the plant bodyacademic.uprm.edu/lrios/3052/life11e_ch33_lecture.pdf · 2019-03-12 · 33.2 plant...

33The Plant Body

Chapter 33 Key Concepts

33.1 The Plant Body Is Organized in a

Distinctive Way

33.2 Plant Organs Are Made Up of Three

Tissue Systems

33.3 Meristems Build a Continuously

Growing Plant

33.4 Domestication Has Altered Plant

Form

Investigating Life: Bread of the Tropics

How might plant biologists improve

the cassava plant for human use?

Roots of the cassava plant store starch,

and it is important in the diets of over

800 million people around the world.

But cassava is a poor source of protein,

and the roots contain cyanide so they

must be carefully prepared.

Key Concept 33.1 Focus Your Learning

• Plant growth patterns reflect challenges

imposed by scarce resources and an

inability to move.

• Plant development is influenced by

apical meristems, totipotency,

vacuoles, and cell walls.

• Two basic patterns that develop early

in plant embryogenesis are apical–

basal polarity and radial symmetry.

33.1 The Plant Body Is Organized in a Distinctive Way

Plants must harvest energy from sunlight

and collect water and mineral nutrients

from the soil.

Stems, leaves, and roots enable plants

anchored in one spot to capture scarce

resources.

Plants can grow throughout their lifetimes

and can redirect growth to respond to

environmental cues.


All vascular plants have essentially the

same structural organization. This

chapter describes the basic structure of

the angiosperms.

Three types of vegetative organs: roots,

stems, and leaves

• Organized into shoot systems and

root systems

Figure 33.1 Vegetative Organs and Systems


Root system: Anchors plant, absorbs

water and mineral nutrients, stores

products of photosynthesis.

Extreme branching of roots provides large

surface area for absorption.


Shoot system: Stems, leaves, flowers.

• Leaves are the main organs of

photosynthesis.

• Stems hold and display leaves in the

sun; connect roots and leaves.


Shoots and roots are composed of

repeating modules called phytomers.

A shoot phytomer consists of node,

internode, and axillary buds.

A bud can develop into a leaf, a

phytomer, a flower, or a flowering stem.

The terminal bud is at the end of a stem

or branch.


Two major clades of angiosperms:

• Monocots—narrow-leaved plants such

as grasses, lilies, orchids, and palms.

• Eudicots—broad-leaved plants such as

soybeans, roses, sunflowers, and

maples.

Figure 33.2 Comparing the Two Major Angiosperm Clades


Processes of plant development:

• Determination—commitment of cells

to their ultimate fates

• Differentiation—cell specialization

• Morphogenesis—organization of cells

into tissues and organs

• Growth—increase in body size


Development is influenced by 4 features:

1. Meristems—regions of

undifferentiated cells where cell

division occurs.

Apical meristems occur at tips of

shoots and roots; allow plants to

grow throughout their lives.


2. Totipotency

Totipotent: cells can differentiate

into any type of cell in the body.

Some differentiated plant cells can

dedifferentiate and become

totipotent.

A plant can repair damage caused

by the environment or herbivores.


3. Vacuoles

Mature plant cells usually have a

central vacuole containing a high

concentration of solutes.

The solutes are pumped into the

vacuole by transporter proteins in

the tonoplast, the vacuolar

membrane.


Active accumulation of solutes

provides osmotic force for water

uptake into the vacuole.

As the vacuole expands, it exerts

turgor pressure on the cell wall,

which keeps plants upright and is

essential for plant growth.


4. Cell walls

Each plant cell is surrounded by a

rigid cell wall.

Morphogenesis is controlled by the

planes of cell division, which

determine the direction in which a

piece of tissue will grow.

Figure 33.3 Cytokinesis and Morphogenesis


Plants grow by cell expansion.

Proteins called expansins in the cell wall

help loosen it by disrupting noncovalent

bonds between cellulose microfibrils and

other polysaccharides.

This is followed by assembly of new

polysaccharides and microfibrils,

allowing the cell wall to grow.


Primary cell wall—wall of a growing cell.

When cell expansion stops, some plants

deposit more cellulose layers to form a

rigid secondary cell wall.

Secondary walls cannot expand. They

contain lignin, a complex polymer that is

a major component of wood.

