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

• 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

Fig. 35-1

• 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:

• Below ground (water and minerals)

• Above ground (sunlight and CO2)

– 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

• Nonphotosynthetic roots rely on sugars

produced by photosynthesis (photosynthates) in the shoot system

• Shoots rely on water and minerals absorbed by the root system

Fig. 35-2

Reproductive shoot (flower)

Apical bud

Node

Internode

Apicalbud

Shoot

systemVegetative

shoot

LeafBlade

Petiole

Axillary

bud

Stem

Taproot

Lateral

branch

roots

Rootsystem

Roots

• Roots are multicellular organs with important functions:

– Anchoring the plant

– Absorbing minerals and water

– Storing organic nutrients, like carbohydrates

• Most eudicots and gymnosperms have a taproot system that penetrates deeply into the soil, consisting of:

• One main vertical root that develops from an embryonic root, known as the taproot

• 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

Fig. 35-2

Reproductive shoot (flower)

Apical bud

Node

Internode

Apicalbud

Shoot

systemVegetative

shoot

LeafBlade

Petiole

Axillary

bud

Stem

Taproot

Lateral

branch

roots

Rootsystem

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

• Lateral roots are multicellular organs

• Root hairs are short-lived and are thus being

constantly replaced

Fig. 35-3

• Many plants have modified roots:

– Prop roots: support tall, top-heavy plants

• The roots of mature maize plants are all adventitious after the original root

dies

– Pneumatophores (air roots): project above the water’s surface, allowing the

root system to obtain the CO2 lacking in thick, waterlogged mud

• Produced by trees that

inhabit tidal swamps

– “Strangling” aerial roots:

snake-like roots wrap around a

host tree or other objects

• Eventually, a host tree will die

of shading by this

tree’s leaves

– Buttress roots: aerial roots

that support the tall trunks of

some tropical trees

– Storage roots: store food and water

Fig. 35-4

Prop roots

“Strangling”aerial roots

Storage roots

Buttress roots

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

– An axillary bud is a structure that

has the potential to form a lateral

shoot, or branch

• 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

Fig. 35-2

Reproductive shoot (flower)

Apical bud

Node

Internode

Apicalbud

Shoot

systemVegetative

shoot

LeafBlade

Petiole

Axillary

bud

Stem

Taproot

Lateral

branch

roots

Rootsystem

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

• In such cases, the growing

axillary bud gives rise to a

lateral shoot, complete with

its own apical bud, leaves,

and axillary buds

• This is why pruning trees

and shrubs will actually

make them bushier

Fig. 35-2

Reproductive shoot (flower)

Apical bud

Node

Internode

Apicalbud

Shootsystem

Vegetativeshoot

LeafBlade

Petiole

Axillarybud

Stem

Taproot

Lateral

branch

roots

Rootsystem

• 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

– Bulbs: vertical underground shoots

consisting mostly of the enlarged bases

of food-storing leaves

– Stolons: horizontal shoots that grow

along the soil surface

• These “runners” allow plants to

reproduce asexually, as plantlets

form at nodes along each runner

– Tubers: enlarged ends of rhizomes or

stolons specialized for storing food

Fig. 35-5Rhizomes

Bulbs

Storage leaves

Stem

Stolons

Stolon

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

– Most monocots have parallel veins

– Most eudicots have branching veins

• 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

Fig. 35-6

(a) Simple leaf

Compoundleaf

(b)

Doublycompoundleaf

(c)

Petiole

Axillary bud

Leaflet

Petiole

Axillary bud

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 its base

• Doubly compound leaf: each leaflet is divided

into smaller leaflets

– (Doubly) compound leaves are a

structural adaptation that allow these

large leaves to withstand strong wind

with less tearing

• It may also confine pathogens that

invade a leaf to a single leaflet,

rather than allowing them to spread

to the entire leaf

Fig. 35-6

(a) Simple leaf

Compoundleaf

(b)

Doublycompoundleaf

(c)

Petiole

Axillary bud

Leaflet

Petiole

Axillary bud

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

plant closer to the support

• 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

Fig. 35-7

Tendrils

Spines

Storageleaves

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

– 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

Fig. 35-8

Dermaltissue

Groundtissue 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

plants

• 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

Fig. 35-9

Very hairy pod(10 trichomes/

mm2)

Slightly hairy pod(2 trichomes/

mm2)

Bald pod(no trichomes)

