Chapter 5
Lecture Outline
Leaves
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Outline

Introduction

Leaf Arrangements and Types

Internal Structure of Leaves

Stomata

Mesophyll and Veins

Specialized Leaves

Autumnal Changes in Leaf Color

Abscission

Human and Ecological Relevance of Leaves
Introduction

All leaves originate as primordia in buds.

At maturity, most leaves have:
• Stalk = petiole
–
Leaves sessile if lacking petiole (monocots)
• Flattened blade = lamina
• Network of veins = vascular bundles
• Stipules at base of petiole

Leaves of flowering plants associated with
leaf gaps and have axillary buds at base.
Introduction

Leaves may be simple or
compound.
• Simple leaves - With a
single blade
• Compound leaves - Blade
divided into leaflets
–
Pinnately compound leaves Leaflets in pairs along rachis
(petiole)
o
–
Bipinnately
compound
leaf
Bipinnately compound leaf Leaflets subdivided
Palmately compound leaves All leaflets attached at same
point at end of petiole.
Palmately
compound leaf
Introduction

Green leaves capture light energy by means
of photosynthesis.
• Photosynthesis - Trapping and storing of energy
in sugar molecules that are constructed from
water and carbon dioxide

Stomata - Tiny pores on lower surfaces of
leaves
• Allow carbon dioxide to enter and oxygen to
diffuse out
• Water vapor also escapes via stomata.
–
Guard Cells control water loss by opening or closing
pore of stomatal apparatus.
Introduction

Other functions of leaves:
• Wastes from metabolic processes accumulate in
leaves and are disposed of when leaves are
shed.
• Play major role in movement of water absorbed
by roots
–
Transpiration occurs when water evaporates from leaf
surface.
–
Guttation - Root pressure forces water out hydathodes
at tips of leaf veins in some plants.
Leaf Arrangements and Types

Leaves are attached to stems at nodes, with
stem regions between known as internodes.
• Phyllotaxy - Arrangement of leaves on stem
–
Alternate - One leaf per node
–
Opposite - Two leaves per node
–
Whorled - Three of more leaves at a node
Alternate
Opposite
Whorled
Leaf Arrangements and Types

Venation - Arrangement of veins in a leaf or
leaflet blade
• Pinnately veined leaves - Main midvein included
within enlarged midrib.
–
Secondary veins branch from midvein.
• Palmately veined leaves - Several primary veins
fan out from base of blade.
Pinnate
venation
Palmate
venation
Leaf Arrangements and Types

Monocots - Primary veins parallel = Parallel
venation

Dicots - Primary veins divergent in various
ways = netted or reticulate venation.

Dichotomous venation - Veins fork evenly and
progressively from base of blade.
Parallel
venation
Reticulate venation
Dichotomous
venation
Internal Structure of Leaves

Three regions: Epidermis, mesophyll, veins
(vascular bundles)

Epidermis - Single layer of cells covering the
entire surface of the leaf
• Devoid of chloroplasts
• Coated with cuticle (with cutin)
• Functions to protect tissues inside leaves
• Waste materials may accumulate in epidermal
cells.
• Different types of glands may also be present in
the epidermis.
Stomata

Lower epidermis typically has thinner layer of
cutin and is perforated by numerous stomata.
• Stomata bordered by two guard cells.
–
Guard cells originate from the same parent cell, and
contain chloroplasts.
o Primary functions:
« Regulate gas exchange between leaf interior and
atmosphere
« Regulate evaporation of water
o Changes in amount of water in guard cells cause
them to inflate or deflate.
« Inflate - Stomata open
« Deflate - Stomata close
Mesophyll and Veins

Most photosynthesis takes place in the
mesophyll between the two epidermal layers.
• Palisade Mesophyll
– Compactly
stacked, barrelshaped
parenchyma
cells, commonly
in two rows
– Contains most of
leaf’s chloroplasts
• Spongy Mesophyll
–
Loosely arranged parenchyma cells with abundant air
spaces
Mesophyll and Veins

Veins (vascular bundles) are scattered
throughout mesophyll.
• Consist of xylem and phloem tissues surrounded
by bundle sheath of thicker-walled parenchyma
Mesophyll and Veins

Monocots have some differences:
• Usually do not have mesophyll differentiated into
palisade and spongy layers
• Often have bulliform
cells on either side
of main central vein
–
Bulliform cells partly
collapse under dry
conditions.
o
Causes leaf to fold
or roll, reducing
transpiration
Monocot leaf cross section
Specialized Leaves

