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)