
Bark
Cork
Layers of
periderm
Fig. 35-19b
Secondary
xylem
Secondary xylem
Secondary phloem
Vascular cambium
Late wood
Early wood
Bark
Cork
cambium Periderm
Cork
0.5 mm
•
Vascular ray
0.5 mm
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
– It increases in circumference and also adds layers of
secondary xylem to its interior and secondary phloem to its
exterior
– Each layer has a larger diameter than the previous layer,
thickening roots and stems
• The vascular cambium develops from undifferentiated
parenchyma cells
– In woody stems, these cells are located outside the pith
and primary xylem and to the inside of the cortex and
primary phloem
– In woody roots, the vascular cambium forms to the exterior
of the primary xylem and interior to the primary phloem and
pericycle
–
C = vascular cambium
–
X = xylem
–
P = phloem
As the meristemic cells of the vascular cambium divide, they:
–
Increase
the circumference of the vascular cambium
Fig. 35-20
–
Add secondary xylem to the inside of the cambium
–
Add secondary phloem to the outside of the cambium
Vascular cambium
Growth
X X C P P
X X C P
C
•
In cross section, the vascular cambium appears as a ring of initials
Vascular
cambium
Secondary
xylem
Secondary
phloem
X C P
C
•
C C C
X C
C
C
C
C
After one year
of growth
After two years
of growth
•
Some initials are elongated and oriented with their long axis parallel to the
axis of the stem or root
–
•
These initials produce tracheids, vessel elements, fibers of the xylem,
sieve-tube elements, companion cells, parenchyma, and fibers of the
phloem
The other initials are shorter and oriented perpendicular to the axis of the
stem or root
–
These initials produce radial files of cells called vascular rays that
connect the secondary xylem with
the secondary phloem
Fig. 35-19a2
Pith
Primary xylem
Vascular cambium
Primary phloem
(a) Primary and secondary growth
in a two-year-old stem
Epidermis
Cortex
Cortex
Epidermis
Primary phloem
• The cells of a vascular ray move
water and nutrients between the
secondary xylem and phloem,
store carbohydrates, and aid in
wound repair
Vascular cambium
Primary xylem
Pith
w th
G ro
Vascular ray
Secondary xylem
Secondary phloem
First cork cambium
Cork
Periderm (mainly
cork cambia
and cork)
Secondary phloem
Secondary
xylem
• Over many years, secondary xylem accumulates as wood, and
consists of tracheids, vessel elements (only in angiosperms),
and fibers
– The walls of secondary xylem cells are heavily lignified and
account for the hardness and strength of wood
• Wood that develops in the early spring in temperate
regions, however, has thin cells walls to maximize water
delivery to new, growing leaves
– This type of wood is called early wood
• Wood produced later in the growing season is
composed of thick-walled cells that do not transport as
much water but contribute more to stem support
– This type of wood is called late wood
•
In temperate regions, the vascular cambium of perennials is dormant
through the winter
–
•
After growth resumes in the spring, there is a large contrast between
the large cells of new early wood and last year’s smaller cells of late
wood
A year’s growth appears as a distinct ring in cross sections of most tree
trunks and roots
–
Researchers can therefore estimate a tree’s age by counting its annual
rings
• Dendrochronology: the science of analyzing tree ring growth
patterns
•
Rings can vary in thickness depending on seasonal growth:
–
Trees grow well in wet and warm years but may hardly grow in cold or
dry years
• Scientists can thus also use ring patterns to study climate changes
– Thick ring = warm year
– Think ring = cold year
Fig. 35-23
•
As a tree or woody shrub ages, the older layers of
secondary xylem no longer transport water and
minerals, a solution called xylem sap
–
These layers are called heartwood because
they are closer to the center of a stem or root
• A large tree can thus survive even if the
center of its trunk is hollow
–
The newest, outer layers of secondary
xylem still transport xylem sap and are
therefore known as sapwood
Fig. 35-22
• Because each new layer of
secondary xylem has a larger
circumference, secondary
growth allows the xylem to
transport increasingly more
sap each year, supplying an
increasing number of leaves
Growth
ring
Vascular
ray
Heartwood
Secondary
xylem
Sapwood
Vascular cambium
Secondary phloem
Bark
Layers of periderm
• Heartwood is generally darker than sapwood
– This is due to resins and other compounds that help protect
