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32
Leaf Structure and Function
Lecture Outline
I.
Foliage leaves vary greatly in external form and size, ranging from 20m (65 ft) to
less than 2.5 cm (1 in) long
A. Leaves are so different that specific terms have been developed to describe
their shapes, margins, venation, and stem attachment
1. Leaves are typically the first structure examined when attempting to
identify a plant species
B. Most leaves have a blade, a petiole, and a stipule
1. The blade is the flat portion of the leaf that functions in photosynthesis
2. The petiole attaches the blade to the stem
3. The stipules are leaflike growths that may occur at the base of the petioles
C. Simple leaves have a single blade; compound leaves have blades divided into
leaflets
1. Axillary buds develop in the leaf axil (between petiole and stem)
2. Axillary buds never develop at the base of leaflets, and can be used to
differentiate leaflets from leaves
D. Leaves may occur in an alternate arrangement (one leaf per node), an opposite
arrangement (two leaves per node), or whorled arrangement (more than two
leaves per node)
E. Venation may be parallel (typical of monocots) or netted (common in dicots)
1. Netted venation may be palmate (several major veins radiate from one
point) or pinnate (veins radiate from a central major vein)
II.
The leaf consists of an epidermis, ground tissue that is photosynthetic, and
vascular tissue
A. The leaf has evolved to optimize photosynthesis
1. The upper and lower epidermis are primarily composed of nonphotosynthetic parenchyma cells
2. The cuticle, permeated with waxy cutin, helps to prevent water loss
a) The cuticle of plants adapted to live in hot, dry climates is thicker than
that of plants adapted to live in moist or cooler climates
3. Trichomes may retard water loss, prevent herbivores from grazing, reflect
sunlight to prevent overheating, or excrete excess salts
4. Stomata are surrounded by two epidermal guard cells that are
photosynthetic
a) Stomata are typically more abundant on the lower surface of the leaf,
and sometimes found only there (except for floating aquatic plants like
water lilies)
b)
Subsidiary cells may be associated with stomata, and are involved in
stomatal opening and closing
5.
The mesophyll is the tissue between the epidermal layers
a) Mesophyll is parenchyma specialized for photosynthesis
b) The upper cells form the palisade layer, and cells are closely packed
together
(1) The palisade layer is composed of a great number of layers in
leaves exposed to direct sunlight
c) Cells in the lower part of the leaf are loosely arranged in the spongy
layer and allow for diffusion of carbon dioxide
6. Veins (vascular bundles) carry xylem and phloem throughout the
mesophyll
a) Xylem is typically located in the upper side of the vein, and phloem is
located in the lower side of the vein
b) Vascular bundles are surrounded by bundle sheath cells, which are
either parenchyma or sclerenchyma cells
c) Bundle sheaths often run from the two epidermal surfaces to support
the vascular bundle
B. Leaf structure differs in dicots and monocots
1. Monocot leaves have parallel venation and typically lack a petiole
2. The mesophyll of some monocots (corn and other grasses) is not
differentiated into palisade and spongy layers
3. In cross section, the leaves of monocots show evenly spaced vascular
bundles because of the parallel venation
4. In dicots and some monocots, guard cells are shaped like kidney beans
a) Some grasses and related monocots have guard cells shaped like
dumbbells
b) Guard cells may be associated with subsidiary cells that aid them in
changing shape
III.
Leaf structure is related to function
A. Everything about a leaf is adapted to maximize photosynthesis and reduce
water loss
1. The epidermis is relatively transparent and lets light reach the
photosynthetic mesophyll cells
2. Stomata allow exchange of gases, bringing carbon dioxide in and letting
oxygen out
3. Xylem brings water to the leaf
4. Phloem carries sugars to other parts of the plant
5. Bundle sheaths and bundle sheath extensions help support the leaf
B. The leaves of each type of plant help it survive in the environment to which it
is adapted
1. Aquatic plants like lilies may have long petioles, which allow the leaves to
float on the water surface and gather air into the spongy mesophyll
2.
IV.
Leaves (needles) of conifers are adapted to save water, as they may live in
arid regions, or areas in which available water is frozen during the winter
a) They are evergreen and produce and lose leaves throughout the year
b) They are very waxy, with thick cuticles, especially in winter when
water is not available and humidity is low
Stomatal opening and closing are due to changes in guard cell turgidity
A. Stomata must be open for photosynthesis to occur (except in CAM plants)
1. When water flows into the guard cells, they become swollen, and the inner
walls bend outward, producing a pore
2. Stomatal opening and closing is triggered by light levels, as well as carbon
dioxide concentration within the leaf, dehydration, hormones, and a
biological clock
B. Stomata open and close in response to the movement of H+ and K+ across the
guard cells’ plasma membranes
1. Blue light stimulates this opening and closing and is detected by yellow
pigment in the guard cells, probably in their plasma membranes
2. Blue light activates ATP synthase
