The Shoot System II: The Form and Structure of Leaves

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 *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

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

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

The Shoot System II: The Form and Structure of Leaves