Very hairy pod:10% damage

Slightly hairy pod:25% damage

Bald pod:40% damage

EXPERIMENT

RESULTS

• 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

• 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:

– 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

Fig. 35-10a

Parenchyma cells in Elodea leaf,with chloroplasts (LM) 60 µm

• 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

is a petiole)

• These cells elongate

with the stems and

leaves they support

Fig. 35-10b

Collenchyma cells (in Helianthus stem) (LM)

5 µm

• Sclerenchyma cells also function as supprting elements in the plant

• 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)

• 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

Fig. 35-10c

5 µm

25 µm

Sclereid cells in pear (LM)

Fiber cells (cross section from ash tree) (LM)

Cell wall

• 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

• They are found in the xylem of almost all vascular plants

• 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

Fig. 35-10d

Perforationplate

Vesselelement

Vessel elements, withperforated end walls Tracheids

Pits

Tracheids and vessels(colorized SEM)

Vessel Tracheids 100 µm

• 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

• 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

Fig. 35-10d

Perforationplate

Vesselelement

Vessel elements, withperforated end walls Tracheids

Pits

Tracheids and vessels(colorized SEM)

Vessel Tracheids 100 µm

• 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 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

Fig. 35-10e

Sieve-tube element (left)and companion cell:cross section (TEM)

3 µmSieve-tube elements:longitudinal view (LM)

Sieve plate

Companioncells

Sieve-tubeelements

Plasmodesma

Sieveplate

Nucleus ofcompanioncells

Sieve-tube elements:longitudinal view Sieve plate with pores (SEM)

10 µm

30 µm

• 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

• In addition, companion

cells in leaves

sometimes help load

sugars into the sieve-

tube elements,

readying them for

transport to other

parts of the plant

Fig. 35-10e

Sieve-tube element (left)and companion cell:cross section (TEM)

3 µmSieve-tube elements:longitudinal view (LM)

Sieve plate

Companioncells

Sieve-tubeelements

Plasmodesma

Sieveplate

Nucleus ofcompanioncells

Sieve-tube elements:longitudinal view Sieve plate with pores (SEM)

10 µm

30 µm

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:

– Cells that remain in the meristem as sources of new cells are called

initials

– Cells that are displaced

from the meristem and

differentiate to become

part of different tissues

and organs within the

plant are called derivatives

Fig. 35-11

Shoot tip (shootapical meristemand young leaves)

Lateral meristems:

Axillary budmeristem

Vascular cambium

Cork cambium

Root apicalmeristems

Primary growth in stems

Epidermis

Cortex

Primary phloem

Primary xylem

Pith

Secondary growth in stems

Periderm

Corkcambium

Cortex

Primaryphloem

Secondaryphloem

Pith

Primaryxylem

Secondaryxylem

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

• New root cells are produced in this region, including the root cap

– Zone of elongation: region about 1 mm behind the root tip where root cells elongate, sometimes to more than 10X their original length

• Cell elongation pushes the root tip farther into the soil

– Zone of maturation: region where cells complete their differentiation and become distinct cell types

Fig. 35-13

Ground

Dermal

Keyto labels

Vascular

Root hair

Epidermis

Cortex Vascular cylinder

Zone ofdifferentiation

Zone ofelongation

Zone of celldivision

Apicalmeristem

Root cap

100 µm

• 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 the epidermal cells

• 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”

– 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

• The central region is often called pith (not ground tissue like stem pith)

Fig. 35-14Epidermis

Cortex

Endodermis

Vascularcylinder

Pericycle

Core ofparenchyma

cells

Xylem

Phloem100 µm

Root with xylem and phloem in the center(typical of eudicots)

(a)

Root with parenchyma in the center (typical ofmonocots)

(b)

100 µm

Endodermis

Pericycle

Xylem

Phloem

50 µm

Keyto labels

Dermal

Ground

Vascular

• 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

• 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

Fig. 35-15-3

Cortex

Emerginglateral

root

Vascularcylinder

100 µm Epidermis

Lateral root

321

• 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

– 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

• This help grasses tolerate

grazing, since the elevated part

of the leaf blade can be removed

without stopping growth

Fig. 35-16

Shoot apical meristem Leaf primordia

Youngleaf

Developingvascularstrand

Axillary budmeristems

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

• The xylem in each

bundle is located

adjacent to the pith

• The phloem in each

bundle is located

adjacent to the cortex

Fig. 35-17a

Sclerenchyma(fiber cells)

