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)