Plant Organs: Leaves

Chapter 8

LEARNING OBJECTIVE 1

Describe the major tissues of the leaf (epidermis, mesophyll, xylem, and phloem)

Relate the structure of the leaf to its function of photosynthesis

“Typical” Leaf

Blade

Veins

Petiole

Stipules

Axillary bud

Stem

Fig. 8-1, p. 152

KEY TERMS

BLADE

Broad, flat part of a leaf

PETIOLE

Part of a leaf that attaches blade to stem

Leaf Morphology

Parallel

Pinnately

netted

Palmately netted

Bermuda grass

(Cynodon dactylon) Black willow

(Salix nigra)

Sweetgum

(Liquidambar

styraciflua)

(c) Venation patterns.

Stepped Art

Alternate Opposite

Whorled

American beech

(Fagus grandifolia) Sugar maple

(Acer saccharum) Southern catalpa

(Catalpa bignonioides) (b) Leaf arrangement on a stem.

Simple Pinnately compound

Palmately

compound

California white oak

(Quercus lobata) White ash

(Fraxinus americana)

Ohio buckeye

(Aesculus

glabra) (a) Leaf form: simple and compound.

Fig. 8-2, p. 154

KEY TERMS

PHOTOSYNTHESIS

The biological process that includes the capture of light energy and its transformation into chemical energy of organic molecules (such as glucose), which are manufactured from carbon dioxide and water

Tissues in a Leaf Blade

Animation: Leaf Organization

Epidermis

The transparent epidermis allows light to penetrate into the mesophyll, where photosynthesis occurs

KEY TERMS

CUTICLE

Waxy covering over epidermis of aerial parts (leaves and stems) of a plant

Enables the plant to survive in the dry conditions of a terrestrial environment

Trichomes

KEY TERMS

STOMA

Small pores in epidermis of stem or leaf

Permit gas exchange for photosynthesis and transpiration

Flanked by guard cells

GUARD CELL

Two guard cells form a pore (stoma)

Stomata

Stomata typically open during the day, when photosynthesis takes place, and close at night

KEY TERMS

MESOPHYLL

Photosynthetic ground tissue in the interior of a leaf

Contains air spaces for rapid diffusion of carbon dioxide and water into, and oxygen out of, mesophyll cells

Vascular Bundle

Leaf veins have

xylem to conduct water and essential minerals to the leaf

phloem to conduct sugar produced by photosynthesis to rest of plant

KEY TERMS

BUNDLE SHEATH

One or more layers of nonvascular cells (parenchyma or sclerenchyma) surrounding the vascular bundle in a leaf

LEARNING OBJECTIVE 2

Contrast leaf structure in eudicots and monocots

Bundle Sheath Extensions

Lower epidermis

Bundle sheath

extension

Midvein

Bundle sheath

Bundle sheath

extension

Upper epidermis

Fig. 8-5, p. 157

Leaf Cross Sections

Leaf Cross Sections

(a) Privet (Ligustrum vulgare), a eudicot, has a mesophyll with

distinct palisade and spongy sections.

Phloem Xylem Stoma

Lower epidermis

Air space

Spongy mesophyll

Lengthwise view of vein

Palisade mesophyll

Upper epidermis

Midvein

Privet

Fig. 8-6a, p. 158

Parallel vein

Midvein Bundle sheath cells

Mesophyll

Upper

epidermis

Lower

epidermis

Xylem Phloem

Fig. 8-6b, p. 158

Monocot and Eudicot Leaves

Monocot leaves

Usually narrow

Wrap around the stem in a sheath

Have parallel venation

Eudicot leaves

Usually have a broad, flattened blade

Have netted venation

Bulliform Cells

Large, thin-walled cells on upper epidermises of leaves of certain monocots (grasses)

Located on both sides of the midvein

May help leaf roll or fold inward during drought

Bulliform Cells

Midvein

Bulliform

cells

(a) A folded leaf blade.

The inconspicuous

bulliform cells occur in

the upper epidermis

on both sides of the

midvein.

Fig. 8-7a, p. 159

(b) An expanded leaf

blade. A higher

magnification of the

midvein region shows

the enlarged, turgid

bulliform cells.

Midvein

Mesophyll

cell

Bulliform

cells

Fig. 8-7b, p. 159

LEARNING OBJECTIVE 3

Outline the physiological changes that accompany stomatal opening and closing

Variation in Guard Cells

(a) Guard cells of eudicots and many monocots

are bean shaped.

Subsidiary

cells

Closed Open

Guard

cells

Fig. 8-8a, p. 160

Subsidiary

cells

Closed Open

Guard

cells

(b) Some monocot guard cells (those of grasses, reeds, and

sedges) are narrow in the center and thicker at each end.