a) Also includes synthesis of malic acid and hydrolysis of starch
b) ATP synthase pumps H+ out of the guard cells and is facilitated by the
production of malic acid
c) This forms an electrochemical gradient
d) This process is facilitated by diffusion of K+ ions via voltage-activated
K- channels, then Cl- ions are taken into guard cells through ion
channels in plasma membranes
e) K+, Cl- and malate ions accumulate in the vacuoles of guard cells
f) Water then passes into the guard cells by osmosis, and the cell swells
3. The stomata close by K+ in guard cells during the day, increasing as a result
of sucrose (from hydrolysis of starch) and decreasing when the sun goes
down, as sucrose turns into starch (osmotically inactive)
a) Water is lost from the guard cells by osmosis, which lose their
turgidity, and thus the pore closes
b) Other factors affect stomata
(1) A low concentration of carbon dioxide opens the stomata, even in
the dark
(2) Photosynthesis reduces the concentration of carbon dioxide,
triggering the stomata to open
(3) If water levels are low, stomata will remain closed even during the
day
(4) Hormonal control (particularly abscisic acid) is also a factor
(5) An internal biological clock and circadian rhythms can also
regulate stomata opening and closing
V.
Leaves lose water by transpiration and guttation
A. Almost all of the water that a plant absorbs is ultimately lost by evaporation
from leaves
1. Very little water is lost across the cuticle; nearly all is lost through stomata
2. Light, wind, and low humidity increase water losses
3. Transpiration is tied to the upward movement of water through xylem, so
it is a necessary process
a) Transpiration also results in evaporative cooling
b) Further, transpiration results in the uptake of minerals from the soil as
plants take in water to replace that which is lost
(1) Xylem carries this water with dissolved minerals
4. Temporary wilting occurs when a plant loses more water than it takes in
a) This is reversible over night
5. Permanent wilting may result in plant death
6. Transpiration is part of the hydrologic cycle
a) This is especially important in vast forested areas such as the Amazon
B. Some plants exude liquid water
1. When soil moisture is high, and stomata are closed, water moves into the
plant by osmosis, and may be forced out at the leaf margins, giving the
appearance of dew on the leaves (guttation)
VI.
Leaf abscission allows plants in temperate climates to survive an unfavorable
season
A. All trees shed leaves
1. Conifers tend to lose leaves constantly in small numbers
2. Many angiosperm trees abscise in the fall in preparation for winter or dry
period
3. Abscission is initiated by plant hormones, particularly ethylene
a) Chlorophylls break down, and readily available nutrients are
transported from the leaves to the woody tissues (see carotenoids),
and red anthocyanins can accumulate in the vacuoles of epidermal leaf
cells
(1) This may also guard against UV damage in some species
B. In many leaves, abscission occurs at an abscission zone near the base of the
petiole
1. The abscission zone is composed of thin-walled parenchyma cells and is a
weak area with few fibers
2. The middle lamella (located between cell walls) is dissolved by enzymes
and the leaf breaks off
3. A protective layer of cork remains, forming a leaf scar
4. Recently, a gene was discovered that's involved in the development of the
abscission zone in tomato flowers and fruit
VII. Modified leaves may have varied functions
A. Some leaves are formed into spines for defense against herbivores
1. Vines have tendrils for attachment to help support the weight of the plant
(some tendrils are leaves, such as those of peas and squash; others, like the
tendrils of ivy and grapes, are modified stems)
2. Bud scales are modified leaves that protect meristems during winter
3. Bulbs of onions are fleshy leaves adapted for storage of sugars
underground
4. Succulent plants have leaves adapted for storage of water, and are also
photosynthetic
B. Modified leaves of insectivorous plants capture insects
1. Insectivorous plants typically grow in boggy areas that are low in
nitrogen– the insects supply the necessary nitrogen
2. Some insectivorous plants have passive traps, which use enzymes and
acids, like the pitcher plant and the sundew
a) Larvae of some insects and a host of microorganisms flourish in large
pitcher plants
b) Pitcher plants gain nutrients from decomposing carcasses, which
stimulates the leaves to close
3. Some insectivorous plants have leaves modified to form active traps, like
the Venus flytrap
a) Spines on the leaf margins fit closely together and trap prey
b) Digestive glands are located on the inside surface of the trap
Research and Discussion Topics

Relate the structure of the palisade and spongy mesophyll layers to the adaptations
seen in C4 and CAM mechanisms for photosynthesis.

Examine the adaptations seen in aquatic plants (freshwater and the few marine
species) for photosynthesis.

Investigate the unique mechanism by which the leaves of the Venus flytrap utilize
the force of osmosis to close the trap.

Describe various carnivorous plants, such as bladderworts, sundews, and pitcher
plants. Why do they typically grow in bogs? Why do they eat insects? The same
answer explains both questions.

What are the leaves of gymnosperms? Where are stomates found in gymnosperm
leaves? Why is this adaptive?