the core of the tree from fungi and wood-boring insects
• Only the youngest secondary phloem closest to the vascular
cambium functions in sugar transport
– Older secondary phloem sloughs off and does not
accumulate
Fig. 35-22
Growth
ring
Vascular
ray
Heartwood
Secondary
xylem
Sapwood
Vascular cambium
Secondary phloem
Bark
Layers of periderm
The Cork Cambium and the Production of Periderm
•
During the early stages of secondary growth, the epidermis splits, dries, and
falls of the stem or root as it is pushed outward
–
This epidermis is replaced by 2 tissues produced by the cork cambium
and in the outer layer of the pericycle in roots
• Phelloderm is a thin layer of parenchyma cells that forms to the
interior of the cork cambium
• The other tissue is made of cork cells that accumulate to the
exterior of the cork cambium
–
As cork cells mature, they deposit a waxy material called suberin in
their walls and die
• This cork tissue then functions as a barrier that helps protect the
stem or root from water loss, physical damage, and pathogens
–
Each cork cambium and the tissues it produces make up a layer of
periderm
The Cork Cambium and the Production of Periderm
•
•
In most plants, water and minerals are absorbed primarily in the youngest
parts roots
–
The older parts of roots anchor the plant and transport water and
solutes between the soil and shoots
–
Small, raised areas in the periderm called lenticels allow for gas
exchange between living stem or root cells and the outside air
Thickening of the stems and roots also often splits the first cork cambium,
causing it to lose its meristemic activity and differentiate into cork cells
–
A new cork cambium forms to the inside and results in another layer of
periderm
–
As this process continues, older layers of periderm are sloughed off,
which is visible as cracked, peeling bark
• Bark consists of all the tissues external to the vascular cambium,
including secondary phloem and all layers of periderm
Concept Check 35.4
• 1) A sign is hammered into a tree 2 meters from the tree’s
base. If the tree is 10 meters tall and elongates 1 meter each
year, how high will the sign be after 10 years?
• 2) Stomata and lenticels are both involved in gas exchange.
Why do stomata need to be able to close, but lenticels do not?
• 3) Would you expect a tropical tree to have distinct growth
rings? Why or why not?
• 4) If a complete ring of bark is removed around a tree trunk (a
process called girdling), the tree usually dies. Explain why.
Concept 35.5: Growth, morphogenesis, and
differentiation produce the plant body
• Each cell in a plant contains the same set of genes yet
patterns of gene expression cause the cellular
differentiation responsible for the diversity of cell types
– Other processes lead to the development of body
form and organization, known as morphogenesis
• The three developmental processes of growth,
morphogenesis, and cellular differentiation act in
concert to transform the fertilized egg into a plant
Molecular Biology: Revolutionizing the Study of Plants
•
New techniques and model systems are catalyzing explosive progress in our
understanding of plants
–
Arabidopsis, a weed of the mustard family, is a model organism, and
the first plant to have its entire genome sequenced
• Determining the function of many Arabidopsis genes has greatly
expanded our understanding of plant development
• Scientists are attempting to create mutants for each gene in the
genome of this species
DNA or RNA metabolism (1%)
Fig. 35-24
–
By thus identifying each gene’s
function and tracking every
biochemical pathway, researchers
aim to establish a blueprint for
how plants develop
Other
metabolism
(18%)
Unknown
(24%)
Signal transduction (2%)
Development (2%)
Energy pathways (3%)
Cell division and
organization (3%)
Transport (4%)
Transcription
(4%)
Response to
environment
(4%)
Protein
metabolism
(7%)
Other cellular
processes (17%)
Other biological
processes (11%)
Growth: Cell Division and Cell Expansion
•
By increasing cell number, cell division in meristems increases the potential
for growth
–
•
Cell expansion, primarily elongation, accounts for the actual increase in
plant size
The plane (direction) and symmetry of cell division are immensely important
in determining plant form
–
If the planes of division are parallel to the plane of the first division, a
single file of cells is produced
Fig. 35-25
–
If the planes of division vary
randomly, a disorganized
clump of cells results
