Leaf Structure and Function

1
Leaf Structure and Function

• Chapter 32

2
General Leaf Form

• Blade broad, flat portion
• Petiole attaches the blade to the stem
• Stipules leaf-like outgrowths at the base of
  the petiole
• Simple leaves have a single blade
• Compound leaves blade is divided into multiple
  leaflets

3
General Leaf Form

• Node the area of the stem where leaf is
  attached
• Alternate leaf arrangement every other side
• Opposite (MAD dog) two at each node
• Whorled 3 or more at each node
• Venation patter of veins in the leaf
• Parallel side by side
• Pinnately netted branching off long midvein
• Palmately netted branching off a single point
  (like fingers)

4
Leaf Structure

• Upper and lower epidermis cells lack
  chloroplasts secrete a waxy cuticle which
  contain cutin to reduce water loss also may have
  trichome cells which can reduce water loss and
  deter herbivores
• Guard cells on either side of stomata control
  opening and closing associated with transpiration
  and gas exchange

5
Leaf Structure

• Mesophyll middle tissues of the leaf
  photosynthesis occurs here
• Palisade mesophyll columnar cells stacked
  closely together toward the upper epidermis the
  main area of photosynthesis
• Spongy mesophyll irregularly shaped cells
  loosely packed below the palisade mesophyll can
  do photosynthesis but main function is gas
  exchange

6
Leaf Structure

• Vascular bundles (veins) extend through the
  mesophyll and carry both xylem and phloem use
  diffusion to move water to cells and food away
  from cells
• Bundle sheath surrounds the veins and helps
  provide support may have extensions into the
  mesophyll cells

7
Stomata

• Usually open during the day and closed at night
  why? (think about photosynthesis)
• Guard cells change shape to control this
• Water moves into guard cells from surrounding
  cells
• Guard cells swell and bend
• Pore is open
• Water leaves the guard cells
• Cells become flaccid and collapse

8
Transpiration

• Around 99 of water a plant absorbs is lost by
  evaporation from the leaves
• Transpiration plant perspiration
• Most occurs through the open stomata
• Factors affecting transpiration
• Temperature
• Light
• Wind
• humidity

9
Transpiration

• Responsible for water movement in plants without
  it water would not reach the leaves from the soil
• Also serves to cool the plant, just like sweat
  cools us
• It brings dissolved nutrients from the soil into
  the plant and distributes them
• In periods of drought it can lead to wilting and
  even death

10
Guttation

• The loss of liquid water from the leaves of
  plants
• Occurs when transpiration is low and soil
  moisture is high

11
Leaf Abscission

• The way in which plants shed leaves
• Controlled by hormones
• Chlorophyll, sugars, amino acids and many
  essential nutrients are transported from the
  leaves to other parts of the plant
• Abscission zone the area where the petiole
  detaches from the stem

12
Modified leaves

• Spines hard and pointed cactus to deter
  herbivores
• Tendrils help attach vines to other structures
  (peas, squash)
• Bulb underground storage leaves (onions,
  tulips)
• Leaves of insectivorous plants can be either
  passive (pitcher plant) or active (Venus flytrap)