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Plant Form and Function

Chapter 17

Plants

• Herbaceous (nonwoody)

• In temperate climates, aerial parts die back

• Woody

• In temperate climates, aerial parts persist

The Plant Body

Functions of:

Roots

Stem

Leaves

• Flowering plants can be divided into two

groups:

– Monocots: grasses, lilies, palms, and orchids

– Dicots: deciduous trees, bushes, and many

garden flowers

Flowers Leaves Roots Seeds Stems

Flower parts are in

threes or multiples

of three

Flower parts are in

fours or fives or multiples

of four or five

Leaves have smooth

edges, often narrow,

with parallel veins

Leaves are palmate

(handlike) or oval

with netlike veins

Vascular bundles

are scattered

throughout the stem

Monocots have a

fibrous root system

The seed has one

cotyledon (seed leaf)

The seed has

two cotyledons

(seed leaves)

Dicots have a

taproot system

Vascular bundles

are arranged in a

ring around the stem

Monocots

Dicots

embryo

cotyledon

embryo

cotyledons

Fig. 17-2

Tissue Systems

• Integrated throughout the plant body

• provide continuity from organ to organ

• Plant body has 3 tissue systems

1. ground

2. vascular

3. dermal

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Ground Tissue System

• Consists of 3 tissues, many functions

• parenchyma tissue

• collenchyma tissue

• sclerenchyma tissue

Ground Tissue: Parenchyma Tissue

• Composed of living parenchyma cells

• with thin primary cell walls

• Functions

• photosynthesis

• storage

• Secretion

Fig. 32-4a, p. 706

Vacuole

Intercellular

space

Nucleus

Cytoplasm

Cell wall Parenchyma cells

Ground Tissue: Collenchyma Tissue

• Consists of collenchyma cells

• with unevenly thickened primary cell walls

• Provides flexible structural support

• Strings of celery

Fig. 32-4b, p. 706

Thickened corner

of cell wall

Nucleus Cytoplasm Vacuole

Collenchyma cells

Ground Tissue: Sclerenchyma Tissue

• Composed of sclerenchyma cells

• sclereids or fibers

Thick cell walls

• Sclerenchyma cells often dead at maturity

• provide structural support

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Fig. 32-4c, p. 706

Lumen

Cell wall

Sclerenchyma cells

Vascular Tissue System

• Conducts materials throughout plant body

• Provides strength and support

Vascular Tissue: Xylem

• Complex tissue, conducts water and dissolved

minerals

• 2 types of cells of xylem

• tracheids

• vessel elements

Fig. 32-5ab, p. 708

Xylem

End wall with

perforations

Pits

Cell wall

Lumen

(a) Tracheid. (b) Vessel

element

Vascular Tissue: Phloem

• Complex tissue, conducts sugar in solution

• 2 types of cells of phloem

1. sieve tube elements

2. assisted by companion cells

Fig. 32-5cd, p. 708

Phloem

Sieve plate

with pores

Sieve tube

element

Phloem

parenchyma

cells

Lateral sieve

area

Plasmodesma

Companion cell

(c) Sieve tube

element.

(d) Phloem

tissue.

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Dermal Tissue System

• Outer protective covering of plant body

• Epidermis:

• complex tissue

• covers herbaceous plant body

• Periderm:

• complex tissue

• covers woody parts of plant body

Dermal Tissue: Epidermis

• Waxy cuticle reduces water loss

• secreted by epidermis covering aerial

parts

• Stomata permit gas exchange

• between shoot system and atmosphere

• outgrowths or hairs

• many sizes, shapes, and functions

Growth in Plants

• Localized in specific regions (meristems)

• Involves 3 processes:

- cell division

- cell elongation

- cell differentiation

• Primary Growth vs. Secondary Growth

Primary Growth • Increase in stem or root length

• occurs in all plants

• Apical meristems

• at tips of roots and shoots

• within buds of stems

• Responsible for primary growth

Fig. 32-7, p. 710

Root hairs

Area of cell

maturation

Area of cell elongation

Root

cap Apical meristem

(Area of cell division)

Herbaceous Stems

• Epidermis: protective layer covered by a water-

conserving cuticle

• Xylem: conducts water and dissolved minerals

• Phloem: conducts dissolved sugar

• Cortex, pith, and ground tissue:

– function primarily for storage & support

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Basic Tissues in Herbaceous Stems

• Herbaceous eudicot stems

– vascular bundles arranged in a circle (in cross

section)

– distinct cortex and pith

• Monocot stems

– vascular bundles scattered in ground tissue Pith

Cortex

Ground

tissue

Herbaceous Stems

Herbaceous Dicot Stem Monocot Stem

Fig. 34-3a, p. 734

Ground tissue

Vascular

bundles

Epidermis

500 µm

(meristematic)

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Fig. 35-3b, p. 751

Cortex cells

Endodermis

cell

Pericycle cell

Phloem cell

Xylem vessel

elements

25 µm dicot root

Monocot Root

Apical Primary Lateral Secondary

meristems tissue meristems tissues

Meristematic

cells

Primary xylem

Primary phloem

Cortex

Pith

Epidermis

Vascular

cambium

Cork

cambium

Secondary

xylem (wood)

Secondary

phloem

(inner bark)

Periderm

Secondary Growth

• Increase in stem or root girth (thickness)

• Woody plants only!