Plane of
cell division
(a) Planes of cell division
Developing
guard cells
Unspecialized
epidermal cell
(b) Asymmetrical cell division
Guard cell
“mother cell”
The Plane and Symmetry of Cell Division
•
Asymmetrical cell division, in which one daughter cell receives more
cytoplasm than the other during mitosis, is also fairly common in plants
–
Ex) Formation of guard cells typically involves both asymmetrical cell
division and a change in the plane of cell division
• An epidermal cell divides asymmetrically to form a large cell that
remains an unspecialized epidermal cell and a small cell that
becomes a guard cell “mother cell”
• Guard cells form when this
mother cell divides in a
plane perpendicular to
the 1st cell division
Fig. 35-25
Plane of
cell division
(a) Planes of cell division
Developing
guard cells
Unspecialized
epidermal cell
(b) Asymmetrical cell division
Guard cell
“mother cell”
•
The plane in which a cell divides is determined during late interphase
–
The first sign of this spatial orientation is a rearrangement of the
cytoskeleton
–
Microtubules become concentrated into a ring called the preprophase
band
• Though this band disappears before metaphase, it predicts the
future plane of cell division
Fig. 35-26
• Its “imprint” consists of an orderly
array of actin microfilaments that
remain after the microtubules
disperse
Preprophase bands
of microtubules
Nuclei
Cell plates
10 µm
Orientation of Cell Expansion
•
Differences in cell expansion exist between plants and animals
–
Animal cells grow mainly by synthesizing cytoplasm, a metabolically
expensive process
–
Plant cells grow rapidly and “cheaply” primarily by intake and storage of
water in vacuoles
• This rapid extension of shoots and roots is an important
evolutionary adaptation that increases a plant’s exposure to light
and soil
–
In a growing plant cell, enzymes weaken the cross-links in the cell wall
and allow it to expand as water diffuses into the vacuole by osmosis
• Loosening of the wall occurs when hydrogen atoms secreted by the
cell activate cell wall enzymes that break these crosslinks
• Small vacuoles that accumulate most of the incoming water
coalesce and form the cell’s central vacuole
Orientation of Cell Expansion
•
Plant cells rarely expand equally in all directions but do so primarily along
the plant’s main axis
–
Cellulose microfibrils in the cell wall restrict the direction of cell
elongation, causing this differential growth
• The microfibrils do not stretch, so the cell expands mainly
perpendicular to the “grain” of the microfibrils
–
Microtubules play a key role in regulating the plane of cell expansion
Fig. 35-27
• It is the orientation of microtubules in the cell’s outermost
cytoplasm that determines the orientation of the cellulose
microfibrils
Cellulose
microfibrils
Nucleus
Vacuoles
5 µm
Microtubules and Plant Growth
–
The fass mutants have unusually squat cells with seemingly random planes of
cell division
–
Their roots and stems also have no ordered cell files and layers
Fass mutants develop into tiny adult plants with their organs compressed
longitudinally
The stubby form and disorganized tissue arrangement can be traced back to
their abnormal organization of microtubules
Fig. 35-28
•
During interphase,the microtubules are randomly
positioned and preprophase bands do not form
•
As a result, there is no orderly “grain” of cellulose
microfibrils in the cell wall to determine the
direction of elongation
•
This defect gives rise to cells that expand in all
direction and divide without respect to orientation
0.3 mm
–
(b) fass seedling
(a) Wild-type seedling
2 mm
•
Studies of fass mutants of Arabidopsis have confirmed the importance of cytoplasmic
microtubules in cell division and expansion
2 mm
•
(c) Mature fass mutant
Morphogenesis and Pattern Formation
•
Morphogenesis, during which cells are organized into tissues and organs,
must occur for development to proceed properly
–
Pattern formation is the development of specific structures in specific
locations
• It is determined by positional information in the form of signals
indicating to each cell its location within a developing structure
• Each cell within the developing organ responds to this positional
information from neighboring cells by differentiating into a particular
cell type that is oriented in a particular way
–
Positional information may be provided by gradients of specific
molecules, including hormones, proteins, and mRNAs
