Leaf Structure and Function Chapter 32

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

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)

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

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

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

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

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

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

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

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

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)