• Mitosis of meristematic at leteral

meristems (not apical meristems)

• throughout length of older stems and

roots

• Two Lateral Meristems responsible for

secondary growth

1. vascular cambium

2. cork cambium

Fig. 32-9, p. 712

Cork cambium: outer = cork cells; inner = cork parenchyma

cork cells & parenchyma = PERIDERM

Inner bark (secondary phloem)

Bark

Wood

(secondary xylem)

Vascular cambium

Secondary Growth

• Production of secondary tissues, wood, bark

– occurs in some flowering plants (woody

dicots) and all cone-bearing trees

• Vascular cambium divides in two directions

– secondary xylem (to the inside)

– secondary phloem (to the outside)

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Vascular

Cambium

Fig. 34-4a, p. 735

Primary

xylem Epidermis

Cortex Primary

phloem

Vascular

cambium

Pith

Fig. 34-4b, p. 735

Remnant

of cortex

Remnant of

epidermis Remnant

of primary

phloem Secondary phloem

(inner bark)

Secondary xylem

(wood)

Periderm

(outer bark)

Remnant of

primary xylem

Remnant of

pith

Vascular

cambium

Fig. 34-4c, p. 735

Secondary xylem

(wood)

Periderm (outer bark; remnants of primary phloem, cortex and epidermis are gradually crushed or turn apart and sloughed off)

Secondary phloem

(inner bark)

Remnant of

primary xylem

Remnant of

pith Vascular

cambium

Fig. 34-5, p. 736

1X2X3X4X 2P1P

1X2X3X 2P1P

1X2X 2P1P

Secondary xylem Secondary phloem

1X 2X 1P

Second division of vascular cambium forms a phloem cell.

1X 1P

Division of vascular cambium forms two cells, one xylem cell and one vascular cambium cell.

1X

Vascular cambium cell when secondary growth begins.

Vascular cambium cell

Tim

e

Cork Cambium

• Lateral meristem that produces “bark”

– cork parenchyma and cork cells

• Cork cells (cork)

– to outside of cork cambium

• Cork parenchyma

– to inside of cork cambium

– primarily for storage in a woody stem

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Fig. 34-6, p. 737

Primary xylem Pith

Annual ring of

secondary xylem

Secondary

xylem (wood)

Vascular

cambium

Secondary

phloem

Periderm and

remnants of primary

phloem, cortex, and

epidermis

Expanded

phloem ray

Xylem ray

0.5 mm

Fig. 34-8, p. 739

Heartwood

Sapwood

Fig. 34-9, p. 739

Cross section of

3-year-old Tilia

stem

Secondary

phloem

Vascular cambium

Summerwood

Annual

ring of

xylem Springwood

Summerwood of

preceding year 100 µm

Fig. 33-3, p. 718

Palisade

mesophyll

Vein

(vascular

bundle) Cuticle

Spongy

mesophyll

Upper

epidermis

Bundle

sheath

Xylem

Phloem Stoma

Airspace

Lower

epidermis Stoma

Guard cells

Fig. 33-7a, p. 722

Open Closed

Guard

cells Subsidiary

cells

Stoma

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Fig. 34-10, p. 740

Most water that plant

absorbs is transpired

into atmosphere.

Sugar molecules from photosynthesis are transported in phloem throughout plant, including into roots.

Once inside roots, water and minerals are transported upward in xylem to stems, leaves, flowers, fruits, and seeds.

Roots obtain water

and dissolved

minerals from soil.

Stepped Art

Transport Water Movement

• Water and dissolved minerals move from soil into

root tissues (epidermis, cortex)

• Water and minerals move upward, from root xylem to

stem xylem to leaf xylem

• Water entering leaf exits leaf veins and passes into

atmosphere (Transpiration)

Tension–Cohesion Model

• Explains rise of water

– even in the tallest plants!

• Transpiration

– evaporative pull causes tension at top of plant

• Column of water pulled up through the plant remains

unbroken

– due to cohesive (together) and adhesive (others)

properties of water

Sugar Translocation

• Dissolved sugar is moved upward or

downward in phloem

– from source area of excess sugar (usually

a leaf)

– to a sink (area of storage or sugar use:

roots, apical meristems, fruits, seeds)

• Sucrose is predominant sugar transported in

phloem

Pressure–Flow Hypothesis

• Explains movement of materials in phloem

• Companion cells actively load sugar into

sieve tubes at source

– requires ATP

• ATP energy pumps protons out of sieve tube

elements

ATP

Source

Sink

Pressure-flow theory

Sucrose loaded and

unloaded requires ATP

Water moves osmotically