Phloem Xylem

Ground tissueconnecting

pith to cortex

Pith

CortexEpidermis

Vascularbundle

1 mm

Cross section of stem with vascular bundles forminga ring (typical of eudicots)

(a)

Dermal

Ground

Vascular

Keyto labels

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,

helping to strengthen those

stems

– Sclerenchyma cells also

provide support in those

parts of the stems that

are no longer elongating

Fig. 35-17b

Groundtissue

Epidermis

Keyto labels

Cross section of stem with scattered vascular bundles(typical of monocots)

Dermal

Ground

Vascular

(b)

Vascularbundles

1 mm

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

• 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

Fig. 35-18a

Key

to labels

Dermal

Ground

VascularCuticle Sclerenchyma

fibersStoma

Bundle-sheath

cell

Xylem

Phloem

(a) Cutaway drawing of leaf tissues

Guardcells

Vein

Cuticle

Lowerepidermis

Spongymesophyll

Palisademesophyll

Upperepidermis

• 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

• These air spaces are

particularly large near

stomata, where gas

exchange with the

outside air occurs

Fig. 35-18a

Key

to labels

Dermal

Ground

VascularCuticle Sclerenchyma

fibersStoma

Bundle-sheath

cell

Xylem

Phloem

(a) Cutaway drawing of leaf tissues

Guardcells

Vein

Cuticle

Lowerepidermis

Spongymesophyll

Palisademesophyll

Upperepidermis

• 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

• Each vein in a leaf is

enclosed by a

protective bundle

sheath consisting

of one or more

layers of

(parenchyma) cells

Fig. 35-18a

Key

to labels

Dermal

Ground

VascularCuticle Sclerenchyma

fibersStoma

Bundle-sheath

cell

Xylem

Phloem

(a) Cutaway drawing of leaf tissues

Guardcells

Vein

Cuticle

Lowerepidermis

Spongymesophyll

Palisademesophyll

Upperepidermis

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

• Step 3: Some initials of the

vascular cambium give rise

to vascular rays

Fig. 35-19a2

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growthin a two-year-old stem

(a)

Periderm (mainlycork cambia

and cork)

Secondary phloem

Secondaryxylem

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Vascular ray

Secondary xylem

Secondary phloem

First cork cambium

Cork

Growth

• 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

– 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

Fig. 35-19a2

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growthin a two-year-old stem

(a)

Periderm (mainlycork cambia

and cork)

Secondary phloem

Secondaryxylem

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Vascular ray

Secondary xylem

Secondary phloem

First cork cambium

Cork

Growth

• Step 5: In year 2 of secondary growth, the vascular cambium produces

more secondary xylem and phloem

– The cork cambium also produces more cork

• Step 6: As the stem’s diameter increases, the outermost tissues outside of

the cork rupture and are sloughed off

• Step 7: In many cases, the cork cambium reforms deeper in the cortex

– When none of the cortex is left, the cambium develops from phloem

parenchyma cells

Fig. 35-19a3

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growthin a two-year-old stem

(a)

Periderm (mainlycork cambia

and cork)

Secondary phloem

Secondaryxylem

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Vascular ray

Secondary xylem

Secondary phloem

First cork cambium

Cork

Growth

Cork

Bark

Most recent corkcambium

Layers ofperiderm

• Step 8: Each cork

cambium and the

tissues it produces

form a layer of

periderm

• Step 9: Bark consists

of all tissues exterior

to the vascular

cambium

Fig. 35-19a3

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growthin a two-year-old stem

(a)

Periderm (mainlycork cambia

and cork)

Secondary phloem

Secondaryxylem

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Vascular ray

Secondary xylem

Secondary phloem

First cork cambium

Cork

Growth

Cork

Bark

Most recent corkcambium

Layers ofperiderm

Fig. 35-19b

Secondary phloem

Vascular cambium

Secondary xylem

Bark

Early wood

Late wood Corkcambium

Cork

Periderm

0.5

mm

Vascular ray Growth ring

Cross section of a three-year-old Tilia (linden) stem (LM)

(b)

0.5 mm

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

• In cross section, the vascular cambium appears as a ring of initials

– C = vascular cambium

– X = xylem

– P = phloem

• As the meristemic cells of the vascular cambium divide, they:

