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Chapter 32
LEAF STRUCTURE AND FUNCTION
LEAF FORM
Leaves are organs and vary greatly in external form.
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They may range in length from about 20 m (65 ft) to about 0.15 cm (0.06 in).
Their principal parts are blade, petiole and stipules (may be absent).
Leaves can be simple or compound.
Their arrangement along the stem can be alternate, opposite or whorled.
Their venation may be netted or parallel.
Veins can be arranged pinnately or palmately
LEAF TISSUES
1. Epidermis
There are upper and lower epidermises that form the surface of the leaf.
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Made of living parenchyma cells.
Lack chloroplasts.
Covered with a waxy layer, the cuticle.
Have small openings for gas exchange called stomata (sing. stoma).
Guard cells flank each stoma.
Trichomes or hairs may be present.
2. Mesophyll
The photosynthetic tissue found in between the two epidermises is called mesophyll.
It consist of...
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Made of living parenchyma cells.
Abundant chloroplasts.
Usually loosely arranged with many air spaces.
Often arrange in two regions: palisade and spongy mesophyll.
3. Venation
Veins and diffusion cooperate in the movement of materials in veins.
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Veins or vascular bundles extend through the mesophyll.
Each vein contains xylem and phloem tissue.
Xylem is usually restricted to the upper side of the vein, and phloem to the lower side of the vein.
A non-vascular parenchymatous layer of cells called the bundle sheath surrounds veins.
The bundle sheath extensions are support columns of cells that extend from the vein to the upper
and lower epidermis.
May be composed of parenchyma, collenchyma or sclerenchyma cells.
Monocot leaves usually have parallel venation and it is not differentiated into palisade and spongy
mesophyll; the guard cells are shaped like dumbbells
Dicot leaves have netted venation, mesophyll differentiated into two regions, and the guard cells are
bean-shaped.
Subsidiary cells are epidermal cells associated with the stomata and are involved in the opening and
closing of the stomata.
Gymnosperms normally have parallel or free venation (not netted).
FUNCTION
Photosynthesis is the primary function of leaves.
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A single red maple leaf fixes about 454 kg (1,000 pounds) of CO2 into carbohydrates in one
summer.
Transparent epidermal cells allow sunlight penetrate to the palisade mesophyll, where most of the
photosynthetic activity takes place.
Open stomata allow CO2 to enter the air spaces and O2 to escape into the atmosphere.
Loose arrangement of the spongy mesophyll allows gas exchange to occur readily within the leaf.
Veins and veinlets bring the water needed for photosynthesis and carry away photosynthates.
Support tissue allows the leaf to remain expanded facing the sun.
The anatomy of the leaf reflects the environment in which the plant lives, e.g. hydrophytes have large air
spaces for floatation.
Conifers have adaptations that allow the leaves to survive cold and drought:
 Needle shape with little exposed surface.
 Thick waxy cuticle.
 Sunken stomata.
STOMATAL OPENING AND CLOSING.
Stomata are open or closed according to the physiological needs of the plant.
 Photosynthesis: gas exchange.
 Transpiration: loss of water.
In mesophytes, the stomata are usually open during the day and closed at night.
 CAM plants do the reverse.
Light and dehydration also affect the opening or closing of the stomata.
A low concentration of CO2 in the leaf induces stomata to open even in the dark.
The opening and closing of stomata are controlled by changes in the shape of the guard cells that surround
the pore.
Potassium ion mechanism.
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Light triggers an influx of K+ into the guard cells.
It occurs through active transport; ATP required.
Osmotic pressure decreases and water moves into the guard cells.
The increase turgidity of the cells causes a change in shape and the stoma opens.
Opening of the stomata is most pronounced in blue light, and to a lesser extent in red light.
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Light  proton pump moves H+  K+ actively transported 
out of the guard cell
through specific K channels
water diffuses into 
the guard cells
 guard cells change shape and open the stoma.
The stoma may close by a reversal of the process when light decreases.
Loss of turgidity closes the stoma.
TRANSPIRATION
The loss of absorbed water is 99%.
Only about 1% to 3% of the lost water passes through the cuticle. Most is lost through the stomata.
Environmental factors increase the rate of transpiration, e.g. wind, relative humidity, temperature.
Transpiration is both beneficial and harmful to the plant.
GUTTATION
When transpiration is negligible and soil moisture is high, plants release liquid water through the end of
veinlets near the leaf margin.
LEAF ABSCISSION
Metabolism slows down during the wintertime and there is little need for leaves.
Water absorption from roots slows down or stops altogether during winter. Leaves will continue to lose
water that could not be replaced.
Involves physiological changes initiated by plant hormones.
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Sugars, amino acids and minerals are removed from the leaves.
Chlorophyll breaks down.
The abscission zone is located at the base of the petiole.
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Made of thin walled parenchyma cells.
Lacks fibers and is anatomically weak.
Cork cells developed on the stem side of the abscission zone.
Enzymes dissolve the middle lamella of parenchyma cells.
Protective layer of cork seals off the wound and forms the scar.
LEAF FUNCTIONS
Photosynthesis is principal function of leaves.
Spines are leaves adapted to provide protection.
Succulent leaves are photosynthetic and store water.
Some tendrils are leaves modified for grasping.
Bud scales are leaves modified to protect the apical meristem.
Sepals are modified leaves that protect developing flowers.
Bulbs are made of leaves modified to store food underground.
Some leaves are modified to trap insects.