Plant Structure and Function

Ch. 35

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Morphology of a Flowering Plant

• Root system and shoot system are connected by vascular tissue that is continuous throughout plant

Monocots vs. Dicots

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Nucleus

Plant Cell

Chloroplast

Cell Wall

Roots

• Functions– Anchor plant in the soil

– Absorb water and minerals

– Store food

Root Structure

Key

Dermal

Ground

Vascular

Epidermis

Root hair

Cortex Vascular cylinder

Zone ofmaturation

Zone ofelongation

Zone of celldivision

Apicalmeristem

Root cap

100 µm

Root Systems

Taproot Fibrous root

•Red mangrove growing in seawater

•Adventitious prop roots support and securely anchor this shrub in the mud and loose sand of tidal waters.

•Close-up view of prop roots •Numerous pores called lenticels which provide gas exchange and an additional source of oxygen for the submersed roots.

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Cortex

Epidermis

Developing lateral root

Vascular cylinder -xylem -phloem

Modified Roots

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Stems

• Functions– Support

– Transport

– Storage

Monocot arrangement

Dicot arrangement

• Proximity of terminal bud inhibits growth of axillary buds (Apical dominance)

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Modified Stems

Storage leaves

Stem

Roots

Node

Root

Rhizome

Stolons—allow asexual reproduction

Bulbs—store food

Tubers—store foodRhizomes—horizontal stem

Leaves

• Leaf structure– Shape

– Size

– Edges

Leaf Structure

Mes

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ll

Stomata

Modified Leaves

• Tendrils—allow plant to cling to support

• Spines—reduces water loss

Modified Leaves

• Storage—modified for water storage (succulents)

• Bracts—attracts pollinators

Modified Leaves

• Reproductive leaves—produce adventitious plantlets which fall off and take root

Tissue Systems

--cube-shaped, thin and flexible cell walls--function in photosynthesizing and storing organic products and wound healing

--elongated, thicker cell walls--cells grouped in strands or cylinders to support leaves and stems (parts that are still growing)

--cells have rigid, thick walls with lignin--at maturity, consists of dead cells--supports and strengthens plant

1. Ground System•Parenchyma

•Collenchyma

•Sclerenchyma

Tissue Systems

2. Vascular SystemXylem-Conducts water and minerals

from roots to plant-composed of dead cells that form water-pipe system

Phloem-Conducts food throughout plant-composed of living cells arranged into tubules

Water-conducting Cells of XylemPARENCHYMA CELLS

Parenchyma cells in Elodealeaf, with chloroplasts (LM) 60 µm

80 µmCortical parenchyma cells

Collenchyma cells (in cortex of Sambucus, elderberry; cell walls stained red) (LM)

COLLENCHYMA CELLS

SCLERENCHYMA CELLS

SUGAR-CONDUCTING CELLS OF THE PHLOEM

WATER-CONDUCTING CELLS OF THE XYLEM

5 µm

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

25 µm

Sclereid cells in pear (LM)

Cell wall

Sieve-tube members:longitudinal view

30 µm

15 µm

Companioncell

Companioncell

Sieve-tubememberPlasmodesma

Sieveplate

Sieve plate with pores (LM)

Nucleus

Cytoplasm

Sieve-tube members:longitudinal view(LM)

Vessel elements withperforated end walls

Vesselelement

Tracheids

Pits

Tracheids and vessels(colorized SEM)

TracheidsVessel 100 µm

Sugar-conducting Cells of Phloem

PARENCHYMA CELLS

Parenchyma cells in Elodealeaf, with chloroplasts (LM) 60 µm

80 µmCortical parenchyma cells

Collenchyma cells (in cortex of Sambucus, elderberry; cell walls stained red) (LM)

COLLENCHYMA CELLS

SCLERENCHYMA CELLS

SUGAR-CONDUCTING CELLS OF THE PHLOEM

WATER-CONDUCTING CELLS OF THE XYLEM

5 µm

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

25 µm

Sclereid cells in pear (LM)

Cell wall

Sieve-tube members:longitudinal view

30 µm

15 µm

Companioncell

Companioncell

Sieve-tubememberPlasmodesma

Sieveplate

Sieve plate with pores (LM)

Nucleus

Cytoplasm

Sieve-tube members:longitudinal view(LM)