– Increase the circumference of the vascular cambium

– Add secondary xylem to the inside of the cambium

– Add secondary phloem to the outside of the cambium

Fig. 35-20

Vascular cambium Growth

Secondaryxylem

After one yearof growth

After two yearsof growth

Secondaryphloem

Vascularcambium

X X

X X

X

X

P P

P

P

C

C

C

C

C

C

C C C

C C

CC

• 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

• The cells of a vascular ray move

water and nutrients between the

secondary xylem and phloem,

store carbohydrates, and aid in

wound repair

Fig. 35-19a2

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growthin a two-year-old stem

(a)

Periderm (mainlycork cambia

and cork)

Secondary phloem

Secondaryxylem

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Vascular ray

Secondary xylem

Secondary phloem

First cork cambium

Cork

Growth

• 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

• 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

• 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

Fig. 35-22

Growthring

Vascularray

Secondaryxylem

Heartwood

Sapwood

Bark

Vascular cambium

Secondary phloem

Layers of periderm

Fig. 35-23

• 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

Growthring

Vascularray

Secondaryxylem

Heartwood

Sapwood

Bark

Vascular cambium

Secondary phloem

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

• 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

– By thus identifying each gene’s

function and tracking every

biochemical pathway, researchers

aim to establish a blueprint for

how plants develop

Molecular Biology: Revolutionizing the Study of Plants

Fig. 35-24

DNA or RNA metabolism (1%)

Signal transduction (2%)

Development (2%)

Energy pathways (3%)

Cell division andorganization (3%)

Transport (4%)

Transcription(4%)

Response toenvironment(4%)

Proteinmetabolism(7%)

Other biologicalprocesses (11%)

Other cellularprocesses (17%)

Othermetabolism(18%)

Unknown(24%)

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

– If the planes of division vary

randomly, a disorganized

clump of cells results

Fig. 35-25

Plane ofcell division

(a) Planes of cell division

Developingguard cells

Guard cell“mother cell”

Unspecializedepidermal cell

(b) Asymmetrical cell division

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 ofcell division

(a) Planes of cell division

Developingguard cells

Guard cell“mother cell”

Unspecializedepidermal cell

(b) Asymmetrical cell division

• 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

• Its “imprint” consists of an orderly

array of actin microfilaments that

remain after the microtubules

disperse

Fig. 35-26

Preprophase bandsof microtubules

10 µm

Nuclei

Cell plates

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

• It is the orientation of microtubules in the cell’s outermost

cytoplasm that determines the orientation of the cellulose

microfibrils

Fig. 35-27

Cellulosemicrofibrils

Nucleus Vacuoles 5 µm

Microtubules and Plant Growth

• Studies of fass mutants of Arabidopsis have confirmed the importance of cytoplasmic

microtubules in cell division and expansion

– 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

• 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

Fig. 35-28

(a) Wild-type seedling

(b) fass seedling

(c) Mature fass mutant

2 m

m

2 m

m

0.3

mm

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

– The first cell division of the zygote

is abnormal because it is

symmetrical

– This results in ball-shaped plants

with neither roots nor leaves

Fig. 35-29

• 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

mature hairless cells

– 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

Fig. 35-31

Corticalcells

20 µ

m

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

– 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

Fig. 35-32

Leaves producedby adult phase

of apical meristem

Leaves producedby 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

• Ex) Petals growing in place

of stamens (see photo)

Fig. 35-33

(a) Normal Arabidopsis flower

CaSt

Pe

Se

Pe

Pe

Pe

Se

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)

– 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)

Fig. 35-34a

Sepals

Petals

Stamens

CarpelsA

BC

A + Bgene

activity

B + Cgene

activity

C geneactivity

A geneactivity

(a) A schematic diagram of the ABC hypothesis

Carpel

Petal

Stamen

Sepal

• Sepals arise from those parts of the

floral meristems in which only the

A genes are active

– Petals arise where A and B

genes are active

– Stamens arise where B and C

genes are active

• 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:

– If A or C is missing, the other activity occurs through all four whorls

Fig. 35-34a

Sepals

Petals

Stamens

CarpelsA

BC

A + Bgene

activity

B + Cgene

activity

C geneactivity

A geneactivity

(a) A schematic diagram of the ABC hypothesis

Carpel

Petal

Stamen

Sepal

Fig. 35-34b

Activegenes:

Whorls:

StamenCarpel

Petal

Sepal

Wild type Mutant lacking A Mutant lacking B Mutant lacking C

(b) Side view of flowers with organ identity mutations

A A A AC C C CB B B B B B

C C C C C C C C C C C CA A A A A A A AB B B BB A A A AB

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