• Ex) Diffusion of a growth-regulating molecule produced in the shoot
apical meristem “informs” cells below it of their distance from the
shoot tip
– A second chemical signal from the outermost cells also allows
these cells to “gauge” their radial position within the developing
organ
Morphogenesis and Pattern Formation
• One type of positional information is associated with polarity
– Polarity is the condition of having structural or chemical
differences at opposite ends of an organism
• One of the most obvious examples of polarity is the
morphological difference between the two ends of a
plant, one side containing a root and the other, a shoot
• Though less obvious, this polarity is also manifest in
physiological properties
– Ex) The emergence of of adventitious roots within
the root end of a stem cutting and adventitious
shoots from the shoot end, which is due to the
hormone auxin
• The first division of a plant zygote is normally asymmetric,
which initiates polarization of the plant body into shoot and root
– The proper establishment of axial polarity is a critical step
in a plant’s morphogenesis, as evidenced by Arabidopsis
mutants:
• In the gnom mutant of Arabidopsis, the establishment of
polarity is defective
Fig. 35-29
– The first cell division of the zygote
is abnormal because it is
symmetrical
– This results in ball-shaped plants
with neither roots nor leaves
• Morphogenesis in plants, as in other multicellular organisms, is often
controlled by homeotic genes
–
Recall: homeotic genes are master regulatory genes that
mediate many major events in an individual’s development
• Ex) The protein product of the homeotic gene KNOTTED-1
is important in the development of leaf morphology, including
the production of compound leaves
– If this gene is expressed in greater quantity in the
genome of tomato plants, the normally compound leaves
become “super-compound”
Fig. 35-30
Gene Expression and Control of Cellular Differentiation
In cellular differentiation, cells of a developing organism synthesize different proteins
and diverge in structure and function even though they have a common genome
–
Cellular differentiation to a large extent depends on positional information
(where a particular cell is located relative to other cells), and is further affected
by homeotic genes
•
Ex) Two distinct cell types are formed in the root epidermis of Arabidopsis:
root hair cells and hairless epidermal cells
– Immature epidermal cells in contact with two underlying cortical cells
differentiate into root hair cells, while those in contact with only one
cortical cell differentiate into
Cortical
cells
mature hairless cells
Fig. 35-31
– Differential expression of a homeotic
gene called GLABRA-2 is also
required for appropriate root hair
distribution
•
The GLABRA-2 gene (blue) is
normally expressed only in
epidermal cells that will not
develop into root hairs
20 µm
•
Location and a Cell’s Developmental Fate
• Positional information underlies all the processes of
development: growth, morphogenesis, and differentiation
– One way to study the relationships among these processes
is clonal analysis
• During this process, the cell lineages derived from each
cell in an apical meristem are mapped during organ
development
– Researchers use mutations to distinguish a specific
cell from neighboring cells in the shoot tip
– All cells created by this mutant cell via cell division
will therefore be “marked”
• Ex) If the mutation prevents chlorophyll
synthesis, the mutant and all its descendants will
be albino
Location and a Cell’s Developmental Fate
• Cells are not dedicated early to forming specific tissues and
organs
– Random changes in rates and planes of cell division can
reorganize the meristem
• Ex) The outermost cells of the apical meristem usually
divide perpendicular to the surface of the shoot tip,
becoming part of the dermal tissue
– Occasionally, however, one of the outermost cells
divides parallel to the surface of the shoot tip,
placing this cell below, among cells derived from
different lineages
– Thus, the cell’s final position determines what kind of cell it
will become
Shifts in Development: Phase Changes
•
Plants pass through developmental phases, called phase changes,
developing from a juvenile phase to an adult phase
–
Unlike phase changes in animals, plant developmental phases occur
within a single region of the plant, the shoot apical meristem
• The most obvious morphological changes typically occur in leaf
size and shape
