Chapter_7_Leaves-1

Chapter 7: Leaves

Introduction to Leaves

  • Origin: All leaves originate as primordia in buds.

  • Mature Leaf Structure:

    • Stalk = petiole

    • Leaves are sessile if lacking a petiole (common in monocots).

    • Flattened blade = lamina.

    • Network of veins = vascular bundles.

    • Stipules present at the base of the petiole.

  • Deciduous Trees: Shed leaves after one growing season.

  • Association: Leaves of flowering plants are associated with leaf gaps and have axillary buds at their base.

Simple and Compound Leaves

  • Simple and Compound Classification:

    • Simple Leaves: Have a single blade.

    • Compound Leaves: Blade is divided into leaflets.

    • Pinnately Compound Leaves: Leaflets in pairs along the rachis (petiole).

    • Bipinnately Compound Leaves: Leaflets are subdivided.

    • Palmately Compound Leaves: All leaflets attached at the same point at the end of the petiole.

Photosynthesis

  • Function of Leaves: The flat surfaces of green leaves capture light energy.

  • Definition of Photosynthesis: Trapping and storing energy in sugar molecules, constructed from water and carbon dioxide.

  • Stomata: Tiny pores on the lower surfaces of leaves that allow:

    • Carbon dioxide to enter.

    • Oxygen to diffuse out.

    • Water vapor to escape.

  • Guard Cells: Control water loss by opening or closing the stomatal apparatus.

Other Functions of Leaves

  • Accumulate waste from metabolic processes which are disposed of when leaves are shed.

  • Major role in the movement of water absorbed by roots.

  • Transpiration: Occurs when water evaporates from leaf surfaces.

  • Guttation: Root pressure forces water out of hydathodes at the tips of leaf veins in some plants.

Leaf Arrangements and Types

  • Variation in leaf shapes and sizes:

    • Duckweed leaves: Less than 1 mm wide.

    • Giant Water Lily leaves: Can reach 2 meters in diameter.

Leaf Nodes

  • Nodes and Internodes: Leaves attach to stems at nodes, with regions in between called internodes.

  • Phyllotaxy: Arrangement of leaves on the stem:

    • Alternate: One leaf per node.

    • Opposite: Two leaves per node.

    • Whorled: Three or more leaves at a node.

Leaf Venation

  • Venation: Arrangement of veins within a leaf or leaflet blade.

    • Pinnately Veined Leaves: Main midvein included in an enlarged midrib with secondary veins branching from it.

    • Palmately Veined Leaves: Several primary veins fan out from the base of the blade.

Venation of Monocots and Dicots

  • Monocots: Characterized by parallel venation where primary veins run parallel to each other.

  • Dicots: Exhibit netted or reticulate venation with primary veins diverging in various ways.

  • Dichotomous Venation: Veins fork evenly and progressively from the base of the blade.

Internal Structure of Leaves

  • Composed of three regions: Epidermis, Mesophyll, Veins (Vascular Bundles).

    • Epidermis:

    • Single layer of cells covering the leaf.

    • Devoid of chloroplasts.

    • Coated with a cuticle containing cutin, may secrete waxy substances.

    • Protects internal leaf tissues.

    • Waste materials may accumulate in epidermal cells.

Stomata

  • Thinner layer of cutin on the lower epidermis, perforated by numerous stomata.

  • Guard Cells: Border stomata, originating from the same parent cell and containing chloroplasts.

    • Primary Functions:

    • Regulate gas exchange between the leaf interior and the atmosphere.

    • Regulate water evaporation.

    • Changes in guard cell water content affect stomatal opening (inflate = open; deflate = close).

Mesophyll and Veins

  • Most photosynthesis occurs in the mesophyll between epidermal layers.

    • Palisade Mesophyll: Compactly stacked, barrel-shaped parenchyma cells in two rows; contains most of leaf's chloroplasts.

    • Spongy Mesophyll: Loosely arranged parenchyma cells with abundant air spaces, facilitating gas exchange.

  • Veins (Vascular Bundles): Scattered throughout mesophyll, consisting of xylem and phloem tissues surrounded by a bundle sheath of thicker-walled parenchyma.

Monocot Mesophyll and Veins

  • Monocots exhibit differences:

    • Mesophyll is not differentiated into palisade and spongy layers.

