The Shoot System II: The Form and Structure of Leaves

Chapter 8

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

• Simple leaves– Leaves with a single blade– Examples

• Poplar• Oak • Maple

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

• Originate from meristems

• Leaf primordia – early stages of development

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

Environmental Abscission Controls

• Cold temperatures

• Short days– Induce hormonal changes that affect

formation of abscission zone– Leaves move nutrients back into stem– Leaves lose color– Leaves fall off tree– Leaves decompose and recycle nutrients