In-Text Art, Ch. 33, p. 718 (1)

In-Text Art, Ch. 33, p. 718 (2)


Two basic patterns are established in the

plant embryo:

• Apical–basal axis: Arrangement of cells

and tissues along the main axis from

root to shoot.

• Radial axis: Concentric arrangement of

the tissue systems.

Figure 33.4 Two Patterns for Plant Morphogenesis


The first division of a zygote results in

uneven distribution of the cytoplasm,

which establishes polarity.

One cell produces the embryo, the other

produces a supporting structure, the

suspensor.

In eudicots, the cotyledons begin to

develop in the heart stage. Elongation

results in the torpedo stage.

Figure 33.5 Plant Embryogenesis


The shoot apical meristem develops

between the cotyledons.

At the other end of the axis, the root

apical meristem forms.

By the end of embryogenesis, radial

symmetry has been established; the 3

tissue systems are arranged

concentrically.

Key Concept 33.1 Learning Outcomes

• Identify ways in which plants have

overcome the problems of scarce

resources and an inability to move.

• Analyze major differences in plant and

animal development.

• Examine the process by which a

zygote develops into an embryo with an

apical−basal axis.


• Ground tissue forms most of the plant

body and includes parenchyma,

collenchyma, and sclerenchyma. The

dermal and vascular systems have

parenchyma and sclerenchyma.

• Vascular tissue consists of xylem and

phloem, which are the plant’s transport

system.

33.2 Plant Organs Are Made Up of Three Tissue Systems

Plant tissues are grouped into 3 tissue

systems: dermal, ground, and vascular.

These ultimately extend throughout the

plant body in a concentric arrangement.

Figure 33.6 Three Tissue Systems Extend throughout the Plant Body


Dermal tissue system:

• Forms the epidermis, or outer covering

• Usually a single layer of cells

• Stems and roots of woody plants have

a dermal tissue called periderm


Epidermal cells can differentiate to form:

• Stomatal guard cells—form stomata

(pores) for gas exchange

• Trichomes (leaf hairs)—protection

against insects and damaging solar

radiation

• Root hairs—increase root surface area

for uptake of water and mineral

nutrients


Above ground epidermis secretes a waxy

extracellular cuticle.

Made up of cutin, a complex mixture of

waxes and cell wall polysaccharides.

It limits water loss, protects against

damaging solar radiation, and is a

barrier to pathogens.


Ground tissue system:

• Makes up most of the plant body

• Functions in storage, support, and

photosynthesis

• Has three cell types:

Collenchyma

Parenchyma

Schlerenchyma


Parenchyma cells:

• Thin primary walls, large central

vacuoles

• Middle lamella—layer of pectin that

cements adjacent cells together

• Sites of photosynthesis and storage

(e.g., starch in roots)

• Many can divide and can give rise to

new cells (e.g., to heal a wound)

Figure 33.7 Ground Tissue Cell Types (Part 1)


Collenchyma cells:

• Primary walls thickened by pectins;

usually elongate

• Provide support in leaf petioles,

nonwoody stems, and growing organs

• Tissue is flexible; can bend without

snapping

• Celery “strings” are collenchyma cells


Sclerenchyma cells:

• Thickened secondary walls; many

undergo apoptosis after secondary wall

is laid down.

• Fibers: Elongated cells provide rigid

support; often in bundles.

• Sclereids may be densely packed as

in nut shells, or in clumps as in stone

cells in pears.


Vascular tissue system:

• Xylem distributes water and minerals

taken up by roots to all parts of the

plant.

• Phloem transports carbohydrates from

site of production (sources) to sites of

utilization or storage (sinks).


Xylem: Mature cells are dead.

Two types of tracheary elements:

1. Gymnosperms have tracheids with

pits in the secondary walls that allow

materials to move freely; major cell

type in gymnosperm wood.

Figure 33.8 Vascular Tissue Cell Types (Part 1)


2. Flowering plants have vessels made

of vessel element cells end-to-end,

also with pits.

Pits larger diameter than tracheids.

End walls break down before death,

forming hollow tubes.

Xylem of many angiosperms also

contains tracheids.


Phloem: Mature cells are living.

