BOTA 205 Lecture 10 Leaves

Blade (Lamina)

Broad, flat portion of the leaf rich in chloroplasts; main site of photosynthesis.

Petiole

Stalk connecting the leaf blade to the stem; may be absent in sessile leaves.

Veins (Venation)

Vascular bundles that support the leaf and transport water, nutrients, and sugars.

Stipules

Small, leaf-like appendages at the base of the petiole; diagnostic trait in some species.

Axillary Bud

Bud located where the leaf meets the stem; can develop into a branch or flower.

Simple Leaf

Single, undivided blade attached to a petiole.

Compound Leaf

Blade divided into leaflets; axillary bud occurs only at base of petiole, not at leaflet bases.

Pinnately Compound

Leaflets arranged along an axis that continues from the petiole.

Palmately Compound

Leaflets all arise from one point at the end of the petiole.

Phyllotaxy

Arrangement of leaves on a stem.

Alternate Arrangement

One leaf per node, alternating sides; zig-zag pattern (e.g., sunflower).

Opposite Arrangement

Two leaves per node, directly across; often form cross pattern (e.g., maples, ashes).

Whorled Arrangement

Three or more leaves at a single node forming a circular whorl (e.g., oleander).

Parallel Venation

Veins run parallel; typical of monocots.

Netted Venation

Veins form a branching, web-like network; typical of dicots.

Cuticle

Waxy outer layer that reduces water loss from aerial plant parts.

Upper & Lower Epidermis

Protective outer layers; transparent, lack chloroplasts; lower side usually has more stomata.

Palisade Mesophyll

Columnar, chloroplast-rich cells beneath upper epidermis; primary photosynthetic layer.

Spongy Mesophyll

Loosely arranged cells with air spaces; aid in gas diffusion within the leaf.

Vascular Bundles (Veins)

Contain xylem (water transport) and phloem (sugar transport); embedded in mesophyll.

Bundle Sheath

Cells surrounding vascular bundles that regulate material exchange; may have supporting extensions.

Stomata

Adjustable pores flanked by guard cells that control gas exchange and water loss.

Guard Cells

Specialized cells that regulate stomatal opening and closing.

Stomatal Opening Triggers

Low CO₂ and blue light open stomata; dehydration causes closure.

Transpiration

Process of water loss through stomata; cools plant and drives mineral transport from roots.

Factors Affecting Transpiration

Increased by heat, light, wind, and dry air; decreased by high humidity.

Guttation

Exudation of water droplets from hydathodes at leaf tips due to root pressure under humid conditions.

Hydathodes

Specialized pores through which guttation occurs.

Monocot Leaf Venation

Typically parallel.

Dicot Leaf Venation

Typically netted (pinnate or palmate).

Monocot Petiole/Base

Usually lacks a petiole; leaf base sheaths stem.

Dicot Petiole/Base

Has a distinct petiole and broad blade.

Monocot Mesophyll

Lacks distinct palisade and spongy layers.

Dicot Mesophyll

Has distinct palisade and spongy layers.

Monocot Stomata

Present on both leaf surfaces.

Dicot Stomata

Usually only on the undersurface.

Bulliform Cells

Large, bubble-like cells on monocot leaves; may help fold/unfold leaves to reduce water loss.

Leaf Abscission

Process of leaf shedding at the abscission zone; involves enzyme activity and cork layer formation.

Deciduous Plants

Lose all leaves seasonally (autumn).

Evergreen Plants

Continuously replace leaves; some shed throughout the year.

Abscission Zone

Region at leaf base where cells separate and leaf detaches; forms protective cork layer.

Tendrils

Slender coiling leaves or leaflets used for climbing and support (e.g., peas).

Spines

Modified leaves for defense and water conservation (e.g., cacti).

Thorns

Hard, pointed modified stems arising from axils; serve defense role.

Prickles

Sharp epidermal outgrowths without vascular tissue (e.g., roses).

Glochids

Tiny, barbed, hair-like spines on cacti areoles; detach easily for defense.

Spurs

Short stems bearing leaves, flowers, or fruit; may store nutrients.

Carnivorous Leaves

Modified to trap and digest prey to supplement nutrient intake.