8: Leaves

-phyll= leaf

(phylloplane= surface of the leaf)

(phyllosphere= microclimate of the leaf)

A. SOME LEAF FUNCTIONS—WHAT PROCESSES OR ACTIVITIES OCCUR IN THIS PLANT ORGAN?

  • photosynthesis

  • storage

  • protection (spikes on cacti)

  • transpiration → loss of water vapor via stomata

  • climbing → tendrils

  • prey capture (carnivorous)

B. VARIATIONS IN LEAF MORPHOLOGY

LEAF MORPHOLOGY:

  • Some leaves may have leaf-like appendages at their bases called stipules.

  • Not all leaves have a distinct petiole.

Leaves are perhaps the most varied plant structures. Botanists have developed ways to describe them based on certain characteristics. Among the most common, in addition to their arrangement on the stem, are the following:

  1. BLADE SHAPE (many types)

    1. specific appearances of the base and/or the tip of the blade may be used

  2. BLADE MARGINS (again, many types)

  3. VENATION: What are veins in a leaf? Are they similar to human veins?

    1. Pinnate, Palmate, (Netted/reticulated forms → common in dicots)

    3. (Pinnate. Pinna: feather. Midrib/midvein with lateral veins and minor veins)

    4. (palmate: palm-like, several major veins radiate from base)

    5. and Parallel/Linear (More common in monocots)

  4. COMPLEXITY

    1. Simple: One intact blade

    2. Compound: Blade separated into leaflets

C. LEAF ANATOMY

Epidermis (lower and upper): outer coverings of leaf that produces cuticle

  • numerous stomata regulated by guard cells

Cuticle: prevents water loss

  • waxy, hydrophobic

  • provides protection

Trichomes: hairs on leaf → reflect light, decreases air flow across leaf, may be defensive, reduce water loss

Mesophyll: bulk of blade, ground tissue, may be columnlike in shape → if so, palisade

  • may be air spaces between cells → sponge

Vascular Bundle: veins: xylem and phloem surrounded by bundle sheath cells

Some anatomical differences between dicot and monocot leaves:

Mesophyll Composition:

  • Monocots: may be spongy, typically not palisade

  • Dictors: often have palisade and spongy mesophyll

Stomatal Density (average number of stomata per cm²):

  • Monocots:

    • Corn → Upper: 5,500, Lower, 6,800

    • Oat → Upper: 2,500, Lower: 2,300

    • Wheat → Upper: 3,300, Lower: 1,400

  • Dicots: (lower is usually much higher)

    • Bean → Upper: 4,000, Lower: 28,000

    • Coleus → Upper: 0, Lower: 14,100

    • Geranium → Upper: 1,900, Lower, 5,900