Fig. 8-8b, p. 160

Fig. 8-8d, p. 160

Stomatal Opening 1

1. Blue light activates proton pumps

in guard-cell plasma membrane

2. Protons (H+) are pumped out of guard cells, forming a proton gradient

Charge and concentration difference on two sides of the guard-cell plasma membrane

KEY TERMS

PROTON GRADIENT

Difference in concentration of protons on the two sides of a cell membrane

Contains potential energy that can be used to form ATP or do work in the cell

Stomatal Opening 2

3. Gradient drives facilitated diffusion of potassium ions into guard cells

4. Chloride ions also enter guard cells through ion channels

Ions accumulate in vacuoles of guard cells

Solute concentration becomes greater than that of surrounding cells

KEY TERMS

FACILITATED DIFFUSION

Diffusion of materials from a region of higher concentration to a region of lower concentration through special passageways in the membrane

Stomatal Opening 3

5. Water enters guard cells from surrounding epidermal cells by osmosis

Increased turgidity changes the shape of guard cells, causing stoma to open

Stomatal Opening

Blue light

activates

proton

pumps.

Protons are

pumped

out of guard

cells,

forming proton

gradient.

Potassium

ions enter

guard cells

through

voltage-

activated ion

channels.

Chloride ions

also enter guard

cells through

ion channels.

Water enters

guard cells by

osmosis,and

stoma opens.

1 4 5 3 2

Fig. 8-9, p. 162

Stomatal Closing

As evening approaches, sucrose concentration in guard cells declines

Sucrose is converted to starch (osmotically inactive)

Water leaves by osmosis, guard cells lose their turgidity, pore closes

Adaptations to Environment

Blade Petiole

Fig. 8-10, p. 163

Mesophyll cell

(photosynthetic

parenchyma cell)

Vascular

bundle Phloem

Xylem

Endodermis

Resin duct

Epidermis and cuticle

Guard cells of

sunken stoma

Fig. 8-11, p. 164

LEARNING OBJECTIVE 4

Discuss transpiration and its effects on the plant

KEY TERMS

TRANSPIRATION

Loss of water vapor from a plant’s aerial parts

Transpiration

Occurs primarily through stomata

Rate of transpiration is affected by environmental factors

temperature, wind, relative humidity

Both beneficial and harmful to the plant

Transpiration

75% Water recycled by transpiration

and evaporation

25% Water seeps into ground or runs off

to rivers, streams, and lakes

p. 165

Wilting

Guttation

LEARNING OBJECTIVE 5

Define leaf abscission

Explain why it occurs and what physiological and anatomical changes precede it

KEY TERMS

ABSCISSION

Normal (usually seasonal) falling off of leaves or other plant parts, such as fruits or flowers

Leaf Abscission

In temperate climates, most woody plants with broad leaves shed leaves in fall

Helps them survive low temperatures of winter

Involves physiological and anatomical changes

Processes of Abscission 1

As autumn approaches, plant reabsorbs sugar

essential minerals are transported out of leaves

Chlorophyll is broken down

red water-soluble pigments are synthesized and stored in vacuoles of leaf cells (in some species)

Processes of Abscission 2

A protective layer of cork cells develops on the stem side of the abscission zone

Area where leaf petiole detaches from stem, composed primarily of thin-walled parenchyma cells

Processes of Abscission 3

Enzymes dissolve middle lamella in abscission zone

(“cement” that holds primary cell walls of

adjacent cells together)

After leaf detaches, protective layer of cork seals off the area, forming a leaf scar

Abscission Zone

Stem

Abscission

zone

Petiole

Bud scales

Axillary bud

Fig. 8-14, p. 167

LEARNING OBJECTIVE 6

List at least five modified leaves, and give the function of each

KEY TERMS

BUD SCALE

Modified leaf that covers and protects delicate meristematic tissue of winter buds

SPINE

Leaf modified for protection, such as a cactus spine

KEY TERMS

BRACT

Modified leaf associated with a flower or inflorescence but not part of the flower itself

TENDRIL

Leaf or stem that is modified for holding on or attaching to objects

Supports weak stems

KEY TERMS

BULB

A rounded, fleshy, underground bud that consists of a short stem with fleshy leaves

Specialized for storage

Leaf Modifications

Fig. 8-15a, p. 168

Fig. 8-15b, p. 168

Fig. 8-15c, p. 168

Fig. 8-15d, p. 168

Fig. 8-15e, p. 168

Fig. 8-15f, p. 168

Epiphytes

Flowerpot Plant

(a) The leaves of the flowerpot plant (Dischidia

rafflesiana) are modified to hold water and organic

material carried in by ants.

Pot (modified

leaf)

Stem

Fig. 8-16a, p. 169

(b) A cutaway view of a pot removed from

a plant reveals the special root that absorbs

water and dissolved minerals inside the pot.

Root

Fig. 8-16b, p. 169

Carnivorous Plants

Leaves modified to trap insects