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
Axillary bud
Stipules
Stem
Fig. 8-1, p. 152
Animation: Simple and
Compound Leaves
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TO PLAY
KEY TERMS

BLADE


Broad, flat part of a leaf
PETIOLE

Part of a leaf that attaches blade to stem
Leaf Morphology
Simple
Pinnately compound
Palmately
compound
California white oak
(Quercus lobata)
(a) Leaf form: simple and compound.
Alternate
American beech
(Fagus grandifolia)
(b) Leaf arrangement on a stem.
Parallel
Bermuda grass
(Cynodon dactylon)
(c) Venation patterns.
Ohio buckeye
(Aesculus
glabra)
White ash
(Fraxinus americana)
Opposite
Whorled
Sugar maple
(Acer saccharum)
Pinnately
netted
Southern catalpa
(Catalpa bignonioides)
Palmately netted
Sweetgum
(Liquidambar
styraciflua)
Black willow
(Salix nigra)
Stepped Art
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
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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
Animation: Monocot and
Dicot Leaves
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Bundle Sheath Extensions
Upper epidermis
Bundle sheath
extension
Bundle sheath
Midvein
Bundle sheath
extension
Lower epidermis
Fig. 8-5, p. 157
Leaf Cross Sections
Leaf Cross Sections
Upper epidermis
Palisade
mesophyll
Midvein
Lengthwise
view of vein
Spongy
mesophyll
Privet
Air space
Lower
epidermis
Stoma
Xylem
Phloem
(a) Privet (Ligustrum vulgare), a eudicot, has a mesophyll with
distinct palisade and spongy sections.
Fig. 8-6a, p. 158
Midvein
Bundle sheath cells
Mesophyll
Parallel vein
Upper
epidermis
Lower
epidermis
Phloem
Xylem
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
(a) A folded leaf blade.
The inconspicuous
bulliform cells occur in
the upper epidermis
on both sides of the
midvein.
Bulliform
cells
Midvein
Fig. 8-7a, p. 159
Bulliform
cells
(b) An expanded leaf
blade. A higher
magnification of the
midvein region shows
the enlarged, turgid
bulliform cells.
Mesophyll
cell
Midvein
Fig. 8-7b, p. 159
LEARNING OBJECTIVE 3

Outline the physiological changes
that accompany stomatal opening
and closing
Variation in Guard Cells
Open
Guard
cells
Closed
Subsidiary
cells
(a) Guard cells of eudicots and many monocots
are bean shaped.
Fig. 8-8a, p. 160
Open
Guard
cells
Closed
Subsidiary
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
Animation: Stomata
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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
1 Blue light
activates
proton
pumps.
2 Protons are
3
pumped
out of guard
cells,
forming proton
gradient.
4 Chloride ions 5
Potassium
ions enter
also enter guard
guard cells
cells through
through
ion channels.
voltageactivated ion
channels.
Water enters
guard cells by
osmosis,and
stoma opens.
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
Guard cells of
sunken stoma
Epidermis and cuticle
Resin duct
Endodermis
Xylem
Vascular
Phloem bundle
Mesophyll cell
(photosynthetic
parenchyma cell)
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
Axillary bud
Bud scales
Petiole
Abscission
zone
Stem
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
Stem
Pot (modified
leaf)
(a) The leaves of the flowerpot plant (Dischidia
rafflesiana) are modified to hold water and organic
material carried in by ants.
Fig. 8-16a, p. 169
Root
(b) A cutaway view of a pot removed from
a plant reveals the special root that absorbs
water and dissolved minerals inside the pot.
Fig. 8-16b, p. 169
Carnivorous Plants

Leaves modified to trap insects