Lecture 7 Video Notes

Leaves: Form and Function

• Leaves are primarily for carrying out photosynthesis; they cover the outside of the plant and are optimized for light capture and gas exchange.

• Form follows function: leaves are typically flat to provide a large surface area for light absorption (act like solar panels).

• Trade-off: broad surface area increases water loss via evaporation; evolution solved this with a waxy cuticle to limit water loss while maintaining gas exchange through stomata.

• Stomata (epidermal structures, usually on the leaf underside) regulate CO₂ uptake and water loss; they balance photosynthesis with transpiration. Guard cells control stomatal opening.

• Water supply: leaf cells need hydration; vascular tissue (xylem) delivers water to all leaf cells; phloem exports sugars (primarily sucrose) to other plant parts.

• The leaf is a marvel of engineering — optimized for photosynthesis with coordinated water delivery and carbohydrate export.

Leaf Development and Anatomy: How Leaves Form

• Leaf primordia form on the shoot apical meristem as incipient clusters of cells; these are the first recognizable leaf structures.

• Leaves do not have their own meristem; leaf development is determinate — leaves grow to a certain size and then stop.

• Vascular connections: vascular tissue in the leaf connects to stem tissue via a leaf trace; this connection leaves a break in the stem’s vascular tissue called a leaf gap.

• The pattern and placement of new leaf primordia on the shoot apical meristem are controlled by hormones and genetics, producing definite phyllotaxy (vertical pattern of leaf arrangement).

• Leaves occur at nodes on the stem; axillary buds at nodes have potential to develop into new shoots.

• Phyllotaxy is the spiral (often clockwise or counterclockwise) arrangement of leaves along the stem, determined by precise leaf primordia positioning driven by hormonal/genetic factors.

Parts of a Leaf

• Blade (lamina): the broad, flat part of the leaf.

• Petiole: the stalk that connects the blade to the stem (leaves can be sessile if blade attaches directly to stem).

• Sessile leaves: leaves without a petiole (blade attaches directly to the stem).

• Leaf sheath: in grasses, the blade attaches to the stem via a sheath rather than a distinct petiole.

• Stipules: paired structures at the base of some leaves; not buds and not essential for leaf development; can provide protection during early development.

Leaf Arrangement on the Stem (Phyllotaxy)

• Alternate: one leaf per node.

• Opposite: two leaves per node, directly opposite each other.

• Whorled: three or more leaves per node.

• These patterns are useful for field identification and reflect the developmental patterning on the shoot apical meristem.

Leaf Type: Simple vs Compound

• Simple leaf: blade is a single, undivided piece.

• Compound leaf: blade is divided into multiple leaflets; the stalk that would be the blade’s base is the petiole in simple leaves, but in compound leaves, the stalk between the leaf and stem is the rachis, and individual leaflets attach to the rachis.

• Important note: the entire structure is still a single leaf; the connection point to the stem (with the axillary bud) helps distinguish a true compound leaf from a compound-looking stem endpoint.

• Patterns of leaflets in compound leaves:

  • Pinnately compound: leaflets arranged along the rachis in pairs (e.g., hickory) or a central axis with leaflets on both sides.

  • Palmately compound: all leaflets originate from a single point at the end of the petiole (e.g., Buckeye).

• Compound leaves can have alternate or opposite arrangements on the stem.

• Quick diagnostic: if you see an axillary bud at the attachment point and a single stalk (rachis) with leaflets, you’re looking at a compound leaf, not a simple leaf.

Leaf Venation Patterns

• Pinnate venation: a single main mid vein with lateral veins branching off (major pattern in many dicots).

• Palmate venation: several main veins originate from a single point at the leaf base (e.g., cotton leaves).

• Parallel venation: major veins run parallel to one another, typical of grasses (monocots); small connecting veins exist between major veins.

• Dichotomous venation: repeatedly splits into two equal veins; characteristic of some primitive plants like Ginkgo.

Internal Structure of a Leaf (Cross-Section Overview)

• Leaves have a three-dimensional structure despite being thin.

• Epidermis: a single cell layer on the upper and lower surfaces (epidermal tissue); stomata are located here, as are other epidermal structures like trichomes.

• Mesophyll: ground tissue specialized for photosynthesis; contains most chloroplasts; two regions:

  • Palisade mesophyll: tall, columnar cells rich in chloroplasts near the upper surface.

  • Spongy mesophyll: looser tissue with air spaces, aiding gas diffusion from stomata to palisade cells.

• Chloroplasts are concentrated in mesophyll cells; epidermal cells generally lack chloroplasts (except guard cells).

• Vascular tissue: leaf vascular bundles (xylem and phloem) are connected to stem vascular tissue; typically, xylem faces the upper leaf surface and phloem faces the lower surface; vascular bundles are enclosed by fibers (a form of a closed vascular bundle).

• A typical dicot leaf (example: lilac, Syringa) shows:

  • A single epidermal layer on top and bottom with stomata on the bottom (and guard cells visible).

  • A large mesophyll region divided into palisade (upper) and spongy (lower) layers.

  • Numerous small vascular bundles (veins) distributed through the leaf; a central vascular bundle visible in cross-section.