Shade Leaves
• Receive less total
light than sun
leaves
• Compared to sun
leaves, shade
leaves:
–
–
–
–
Tend to be larger
Tend to be thinner
Have fewer welldefined mesophyll
layers and fewer
chloroplasts
Have fewer hairs
Specialized Leaves

Leaves of Arid Regions
• Arid regions have limited availability of water, wide
temperature ranges, and high light intensities.
• Leaves reduce loss of water by:
–
–
–
–

Thick, leathery leaves
Fewer stomata or sunken stomata
Succulent, water-retaining leaves, or no leaves
Dense, hairy coverings
Leaves of Aquatic Areas
• Less xylem and phloem
• Mesophyll not differentiated into palisade and
spongy layers.
• Large air spaces
Specialized Leaves

Tendrils
• Modified leaves that curl
around more rigid objects,
helping the plant to climb
or to support weak stems
Garden peas
–

Tendrils
Spines
• Modified leaves that reduce leaf
surface and water loss, and protect
from herbivory.
–
Cacti
o Leaf tissue replaced with
sclerenchyma.
o Photosynthesis occurs in stems.
Spine
Specialized Leaves

Thorns - Modified stems
arising in the axils of
leaves of woody plants
Thorn

Prickles - Outgrowths
from epidermis or cortex
Specialized Leaves

Storage leaves
• Succulent leaves are modified for water storage.
–
Have parenchyma cells with large vacuoles
–
Many desert plants
• Fleshy leaves store carbohydrates.
–
Onions, lily
Specialized Leaves

Flower-Pot Leaves
• Leaves develop into
urn-like pouches that
become home of ant
colonies.
• Ants carry in soil and
add nitrogenous wastes
that provide good
growing medium for the
plant’s own roots.
–
Dischidia, an epiphyte of
Australia
Flower-pot leaf sliced lengthwise
Specialized Leaves

Window leaves
• In succulent desert plants of Africa
• Leaves buried in ground, except for exposed end.
–
–
End has transparent, thick epidermis and transparent
water storage cells underneath.
Allows light into leaf, while buried leaves keep plant
from drying out
Specialized Leaves

Reproductive Leaves
• Walking fern - New plants at leaf tips
• Air plant - Tiny plantlets along leaf margins
Air Plant
Specialized Leaves

Floral Leaves (bracts)
• At bases of flowers or flower stalks
• Poinsettia - Flowers do not have petals, instead
brightly colored bracts surround flowers.
• Clary’s sage - Colorful bracts are at top of
flowering stalks above flowers.
Poinsettia
Clary’s sage
Specialized Leaves

Insect-Trapping Leaves
• Grow in swampy areas
and bogs
–
Nitrogen and other
elements are deficient in
soil.
o
Specialized leaves trap
and digest insects.
• Pitcher Plants
–
Insects trapped and
digested inside coneshaped leaves.
Pitcher plant
Specialized Leaves

Insect-Trapping Leaves
• Sundews
–
Have round to oval leaves
covered with glandular
hairs that have a sticky
fluid of digestive enzymes
at tip
• Venus’s Flytraps
–
Only in North Carolina and
South Carolina
–
Blade halves trap insects.
Specialized Leaves

Insect-Trapping Leaves
• Bladderworts
–
Submerged or floating in shallow water
–
Tiny bladders on leaves have trap doors that trap
insects inside bladders.
Bladder of bladderwort
Autumnal Changes in Leaf Color

Chloroplasts of mature leaves contain
several groups of pigments:
• Chlorophylls - Green
• Carotenoids - Yellows
• In fall, chlorophylls break down and other colors
are revealed.

Water soluble anthocyanins (red or blue) and
betacyanins (red) may also be present in the
vacuole.
Abscission

Deciduous plants drop leaves seasonally.

Abscission - Process by which leaves are shed
• Occurs as a result of changes in abscission zone
near base of petiole
–
Protective layer
o
–
Cells coated and
impregnated with
suberin.
Separation layer
o
Pectins in middle
lamella of cells are
broken down by
enzymes.
Human and Ecological Relevance of Leaves

Landscaping - Shade trees

Food - Cabbage, lettuce, celery petioles, spices

Dyes

Perfumes - Oils of orange tree, lavender

Ropes and Twine - Agave, hemp fibers

Drugs - Narcotics, tobacco, marijuana

Beverages - Tea, tequila (agave leaves)

Insecticides - Rotenone

Waxes - Carnauba and caussu waxes

Aesthetics - Floral arrangements, gardens
Review

Introduction

Leaf Arrangements and Types

Internal Structure of Leaves

Stomata

Mesophyll and Veins

Specialized Leaves

Autumnal Changes in Leaf Color

Abscission

Human and Ecological Relevance of Leaves