• Sieve tube elements: Cells meet end-

to-end; plasmodesmata in the end

walls enlarge to form pores—the sieve

plate.

• Some cell components break down, but

companion cells retain all organelles

and act as “life support” for sieve tube

elements.


• List and discuss the importance of the

various functions of parenchyma.

• Define cell characteristics that make

collenchyma useful as plant support

structures.

• Compare collenchyma and

sclerenchyma in terms of their ability to

provide support to plants.


• Describe the water-conducting

elements in plants, and compare these

elements in gymnosperms and

angiosperms.


• Growth in plants can be either

determinate or indeterminate,

depending on the organ structure.

• Growth in terms of cell numbers occurs

at meristems.

• Different apical meristems are where

growth in cell numbers occurs and

gives rise to leaves, stems, flowers,

and roots.


• At the root meristem, zones of cell

division, elongation, and maturation

(differentiation) form the tissues of the

root and root cap.

• The root consists of several tissue

layers outside of the inner vascular

tissues. These tissues have different

arrangements in eudicot and monocot

roots.


• Secondary growth in eudicots causes

increase in diameter and forms wood

and bark.

33.3 Meristems Build a Continuously Growing Plant

Plants grow toward sunlight, and toward

water and dissolved minerals in the soil.

In most animals, growth is determinate—

growth of the individual and all its parts

stops in adult stage.

Shoots and roots have indeterminate

growth—continuous throughout life.


Primary growth: Cell division followed by

cell enlargement; lengthens shoots and

roots.

• Results in the primary plant body: All

non-woody parts of the plant.

Many herbaceous plants consist entirely

of primary plant body.


Secondary growth: Increases plant

thickness.

Trees and shrubs have a secondary plant

body consisting of wood and bark.

Stems and roots thicken as tissues are

laid down.


Meristems: Localized regions of

undifferentiated cells; source of all new

growth in adult plants.

Cells that perpetuate the meristem are

called initials (comparable to animal

stem cells).

When initials divide, some daughter cells

become specialized, others develop into

new initials.


Types of meristems:

• Apical meristems result in primary

growth; give rise to every cell in the

primary body.

• Primary meristems develop from

initials; give rise to the 3 tissue

systems.


• Lateral meristems orchestrate

secondary growth.

Vascular cambium and cork cambium

contribute to the secondary plant

body.

Figure 33.9 Apical and Lateral Meristems (Part 1)


Types of apical meristems:

• Vegetative meristems give rise to

stems, leaves, and roots.

• Inflorescence meristems arise from

shoot apical meristems, and in turn

develop floral meristems.

• Root apical meristems extend roots.


Apical meristems give rise to primary

meristems: protoderm, ground

meristem, procambium.

In-Text Art, Ch. 33, p. 724


Root apical meristem:

• Some daughter cells become the root

cap—protects root tip as it grows

through the soil.

• The cap secretes a muco-

polysaccharide (slime) as a lubricant.

• The root cap detects gravity and

controls downward growth of roots.


A quiescent center in the apical meristem

can become active if needed.

Zone of cell division: Apical and primary

meristems.

Zone of cell elongation: Newly formed

cells elongate, pushing the root farther

into the soil.

Zone of maturation: Cells begin to

differentiate.

Figure 33.10 Tissues and Regions of the Root Tip (Part 1)

Figure 33.10 Tissues and Regions of the Root Tip (Part 2)


Root tissues:

• Arrangement of tissues is different in

monocots and eudicots.

• Protoderm gives rise to the

epidermis—protection and absorption.

Many epidermal cells produce root

hairs, which increase the root surface

area.

Figure 33.11 Products of the Root’s Primary Meristems (Part 1)


Root tissues:

Ground meristem gives rise to the

cortex and endodermis.

Endodermal cell walls have suberin, a

waterproof substance.

Placement of suberin in certain parts

of the cell wall allows control of water

movement and mineral ions into the

vascular tissue system.


Procambium produces the vascular

cylinder or stele.

Pericycle—undifferentiated cells:

• Gives rise to lateral roots

• Gives rise to lateral meristems that

thicken the root

• Membrane transport proteins move

nutrient ions into the xylem

Figure 33.12 Lateral Root Anatomy


In eudicot roots, xylem is at the center,

often in a star shape in cross section;

between the points are bundles of

phloem.

