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Lecture 8: Leaves (Leaf adaptations/ modification (Leaf Abscission…

- - - - An expanded petiole taking on the function of a leaf blade
      - E.g.Acacaia koa
      - Two stages in the transition from a true leaf with many leaflets to a sickle-shaped phyllode
      - Young leaf: Pinnately compound and has a central axis (Rachis) which is branched having tiny leaflets
      - Mature leaf: Develops from the Rachis alone. Fails to branch or produce leaflets
    - - Localised swelling at junctions of leaf parts
      - Acts as hinges to allow movement b/w parts of the leaf. (Lower leaf when wilted or close up when touched)
      - Location
        
        Found at the base of petiole (in Acacia lebbek)
        
        Junction of petiole and lamina (Ischnosiphon sp.)
        
        Base of each leaflets of a compound leaf of Mimosa pudica
        
        Junction of petiole and lamina ofMangifera indica
    - - As seen in the water hyacinth, a free floating aquatic plant. Dark green leaf blades attached to a spongy, inflated petiole
    - - Lathyrus (Pea plant)
      - Leaf lamina modified into tendrils
      - Stipules perform the function of true leaves
  - - - Hard and pointed leaves as seen in desert plants
  - - - Structures developing on leaves as seen in the buds on Bryophyllum daigremontianum
- - - - Without it, water from soil is unable to reach the leaves of tall trees
      - Cohesion-tension theory
        
        Water evaporates from leaf mesophyll cells into the surrounding air via the stomata
        
        Transpiration causes water to move from the xylem into mesophyll cells. Puts the water in xylem in a state of tension that is continuous tension from leaves, down through the stem to root.
        
        As water molecules continue to escape from the leaves, the continuous tension in xylem permits more water molecules to be pulled up and replaced
        
        Columns of water are pulled up by the collective strength of hydrogen bonds b/w water molecules, confined by narrow, tubular xylem tubes
      - Guttation
        
        Some plants exude water as a liquid through special openings at leaf veins tips (Hydathodes)
        
        Occurs when soil moisture is high and stomata closed, especially at night
- - - - Upper epidermis
        
        Single layer of cells with a thick cuticle to reduce water loss
        
        Absence of stomata usually
        
        Covered with trichomes (Hair like structure) that helps to reduce water loss by retaining a layer of moist air. It also reflects excess light to protect the plant form overheating. Some even secretes stinging irritants or excess salts absorbed by salty soil.
      - Lower epidermis
        
        Single layer of cells with thin cuticle
        
        Interspread with many stomata flanked by specialized epidermal cells also known as guard cells. Guard cells may contain some chloroplast
        
        Inside each stoma, there is a large cavity known as the respiratory cavity
      - Mesophyll
        
        Are ground tissue between upper and lower epidermis
        
        Palisade mesophyll
        
        1-3 layer of closely packed cells
        
        Numerous (80%) chloroplast for photosynthesis
        
        Leaves exposed to direct sunlight are thicker with palisade mesophyll as the light is able to penetrate multiple layers of the cells to enable greater photosynthesis
        
        Spongy mesophyll
        
        Oval, rounded, or irregular cells that are loosely arranged
        
        Numerous, large, intercellular spaces and air cavaties
        
        Contains few chloroplasts, as main function to diffuse gases, particularly CO2 within the leaf.
      - Vascular bundles
        
        Xylem
        
        Vessels, Tracheids, fibers, parenchyma
        
        Conducts water and dissolved minerals
        
        Phloem
        
        Sieve-tubes, companion cells and parenchyma
        
        Conducts dissolved sugars from photosynthesis
        
        Each vascular bundle is surrounded by 1 or more layers of non-vascular cells called a bundle sheath
        
        Bundle sheath are composed of parenchyma or sclerenchyma cells. Extends radically towards the upper and lower epidermis frequently, and are called as bundle sheath extensions
  - - - More or less uniform with one surface to another
      - Epidermis on either sides bears more or less equal numbers of stomata and is uniformly thickened and cutinized
      - Mesophyll not differentiated into spongy or palisade mesophyll, and chloroplast are equally distributed
      - Bulliform cells, large thin walled cells on upper epidermis