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Functions of Leaves
• Photosynthesis– Release oxygen, synthesize sugars
• Transpiration– Evaporation of water from leaf surface
• Specialized functions– Water storage– Protection
Comparison of Monocot and Dicot Leaves
Type Shape of blade Venation Description
Monocot
Strap-shaped *blade
Parallel vascular bundles
Leaf bases usually wrap around stem
Dicot
Thin, flat blade Netted pattern of vascular bundles
Petiole holds blade away from stem
*blade – portion of leaf that absorbs light energy
Leaf Blade
• Broad, flat surface for capturing light and CO2
• Two types of leaves– Simple leaves– Compound leaves
Leaf Blade
• Compound leaves– Blade divided into leaflets– Two types
• Palmately compound– Leaflets diverge from a single point– Example: red buckeye
• Pinnately compound– Leaflets arranged along an axis– Examples: black locust, honey locust
Leaf Blade
– Advantages of compound leaves• Spaces between leaflets allow better air flow over
surface– May help cool leaf– May improve carbon dioxide uptake
Petiole
• Narrow base of most dicot leaves
• Leaf without petiole – sessile
• Vary in shape
• Improves photosynthesis– Reduces extent to which leaf is shaded by
other leaves– Allows blade to move in response to air
currents
Sheath
• Formed by monocot leaf base wrapping around stem
• Ligule– Keeps water and dirt from getting between stem and
leaf sheath
• Auricles– In some grass species– Two flaps of leaf tissue– Extend around stem at juncture of sheath and blade
Sheath
Why does grass need mowing so often?
• Grass grows from base of sheath
• Intercalary meristem
• Allows for continued growth of mature leaf
• Stops dividing when leaf reaches certain age or length
Leaf Veins• Vascular bundles composed of xylem and
phloem
Type of venation Example Description
Parallel Monocots
•Several major veins running parallel from base to tip of leaf
•Minor veins perpendicular to major veins
Netted Dicots•Major vein (midvein or midrib) runs up middle of leaf
•Lateral veins branch from midvein
Open dichotomous
Ferns and some gymnosperms
Y-branches with no small interconnecting veins
Epidermis
• Covers entire surface of blade, petiole, and leaf sheath
• Continuous with stem epidermis• Usually a single layer of cells• Cell types
– Epidermal cells– Guard cells– Subsidiary cells– Trichomes
Epidermal Cells
• Appear flattened in cross-sectional view
• Outer cell wall somewhat thickened
• Covered by waxy cuticle– Inhibits evaporation through outer epidermal
cell wall
Stomatal Apparatus
• Cuticle blocks most evaporation
• Opening needed in epidermis for controlled gas exchange
• Two guard cells + pore stoma
• Subsidiary cells – Surround guard cells– May play role in opening and closing pore
Stomatal Apparatus
• Guard cells + subsidiary cells stomatal apparatus
• Functions of stoma– Allows entry of CO2 for photosynthesis
– Allows loss of water vapor by transpiration• Cools leaf by evaporation• Pulls water up from roots
Stomatal Apparatus
• Stomata usually more numerous on bottom of leaf
• Stomata also found in– Epidermis of young stem– Some flower parts
Trichomes
• Secretory– Stalk with multicellular or secretory head– Secretion often designed to attract pollinators
to flowers
• Short hairs– Example: saltbush (Atriplex)– Hairs store water, reflect sunlight, insulate leaf
against extreme desert heat
Trichomes
• Mat of branched hairs– Example: olive tree (Olea europea)– Act as heat insulators
• Specialized trichomes– Leaves modified to eat insects as food
Mesophyll
• Two distinct regions in dicot leaf– Palisade mesophyll– Spongy mesophyll
• Substomatal chamber– Air space just under stomata
Mesophyll
Type Cell type Location Description
Palisade mesophyll
Palisade parenchyma, tightly packed, column shaped, oriented at right angles to leaf surface
Usually on upper surface
Cells tightly packed, absorb sunlight more efficiently
Spongy mesophyll
Spongy parenchyma cells, irregularly shaped, abundant air spaces
Usually located on bottom surface
Irregular cell shape, abundant air spaces allow more efficient air exchange
Mesophyll
• Dicot midrib (midvein)– Xylem in upper part of bundle– Phloem in lower part of bundle
• Bundle sheath– Single layer of cells surrounding vascular
bundle– Loads sugars into phloem– Unloads water and minerals out of xylem
Formation of New Leaves
• Steps in leaf formation– Initiated by chemical signal– Location in leaf depends on plant’s phyllotaxis– Cells at location begin dividing
• Becomes leaf primordium
– Shape of new leaf determined by how cells in primordium divide and enlarge
Cotyledons
• Seed leaves– Primarily storage organs– Slightly flattened, often oval shaped– Usually wither and die during seedling growth
• Example of exception – bean plant• Cotyledons enlarge and conduct photosynthesis
Heterophylly
• Different leaf shapes on a single plant
• Types of heterophylly– Related to age of plant
• Example: ivy (Hedera helix)– Juvenile ivy leaves – three lobes to leaves– Adult ivy leaves – leaves are not lobed
Heterophylly
– Environment to which shoot apex is exposed during leaf development
• Example: marsh plants– Water leaves
» Leaves developing underwater are thin with deep lobes
– Air leaves» Shoot tip above water in summertime develops
thicker leaves with reduced lobing
Heterophylly
– Position of leaf on tree• Shade leaves
– Develop on bottom branches of tree– Mainly exposed to shade– Leaves are thin with large surface area
• Sun leaves– Develop near top of same tree– Exposed to more direct sunlight– Leaves are thicker and smaller
Adaptations for Environmental Extremes
• Xerophytes– Grow in dry climates– Leaves designed to conserve water, store
water, insulate against heat• Sunken stomata• Thick cuticle• Sometimes multiple layers to epidermis
Adaptations for Environmental Extremes
• Xerophytes– Abundance of fibers in leaves
• Help support leaves• Help leaf hold shape when it dries
– Examples• Oleander (Nerium oleander)• Fig (Ficus)• Jade plant (Crassula argentea)
Adaptations for Environmental Extremes
• Hydrophytes– Grow in moist environments– Lack characteristics to conserve water– Leaves
• Thin• Thin cuticle• Often deeply lobed
• Mesophytes – Grow in moderate climates
Leaf Modifications
• Spines– Cells with hard cell wall– Pointed and dangerous to potential predators
• Tendrils– Modified leaflets– Wrap around things and support shoot
Leaf Modifications
• Bulbs– Thick leaves sometimes referred to as bulb
scales• Store food and water
– Modified branches with short, thick stem and short, thick storage leaves
Leaf Modifications
• Plantlets– Leaves have notches along margins– Meristem develops in bottom of each notch
that produce a new plantlet– Plantlet falls off leaf and roots in soil– Form of vegetative (asexual) reproduction– Example
• Air-plant (Kalanchoe pinnata)
Leaf Abscission
• Abscission – separation• Result of differentiation and specialization
at region at base of petiole called abscission zone– Weak area due to
• Parenchyma cells in abscission zone are smaller and may lack lignin in cell walls
• Xylem and phloem cells are shorter in vascular bundles at base of petiole
• Fibers often absent in abscission zone
Leaf Abscission
• Abscission zone weakens
• Cells in vascular bundles become plugged
• Leaf falls off
• Leaf scar – Scar that remains when leaf falls off– Sealed over with waxy materials which block
entrance of pathogens