Botany: Leaf Structure and Characteristics

Exercise 5a: Determination of Leaf Variation

• Topic: Leaf variation in tropical forest plants
• Objective: To determine the patterns of variation in leaves, focusing on composition, phyllotaxy, and blade characteristics.
• Learning Outcomes:
  • Students will develop familiarity with leaf morphology.
  • Students will learn about leaf classification complexity.
  • Students will apply leaf classification knowledge to identify forest trees.
  • Students will observe modified leaves and understand their importance to the plant.

Leaf Structure and Types

• Leaves:
  • Most active and prominent plant organs
  • Highly diverse in form and function
  • Primarily responsible for photosynthesis
• Dicot Leaves:
  • Typically rounded
  • Reticulate venation (veins forming a net-like pattern)
  • Relatively broad lamina
  • Stipules present (small leaf-like structures at the base of the petiole)
• Monocot Leaves:
  • Typically narrow and elongated
  • Parallel venation
  • Often isobilateral (both leaf surfaces are similar)
  • Features like ligules and auricles are present to protect against dirt, dust, etc.
• Gymnosperm Leaves:
  • Mostly small, needle-like, or scale-like.
  • Often held together in fascicles
  • Frequently evergreen (retain leaves year-round)

Leaf Composition

• Simple Leaves:
  • Single, flattened blade per petiole (stalk)
• Compound Leaves:
  • Divided into smaller leaflets
  • Leaflets originate from a single axillary bud
  • Pinnately compound: leaflets arranged along an extension of the petiole.
  • Palmately compound: leaflets attached at the same point at the end of the petiole.

Leaf Arrangement (Phyllotaxy)

• Distycious (Alternate):
  • One leaf per node, and usually opposite each other to maximize light uptake
• Decussate (Opposite):
  • Two leaves per node
  • Arranged perpendicular to their opposite neighbors (top/bottom)
• Whorled:
  • Three or more leaves per node
  • Typically arranged oppositely with each other.

Leaf Venation

• Venation: The arrangement of veins in a leaf
  • Pinnate (veins arranged like a feather)
  • Reticulate (veins form a network)
  • Palmate (veins radiate from a central point)
  • Parallel (veins run parallel to each other)

Leaf Shape

• Various shapes: acicular, subulate, linear, oblong, lanceolate, oblanceolate, ovate, obovate, elliptical, oval, orbicular, cordate, reniform, deltoid, rhombic, spathulate, and flabellate

Leaf Margins

• Different types of margins: entire, revolute, repand, sinuate, crenate, doubly crenate, serrate, doubly serrate, dentate, lobed, pinnati-lobed, palmati-lobed, runcinate, lyrate, cleft, pinnatipartite, palmatipartite, parted, pinnatisect, and palmatisect

Leaf Apex and Bases

• Apex: The tip of the leaf; various terms (acuminate, acute, cuspidate, emarginate, mucronate, obcordate, obtuse, truncate, etc.)
• Base: The part of the leaf attaching to the petiole; various terms (cuneate, acute, obtuse, rounded, truncate, cordate, inequilater, auriculate, sagittate, and hastate)

Modified Leaves

• Scale-like: Small, awns-like to protective
• Needle-like: Long and slender, common in conifers for protection and water conservation
• Bract: Leaf like structure that holds flowers or flowers clusters (usually colored)
• Phyllodes: Modified petiole flattened for photosynthesis
• Bulb: Fleshy, scaled leaves of lilies and daffodils; for storage.
• Leaf Tendril: Succulent leaf modified for support via climbing
• Spine: Leaf or leaf part modified into a sharp point for defense (e.g., cacti).
• Carnivorous: Leaves modified to trap insects in areas with nutrient deficiency (e.g., Venus flytrap)
• Spiny Stipules: Hard, pointed structures at the base of the petiole for protection
• Reproductive/Vegetative: Leaves capable of reproduction by growing roots while attached to the parent plant
• Leaflet Hooks: Three terminal leaflets modified for climbing

Leaf Roots: Adventitious Roots

• Modified leaves for buoyancy
• Modified leaves enable floating on water bodies

Litter-Trappers: Modified Leaves

• Leaves trap and channel decaying materials for nutrient absorption

Adhesive-Discs: Modified Leaves

• Modified leaves for attachment mechanism, such as creeping vines

Cotyledon (Seed Leaves)

• Modified leaf inside a seed for protecting newly emerging true leaves

Exercise 5 Instructions

• Field observation and laboratory analysis; group-based task

Dicot Leaves (Specific Task)

• Observe Binunga and Narra twigs, and identify blade, petiole, midrib, stipules, veins, and veinlets; illustrate and label parts

Monocot Leaves (Specific Task)

• Observe bamboo and Paragis leaves and identify blade, auricle, leaf sheath, and ligule; draw and label in worksheet

Gymnosperm Leaves (Specific Task)

• Observe a pine twig, noting needle-like or aciculate leaves clustered by bud scales; draw and label parts

Palm vs Gymnosperm Leaves (Specific Task)

• Observe Pitogo and Kaong leaves to identify and list any differences in structure

Leaf Classification (Specific Task)

• Examine leaves of 10 forest trees. List and describe each species based on the given guide.

Leaf Metamorphosis (Specific Task)

• Observe 5 forest trees with leaf metamorphosis; illustrate and describe modified parts and their function.

Assessment Items

• Provided a list of assessment components along with the maximum score for each.