The Shoot System II: The Form and Structure of Leaves Chapter 8

Functions of Leaves Photosynthesis Transpiration Specialized functions 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 Short hairs 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 Specialized trichomes 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 Substomatal chamber 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) Bundle sheath 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 Sun 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 Tendrils 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