Examples of Typical Leaf Anatomy (Lilac) and Features

• Lilac leaf cross-section demonstrates:

  • Upper epidermis and lower epidermis (stomatal gaps visible on the lower surface).

  • Palisade mesophyll cells directly beneath the upper epidermis.

  • Spongy mesophyll with air spaces beneath the palisade layer.

  • Veins (vascular bundles) running through the leaf; finer veins interconnect through the mesophyll.

Variations in Leaf Structure by Environment

• Three general environments and their leaf types:

  • Mesophytes: temperate climates with moderate moisture; includes most crop plants and deciduous trees; leaves adapted to intermediate water availability.

  • Hydrophytes: aquatic or submerged leaves; adapted to buoyancy and gas exchange with the atmosphere; features include large air spaces (aerenchyma) and stomata often on the upper surface for gas exchange with the air.

  • Xerophytes: dry/arid environments; leaves have adaptations to reduce water loss; examples include thick epidermis, sunken stomata (stomatal crypts), potential endodermis around leaf vascular tissue, and leaf curling via bulliform cells.

• Oleander (Oleander leaf) as an example of xerophyte leaf cross-section:

  • Very thick upper epidermis (protects against water loss).

  • Stomatal crypts (sunken stomata) on the lower surface.

  • Epithelium with an underlying hypodermis; the stomata are located in stomatal crypts to minimize evaporation.

• Pine needles (conifers) as another xerophyte example:

  • Very thick epidermis with an underlying hypodermis.

  • Sunken stomata and stomatal crypts.

  • Central vascular tissue surrounded by an endodermis (suberin-containing cells) around the vascular bundle, a protective feature preventing passive water loss.

• Bulliform cells in grasses (C4 and other grasses):

  • Large epidermal cells along the midvein; when they lose turgor, leaves curl or roll to minimize surface area and reduce water loss.

  • Common in warm-season grasses like Johnson grass.

• C4 photosynthesis overview (conceptual in this context):

  • Found in many xerophytic grasses (e.g., corn). Involves spatial separation of CO₂ fixation and the Calvin cycle to keep stomata largely closed while maintaining high photosynthetic rates.

  • Two photosynthetic pathways operate in distinct cell types: mesophyll cells and bundle sheath cells; additional biochemical differences accompany anatomical differences (to be covered in later lectures).

Modified Leaves: Diversity of Leaf Functions

• Succulent leaves and spines in desert cacti:

  • Spines along what appears to be a leaf are actually modified leaves; the green photosynthetic tissue is primarily the stem with cladodes or cladophylls.

  • Spines provide protection and reduce water loss.

• Tendrils: climbing support structures that may be modified leaves or modified stems depending on the species:

  • In peas (legumes), the tendril is a modified leaf at the end of the compound leaf.

  • In grapevines, tendrils are modified stems.

• Bracts: modified leaves that surround flowers to attract pollinators and/or protect buds; common examples:

  • Cherry, chestnut, maple trees show bud scales around leaf buds in spring.

  • Bougainvillea and poinsettia display showy bracts around flowers.

  • Dogwood has prominent bracts that resemble petals around the actual flowers.

  • Spathe (and spadix) in aeroids (e.g., it resembles a leaf-like structure around the flower).

• Bud scales: protective scales at the base of leaf buds, especially visible in spring before leaf-out.

• Carnivorous plants: some leaves modify into traps for capturing prey (e.g., pitcher plants, Venus flytrap), providing a dramatic example of leaf specialization.

Important Takeaways and Connections

• Leaves are highly specialized for photosynthesis, but their structure is a balance between light capture and water conservation, with multiple anatomical and developmental adaptations across environments.

• Leaf development is tied to the shoot apical meristem, with leaf primordia and leaf traces connecting leaves to the stem vasculature; phyllotaxy emerges from hormonal and genetic control at the meristem.

• The vascular architecture (xylem/phloem) and the arrangement of mesophyll tissue underpin efficient water delivery and gas exchange for photosynthesis.

• Environmental adaptations (mesophytes, hydrophytes, xerophytes) illustrate how leaf anatomy can be modified dramatically to optimize survival in various habitats (e.g., stomatal crypts in oleander; endodermis in pine needles; bulliform cells in grasses).

• Modified leaves reveal the plasticity of leaf function: spines, tendrils, bracts, bud scales, spathes, and even carnivory show how leaves can evolve away from classical photosynthesis to support protection, support, reproduction, or nutrient acquisition.

• For field identification and anatomy, be mindful of terminology and structural cues:

  • Leaf primordia at the shoot apex indicate determinate leaf development and phyllotactic patterns.

  • The leaf trace and leaf gap are key vascular features in leaf attachment.

  • Distinguishing simple vs compound leaves involves examining the attachment of leaflets and the rachis; recognizing the presence of an axillary bud at the base helps differentiate a leaf from a stem segment.

  • Venation patterns (pinnate, palmate, parallel, dichotomous) provide diagnostic clues for plant groups.

For more examples and details, refer to the posted slides on Blackboard, which include additional illustrations of leaf anatomy, phyllotaxy, and the variety of leaf forms discussed in this lecture.