Fig. 35-32
– Juvenile nodes and
internodes retain their juvenile
status even after the shoot
continues to elongate and the
shoot apical meristem has
changed to the adult phase
– Thus, any new leaves that
develop on branches from
axillary buds at juvenile nodes
will also be juvenile
Leaves produced
by adult phase
of apical meristem
Leaves produced
by juvenile phase
of apical meristem
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 (ex: day
length) and internal signals (ex: hormones)
• Unlike vegetative growth, which is indeterminate, floral
growth is determinate
– The production of a flower by a shoot apical meristem
stops the primary growth of that shoot
–
Transition from vegetative growth to flowering is associated with
the switching on of floral meristem identity genes
• The protein products of these genes are transcription factors
that regulate the genes required for conversion of the
indeterminate vegetative meristems to determinate floral
meristems
• Plant biologists have identified several organ identity genes (plant
homeotic genes) that regulate the development of floral pattern
–
Positional information determines which organ identity genes are
expressed in a particular floral organ primordium
• The result is the development of the developing organ
primordium into a specific floral organ
–
A mutation in a plant organ identity gene can cause abnormal
floral development
Fig. 35-33
• Ex) Petals growing in place
of stamens (see photo)
Pe
Ca
St
Se
Pe
Se
(a) Normal Arabidopsis flower
Pe
Pe
Se
(b) Abnormal Arabidopsis flower
•
By studying mutants with abnormal flowers, researchers have identified
three classes of floral organ identity genes
–
The ABC model of flower formation identifies how these floral organ
identity genes direct the formation of the four types of floral organs
• According to this model, each class of genes is switched on in two
specific whorls (concentric circles) of the floral meristem
– A genes are switched on in the two outer whorls (sepals and
petals)
Sepals
Fig. 35-34a
Petals
– B genes are
switched on in the
two middle whorls
(petals and
stamens)
– C genes are
switched on in the
two inner whorls
(stamens and
carpels)
Stamens
A
B
(a) A schematic diagram of the ABC hypothesis
Carpels
C
A+B
gene
activity
B+C
gene
activity
C gene
activity
Carpel
Petal
A gene
activity
Stamen
Sepal
Fig. 35-34a
•
Sepals
Sepals arise from those parts of the
floral meristems in which only the
A genes are active
–
–
Petals
Stamens
A
B
Petals arise where A and B
genes are active
A+B
gene
activity
Stamens arise where B and C
genes are active
(a) A schematic diagram of the ABC hypothesis
Carpels
C
B+C
gene
activity
C gene
activity
Carpel
Petal
A gene
activity
Stamen
•
Sepal
The phenotype of mutants lacking a
functional A, B, or C organ identity
gene can be explained by combining the ABC model with the rule that:
Fig. 35-34b
–
If A or C is missing, the other activity occurs through all four whorls
Active
genes:
BB
B B
AACCCC AA
BB
BB
CCCCCCCC
A ACCCC AA
AA
AA
ABBAABBA
Mutant lacking A
Mutant lacking B
Mutant lacking C
Whorls:
Carpel
Stamen
Petal
Sepal
Wild type
(b) Side view of flowers with organ identity mutations
Concept Check 35.5
•
1) What attributes of the weed Arabidopsis thaliana make it such a useful
research organism?
•
2) How can 2 cells in a plant have vastly different structures even though
they have the same genome?
•
3) Explain how the fass mutation in Arabidopsis results in stubby plants
rather than normal elongated ones.
•
4) In some species, such as the magnolia on the cover of your textbook,
sepals look like petals, and both are collectively called “tepals.” Suggest an
extension to the ABC model that could hypothetically account for the origin
of tepals.
You should now be able to:
1. Compare the following structures or cells:
– Fibrous roots, taproots, root hairs,
adventitious roots
– Dermal, vascular, and ground tissues
– Monocot leaves and eudicot leaves
– Parenchyma, collenchyma, sclerenchyma,
water-conducting cells of the xylem, and
sugar-conducting cells of the phloem
– Sieve-tube element and companion cell
2. Explain the phenomenon of apical dominance
3. Distinguish between determinate and
indeterminate growth
4. Describe in detail the primary and secondary
growth of the tissues of roots and shoots
5. Describe the composition of wood and bark
6. Distinguish between morphogenesis,
differentiation, and growth
7. Explain how a vegetative shoot tip changes
into a floral meristem