Monocot roots have pith (parenchyma

cells) at the center, which stores

carbohydrates.

Pith is also found in stems of both

monocots and eudicots.


Papyrus is made from strips of pith from

stems of the papyrus plant Cyperus

papyrus.

In-Text Art, Ch. 33, p. 725


Water and minerals enter through the root

system in most plants. The root system is

often larger than the shoot system.

The embryonic root is called the radicle.

In most eudicots the radicle develops into a

primary root or taproot with outgrowth of

lateral roots, forming a taproot system.

Taproots often function as food storage.

Figure 33.13 Root Systems of Eudicots and Monocots (Part 1)


Typical monocot roots arise from the stem

near ground level and are called

adventitious roots.

They form a fibrous root system: many

thin roots of equal diameter originate

from the stem at ground level or below.

• Large surface area; cling to soil well


Prop roots are adventitious roots that

help support the stem in some monocots

(corn, banyan trees, some palms).

These species cannot support

aboveground growth by the thickening of

their stems.


Stem tissues

Shoots are composed of repeating

modules called phytomers; shoots grow

by adding new phytomers.

New phytomers originate from cells in

shoot apical meristems at stem tips and

axillary buds.


Shoot apical meristem forms 3 primary

meristems that give rise to the 3 tissue

systems.

Leaf primordia develop on the sides of

the shoot apical meristem at regular

intervals—these sites become the nodes.

Bud primordia form at the bases of the

leaf primordia. They can become apical

meristems of new shoots.


In young stems, vascular tissue is

arranged in vascular bundles of both

xylem and phloem.

• Eudicots: Vascular bundles form a

cylinder

• Monocots: Bundles are scattered

Figure 33.14 Vascular Bundles in Stems (Part 1)

Figure 33.14 Vascular Bundles in Stems (Part 2)


In eudicots, pith is in the center and

extends between the vascular bundles,

forming pith rays.

The cortex can contain supportive

collenchyma cells with thickened walls.

Pith and cortex constitute the ground

tissue system.

The outermost cell layer is the epidermis.


Stems elevate and support flowers and

leaves. There are many modifications:

• Potato tubers are underground stems;

the “eyes” are axillary buds.

• Many desert plants have enlarged

stems that store water.

• Runners are horizontal stems; roots

grow at intervals and independent

plants can arise from them.

Figure 33.15 Modified Stems (Part 1)


Leaves are produced from apical

meristems called vegetative meristems.

Growth of a leaf is determinate.

Leaf anatomy is adapted to carry out

photosynthesis and exchange of O2 and

CO2 with the environment, while limiting

water losses.

Figure 33.16 The Eudicot Leaf (Part 1)


Two zones of photosynthetic parenchyma

cells make up the mesophyll:

• Palisade mesophyll

• Spongy mesophyll—includes air space

for diffusion of gases


Vascular tissue forms a network of veins

in leaves.

Veins extend to within a few cell

diameters of all the cells, so mesophyll

cells are well supplied with water and

minerals, and the products of

photosynthesis can be conducted to the

phloem.

Investigating Life: Understanding the Synthesis and Transport of Cyanogenic

Glycosides

Leaves may also produce defensive

chemicals, such as cyanide in the

cassava plant.

Hypothesis: Molecules that produce

cyanide in the cassava are made in the

leaves and transported to the

underground root.

Investigating Life: Understanding the Synthesis and Transport of Cyanogenic Glycosides,

Experiment

Investigating Life: Understanding the Synthesis and Transport of

Cyanogenic Glycosides

Conclusion:

Cyanogenic glycosides are made in

leaves and are transported through

petioles to the stem, from which they are

transported to the root.


Epidermal cells are nonphotosynthetic,

and have a waxy cuticle that is

impermeable to water.

The cuticle prevents water loss, but also

prevents diffusion of gases.

Pores called stomata allow gas

exchange. They are opened and closed

by guard cells.


Secondary growth (wood and bark) arises

from two lateral meristems in eudicots:

• Vascular cambium: Elongated cells

that divide often; supplies cells of

secondary xylem and secondary

phloem.