    • Presence of bulliform cells on either side of the main central vein, which collapse in dry conditions, causing the leaf to fold or roll to reduce transpiration.

Specialized Leaves

  • Shade Leaves: Receive less total light than sun leaves, typically:

    • Larger and thinner.

    • Fewer well-defined mesophyll layers and chloroplasts.

    • Fewer hair structures.

  • Leaves of Arid Regions: Adaptations to limited water, temperature variations, and high light:

    • Thick, leathery leaves.

    • Fewer or sunken stomata.

    • Dense, hairy coverings and succulent water-retaining leaves.

Compass Plant

  • Leaves orient to the east and west, with blades perpendicular to the ground, minimizing water loss when the sun is overhead.

Leaves of Aquatic Areas

  • Aquatic Leaves: Characterized by less xylem and phloem; mesophyll not differentiated into palisade and spongy layers with large air spaces.

Tendrils and Spines

  • Tendrils: Modified leaves that curl around rigid objects assisting in climbing or supporting weak stems (e.g. garden peas).

  • Spines: Modified leaves that reduce leaf surface area and water loss, protect against herbivory (e.g. cacti), with photosynthesis occurring in the stems.

Thorns and Prickles

  • Thorns: Modified stems from the axils of leaves of woody plants.

  • Prickles: Outgrowths from epidermis or cortex; e.g. rose thorns are technically prickles.

Storage Leaves

  • Succulent Leaves: Modified for water storage, containing parenchyma cells with large vacuoles (common in desert plants).

  • Fleshy leaves also store carbohydrates (e.g. onions, lily).

Flower-Pot Leaves

  • Develop into urn-like pouches that are homes for ant colonies; ants enhance soil and provide nutrients, forming a symbiotic relationship with the plant (e.g. Dischidia, an epiphyte in Australia).

Window Leaves

  • Found in succulent desert plants; the leaves are mostly buried with the exposed end having a transparent, thick epidermis and transparent water storage cells underneath, allowing light access while retaining moisture.

Reproductive Leaves

  • Walking Fern: Produces new plants at leaf tips.

  • Air Plant: Produces tiny plantlets along leaf margins.

Floral Leaves (Bracts)

  • Located at the bases of flowers or flower stalks. Examples:

    • Poinsettia: Has brightly colored bracts surrounding flowers.

    • Clary’s Sage: Colorful bracts found at the top of flowering stalks.

Insect-Trapping Leaves

  • Can be found in swampy areas or bogs where nitrogen is deficient. Specialized leaves trap and digest insects.

  • Pitcher Plants: Trap and digest insects in cone-shaped leaves, with nectar-secreting glands attracting them.

  • Sundews: Have glandular hairs that secrete a sticky fluid, trapping insects and bending the leaves to capture them.

  • Venus's Flytraps: Located only in North and South Carolina, feature hinged blade halves that close when trigger hairs are touched, trapping insects.

  • Bladderworts: Float in shallow water, possessing tiny bladders that trap insects with trap doors.

Autumnal Changes in Leaf Color

  • Pigments in Leaves:

    • Chlorophylls: Green pigments.

    • Carotenoids: Yellow pigments.

  • In the fall, chlorophylls degrade, revealing other color pigments such as anthocyanins (red/blue) and betacyanins (red).

Abscission

  • Abscission: The process by which deciduous plants shed their leaves seasonally, resulting from changes in the abscission zone near the base of the petiole:

    • Protective Layer: Cells coated and impregnated with suberin.

    • Separation Layer: Pectins in the middle lamella of cells are broken down by enzymes.

Human and Ecological Relevance of Leaves

  • Landscaping: Use of shade trees.

  • Food Resources: Includes cabbage, lettuce, celery petioles, spices.

  • Dyes: Extracts from bearberry, henna.

  • Fuel Sources: From plants like yareta.

  • Oils: Such as eucalyptus and pennyroyal.

  • Perfumes: Derived from oils of orange tree and lavender.

  • Ropes and Twine: Made from agave and hemp fibers.

  • Drugs: Includes narcotics, tobacco, and marijuana.

  • Beverages: Tea and tequila prepared from agave leaves.

  • Insecticides: Such as rotenone.

  • Waxes: Carnauba and caussu waxes are derived from leaves.

  • Aesthetic Uses: In floral arrangements and gardens.