Plant Morphology: Leaf Structure

• The leaf is an organ of a vascular plant, in botanical terms, relating to plant morphology.
• Leaf shape and structure varies among plant species due to climate adaptation, light availability, grazing animals, available nutrients, and ecological competition.

Leaf Structure and Attachment

• Each leaf has a visible bud which develops into a stem or flower bearing stalk.
• Leaf attachment sites on stems are nodes.
• Dicotyledonous plants most often have flattened spoon shaped leaves, with a leaf stalk (petiole) forming a handle, the leaf blade represents the bowl (simple leaf).
• Sessile leaves do not have a leaf stalk.
• Net veined leaves (dicot) may be pinnate or palmate.

Petiole Advantages

• Help rotate the leaf blade to track the sun.
• Provide flexibility in wind and rain.
• Contribute to blade spacing for maximum sunlight exposure.

Leaf Components

• A complete leaf has a leaf base, a pair of stipules, a leaf stalk, and a leaf blade.
• In most monocots, the leaf base expands into a sheath around the stem, known as the leaf-sheath with an upper portion as the leaf-blade.
• The ligule, and a pair of auricles grow on the inner surface of the leaf, between the sheath and the blade.
• The collar is the zone at the junction of the sheath and blade on the outer surface.

Leaf Classification

• Monocots have parallel-veined leaves, such as lawn grass, maize, sugarcane, or bamboo, along with simple, strap-shaped leaf blades that taper to points and attach to the stem.
• Monocotyledons veins are parallel.
• Dicotyledons veins form a netlike pattern.
• Dicotyledons are represented by trees and shrubs.
• A compound leaf appears as a single leaf like a twig that forms a leaflet from the midrib.
• Palmate leaves can also be compound, as seen in the cabbage tree (Cussonia spp).
• Pinnately compound leaves have leaflets arranged along the main or mid-vein.

Pinnate Leaf Types

• Odd pinnate (imparipinnate) includes a terminal leaflet, e.g., Fraxinus (ash).
• Even pinnate (paripinnate) lacks a terminal leaflet.
• Leaves can be compounded more than once, forming bi- or tri-pinnately compound leaves.
• Leaves of dicotyledons can be simple or compound.

How to Tell Leaflets from Leaves

• Buds are in the axils of leaves, both simple and compound, but not in the axils of leaflets or pinna.
• The tip of the rachis in leaflets will never have a terminal bud.

Leaf Arrangement

• Leaf arrangement on the stem is phyllotaxy, which helps prevent leaves from shading each other.
• Leaves can be arranged alternately, oppositely, or in a whorled pattern.
• Leaves can vary in shape from species to species, it is highly dependent on adaptation to climate, and available light, nutrients, ecological competition, and from grazing animals effects

Leaf Components

• Leaf bases can be cordate, cunate, rounded, sagittate, or truncate.
• The leaf apex can be accuminate, aristate, acute, cuspidate, retuse, obtuse, mucreonate, truncate, apiculate, or caudate.
• The edge of a leaf is the leaf margin that varies enormously (smooth, toothed, serrated etc).
• Leaf blades can be dorsiventral (differing top and undersides) or isoventral (same appearance on both sides).
• Stomata, cell guarded openings in the epidermis of leaves, are often plentiful on the underside of dorsiventral leaves.
• Smooth leaves with hair-like outgrowths can be glandular or stinging.

Leaf Structure

• Epidermis generally has no chloroplasts, except in guard cells, and has an extra-thick wall on the outside surface of epidermal cells.
• Cutin and waxes form a cuticle, usually thicker on the upper epidermis.
• Stomata, or stomates, are holes in the epidermis that allow for gas exchange.
• Guard cells border the stomata, controlling stomatal opening.
• Stomata are commonly found only on the lower epidermis in dicots, and on both surfaces in monocots.
• Mesophyll is all internal leaf tissue, excluding some vascular tissue, and is usually organized into two layers in dicots.

Mesophyll Layers

• The upper layer is palisade parenchyma, which has elongated cells perpendicular to the leaf surface with numerous, large chloroplasts, and primary photosynthetic tissue.
• The lower layer, spongy parenchyma, has irregularly shaped cells with conspicuous air spaces and fewer chloroplasts, with critical air spaces for diffusion of gases.

Vascular Bundles Role

• Transport materials to and from the leaf.
• Larger bundles are visible as leaf veins.
• Xylem is on top, and phloem is on the bottom of the vascular bundles.
• Vascular bundles run throughout mesophyll.
• Bundle sheath is a tight sheath of cells around the vascular bundles that control the movement of materials to and from vascular tissue.

Leaf Function

• Photosynthesis harvests light energy and converts it to ATP and NADPH, fixing CO2.
• Transpiration drives xylem transport of water and dissolved nutrients (adhesion & cohesion).
• Transpiration provides evaporative cooling of leaves, when temperatures are high.
• Stomates control gas exchange and transpiration.

Leaf Adaptation

• Leaves evolved to adapt to different environments, a surface structure that avoids moistening by rain and contamination.
• Sliced leaves reduce wind resistance.
• Leaf hairs trap humidity in dry climates, creating a boundary layer that reduces water loss.
• Waxy leaf surfaces reduce water loss.
• Large surface area provides large area for sunlight and shade with plants, minimizes heat and reduce water loss.
• Translucent windows filter light before photosynthesis on inner leaf surfaces.
• Succulent leaves store water and organic acids for CAM photosynthesis.
• Aromatic oils, poisons, and pheromones deter herbivores.
• The inclusions of crystalline minerals deter herbivores (e.g. silica phytoliths in grasses, raphides in Araceae).

Modifications of Leaves

• Petals attract pollinators.
• Spines protect plants (e.g., cacti).
• Insect traps feed carnivorousplant directly.
• Bulbs store food and water (e.g., onions).
• Tendrils allow plants to climb (e.g., peas).
• Bracts and pseudanthia (false flowers) replace flower structures when true flowers are greatly reduced (e.g., Spurges).
• Scale leaves are non-photosynthetic and may be used for underground storage.
• Bracts are modified or specialized leaves associated with reproductive structures such as a flower.
• Cape reeds (Restio) are monocots that produce only leaf sheaths that cannot photosynthesize, so the stem does the photosynthesis.
• Storage/succulent leaves are modified to retain water such as succulent Aloe plants that have water-filled leaves with a bitter substance.
• Water lily (Nymphaea) have large, round blades with stomata on the upper surface.
• Phyllodes refer to the expanded petiole of a leaf where the leaves are leaf-like structures formed from the leaf stalk/petiole.
• Cladodes, such as Opuntia spp. (prickly pear), are branches, not leaves, but function as leaves.

Insectivorous Plants

• There are specialized leaves of insectivorous (insect eating) species capture and digest insects to obtain nitrogen in habitats which are poor in nitrogen.
• Passive traps exist on Nepenthes sp. (pitcher plant), with a lamina forms a tubular shape that secretes a watery digestive fluid.
• Active traps exist, the best example being Venus flytrap (Dionaea muscipula) where trigger hairs projecting from leaves, when touched, cause the blades to close and trap insects.
• Sundew (Drosera spp.) leaves are also active traps, covered with glandular trichomes that secrete a stick digestive liquid, bending towards the victim and curls the leaf bade around the insect.
• Some Cacti develop leaves which reduces to thorns to prevent reduce grazing or browsing.
• Stipules modified into spines, e.g., Acacia sp.
• Leaf midrib forms a tendril.
• Tendrils aid plants in climbing or supporting their stems.