• Cork cambium produces waxy-walled

protective cells; some become part of

the bark.


Woody twigs have both primary and

secondary growth.

Apical meristems are enclosed in buds

protected by bud scales.

Only the buds consist entirely of primary

tissues.

Figure 33.17 A Woody Twig Has Both Primary and Secondary Growth (Part 1)


Vascular cambium is initially a single layer

of cells between primary xylem and

phloem.

Division of these cells produces

secondary phloem cells toward the

outside, and secondary xylem cells

toward the inside.


A continuous cylinder of vascular

cambium runs the length of the stem and

gives rise to complete cylinders of

secondary xylem (wood) and secondary

phloem, which contributes to the bark.

It also produces vascular rays for lateral

transport.


As secondary growth continues, the

epidermis and outer cortex are stretched

and flake away.

Cells near the surface of the secondary

phloem begin to divide, forming a cork

cambium.

Cork has thick-walled cells, waterproofed

with suberin. Cork becomes the

outermost tissue of the stem or root.


Cork cambium sometimes produces cells

toward the inside, which forms the

phelloderm.

The cork, cork cambium, and phelloderm

form a tissue called periderm.

The periderm and secondary phloem— all

the tissues external to the vascular

cambium—constitute the bark.


Lenticels are spongy regions in the

periderm that allow gas exchange for

underlying tissues.

Figure 33.18 Lenticels Allow Gas Exchange through the Periderm


Tree trunks from temperate regions have

annual rings that result from seasonal

conditions.

• Spring—water is plentiful, tracheids or

vessel elements produced have large

diameters.

• Summer—less water, smaller diameter

cells with thicker walls are produced.

Figure 33.19 Annual Rings


Some monocots, such as palms, have

thickened stems, but they do not have

vascular or cork cambiums.

Palms have a wide apical meristem that

produces a wide stem. Dead leaf bases

also contribute to the stem diameter.


• Describe characteristics of determinate

and indeterminate growth in plants.

• Compare meristematic with non-

meristematic cells, and explain the

function of meristematic cells in plant

growth.

• Analyze how apical meristems are able

to produce different organs, including

leaves, stems, flowers, and roots.


• Describe or illustrate a longitudinal

section of the developing root, and

explain the functions of special zones

of cells from the tip upward.

• Explain how different types of meristem

result in the various layers of the

mature root.


• Genetic variation in the structure of

plants is a valuable natural resource for

crop plant evolution.

33.4 Domestication Has Altered Plant Form

Members of the same plant species can

be remarkably diverse in form.

This suggests that minor differences in

genes or gene regulation can underlie

dramatic differences in plant form.


Modern corn was domesticated from the

grass teosinte, which still grows in

Mexico.

Teosinte is highly branched, while

domesticated corn has a single shoot.

This is due to a single gene called

teosinte branched 1 (tb1). The protein

product regulates growth of axillary

buds.

Figure 33.20 Modern Corn Was Domesticated from the Wild Grass Teosinte


A single species, Brassica oleracea (wild

mustard), is the ancestor of many

morphologically diverse crops: kale,

broccoli, Brussels sprouts, cabbage.

Humans selected seed from

morphological variants in the wild

population with the trait they found

desirable.

Figure 20.4 Many Vegetables from One Species


The genomes of plants are still priceless

resources today.

Genetic variation in crop plants and their

wild relatives can be used to improve our

crop plants or adapt them to changing

conditions.

This is especially important as human

activities change the planet and lead to

extinction of plant species.


Various organizations around the world

have developed seed banks, where

seeds of diverse species and variants

are stored.


• Discuss reasons why wild plant

genomes should be preserved.


Many people depend on cassava for food,

but the roots must be processed to

remove the sources of cyanide.

RNA interference has been used to block

cassava leaves from making the cyanide

precursor.

How might plant biologists improve

the cassava plant for human use?


BioCassava Plus is a consortium of

scientists working to improve nutritional

quality and drought resistance.

Cassava has been crossed with a treelike

relative; the resulting plants have edible

roots that grow deep, where they can tap

into deep water supplies.

the plant bodyacademic.uprm.edu/lrios/3052/life11e_ch33_lecture.pdf · 2019-03-12 · 33.2 plant...

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