Vessel elements withperforated end walls

Vesselelement

Tracheids

Pits

Tracheids and vessels(colorized SEM)

TracheidsVessel 100 µm

Sheath of sclerenchyma

phloem

xylem

parenchyma

Tissue Systems 3. Dermal Tissue System

- Forms the outer covering of plants

•Epidermis-outer layer of cells covered by waxy cuticle•Stomata-structures that regulate passage of gases into/out of plant

Meristems: Primary GrowthGrowing region where cells actively divideApical meristems- grow in length at tips of stems and roots

Shoot apicalmeristems(in buds)

Vascularcambium

Corkcambium

Lateralmeristems

Primaryphloem

Periderm

Corkcambium

Secondaryxylem

Primaryxylem

Pith

Pith

Cortex

Secondary growth in stems

Secondaryphloem

Vascular cambium

Primary phloem

Primary xylem

Cortex

Primary growth in stems

Epidermis

Root apicalmeristems

Lateral meristems• Add thickness to woody

plants, a process called secondary growth

• Two lateral meristems– vascular cambium

adds layers of vascular tissue called secondary xylem (wood) and secondary phloem

– cork cambium replaces the epidermis with periderm, which is thicker and tougher

Primary Growth in Roots

Key

Dermal

Ground

Vascular

Epidermis

Root hair

Cortex Vascular cylinder

Zone ofmaturation

Zone ofelongation

Zone of celldivision

Apicalmeristem

Root cap

100 µm

Primary Growth in Shoots

Developingvascularstrand

Axillary budmeristems

0.25 mm

Leaf primordiaApical meristem

•Secondary growth occurs in stems and roots of woody plants but rarely in leaves

Vascular cambium•Produces secondary xylem and phloem

Cork cambium•Produces tough, thick covering for stems and roots

•Replaces epidermis

Anatomy of a Tree Trunk

• As a tree or woody shrub ages, the older layers of secondary xylem, the heartwood, no longer transport water and minerals

• The outer layers, known as sapwood, still transport materials through the xylem

Growth ring

Vascularray

Secondaryxylem

Heartwood

Sapwood

Vascular cambium

Secondary phloem

Layers of periderm

Bark

Growth, morphogenesis, and differentiation produce the plant body

• The three developmental processes of growth, morphogenesis, and cellular differentiation act in concert to transform the fertilized egg into a plant

Growth: Cell Division and Cell Expansion

• By increasing cell number, cell division in meristems increases the potential for growth

• Cell expansion accounts for the actual increase in plant size

The Plane and Symmetry of Cell Division

• 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

Division insame plane

Plane ofcell division

Division inthree planes

Cell divisions in the same plane produce a single file of cells, whereas cell divisions in three planes give rise to a cube.

Single file of cells forms

Cube forms

Nucleus

The Plane and Symmetry of Cell Division

• If the planes of division vary randomly, asymmetrical cell division occurs

Unspecializedepidermal cell

An asymmetrical cell division precedes the development of epidermal guard cells, the cells that border stomata (see Figure 35.17).

Unspecializedepidermal cell

Asymmetrical

cell division

Guard cell“mother cell”

Unspecializedepidermal cell

Developingguard cells

The Plane and Symmetry of Cell Division

• The plane in which a cell divides is determined during late interphase

• Microtubules become concentrated into a ring called the preprophase band

Preprophase bandsof microtubules

Nuclei

Cell plates

10 µm

Genetic Control of Flowering• Flower formation involves a phase change from

vegetative growth to reproductive growth

• It is triggered by a combination of environmental cues and internal signals

• Transition from vegetative growth to flowering is associated with the switching-on of floral meristem identity genes

• Plant biologists have identified several organ identity genes that regulate the development of floral pattern

Normal Arabidopsis flower. Arabidopsisnormally has four whorls of flower parts: sepals (Se), petals (Pe), stamens (St), and carpels (Ca).

Pe

Se

Pe

Se

Pe

Se

Pe

Ca

St

Abnormal Arabidopsis flower. This flower has an extra set of petals in place of stamens and an internal flower where normal plants have carpels.

• The ABC model of flower formation identifies how floral organ identity genes direct the formation of the four types of floral organs

Sepals

Petals

Stamens

AB

CCarpels

C geneactivity

B + Cgene

activity

A + Bgene

activity

A geneactivity

A schematic diagram of the ABChypothesis