leaves

• Functions of the Leaf

  • Functions of the
  • Gaseous exchange
  • Respiration (take in O2 -> release CO2)
  • Photosynthesis (take in CO2 -> release O2)
  • Transpiration - water vapour getting lost from surface of leaf
• Xerophytes (desert plants)

  • Thick and reduced leaves
  • Stomata opens at night
  • To conserve water
  • Thick waxy layer
• Floating leaves (lily pad)


  • Stomata on upper epidermis
  • Submerged leaves - no stomata
• Conifers

  • Thick waxy needle leaves
  • Evergreen
  • Thick, waxy cuticle
  • Sunken stomata
• Simple leaves


  Single blade per petiole
• Compound leaves


  Divided into smaller leaflets but originate from a single axillary bud
• Pinnately compound


  Leaflets in pairs along an extension of the petiole
• Palmately compound


  Leaflets attached at same point on end of petiole
• Leaf Outline

  • Linear - thin
  • Lanceolate - like spear
  • Ovate - oval shape
  • Cordate - heart
• Parallel venation


  Veins running parallel to each other (monocot)
• Pinnately veined


  One primary vein (midvein) which is included within enlarged midrib
• Palmately veined


  Several primary veins fan out from base of blade
• Phyllotaxy : leaf arrangement on stem


  • Alternate - leaves are attached alternately in spiral along a stem with one leaf per node
  • Opposite - 2 leaves arise from opposite node
  • Whorled - 3+ leaves occur at a node
  • Rosetted/Basal - leaves are at base of plant
• Cuticle


  Waxy covering over epidermis of leaves and stems of plant<|>Enables plant to survive in dry conditions of terrestrial environment<|>Minimizes water loss and reduce pathogen entry due to waxy secretion<|>Consists of lipid and hydrocarbon polymers<|>Synthesized by epidermal cells
• Epidermis


  Allows light to penetrate into mesophyll (where photosynthesis occurs)<|>3 kinds of epidermal cells: Ordinary cells, Trichomes, Guard cells
• Stomata


  Pore/opening in lower epidermis<|>Only passageway of atmospheric gases in and out of leaves<|>Opening and closing based on movement of guard cells<|>100 per sq mm.
• Factors Affecting Opening and Closing of Stomata

  • Light - causes it to open and darkness closes it
  • Water / Moisture availability - causes to close to prevent further water loss
  • Temperature changes - higher temperature = stomata closes to prevent water loss
• Eudicot leaves


  • Broad, flattened blade
  • Netted venation
• Monocot leaves


  • Narrow
  • Wrap around stem in a sheath
  • Has parallel venation
  • Bulliform cells - large, thin walled cells located at upper epidermis; helps reduce water loss
  • Ligule - collar extension of sheath curving around stem
  • Auricle - appendage that surrounds stem at junction of blade and sheath
• Modified Leaves

  • Cotyledons - leaves produced by germinating seed; often contain store of food to help seedling become established
  • Coloured Bracts - petal - like leaves to attract pollinators
  • Insect-trapping leaves - digests insect bodies to obtain nitrogen and nutrients; lives in nutrient-poor conditions
  • Tendrils - blade of leaves/leaflets are reduced in size; allows plant to cling onto other objects
  • Spines - Leaves are reduced to discourage herbivory and reduce water loss; stems are primary organ of photosynthesis
  • Storage leaves - retain water in large vacuoles (succulent leaves)
  • Absorptive leaf - absorbs water
  • Window leaves - Plant is buried in soil with transparent part exposed to light; being buried reduces loss of water in arid environments
  • Flower pot leaves - Structure to catch water and debris for nutrient collection
• Leaf Abscission

  1. As autumn approaches, plants reabsorbs sugar

  2. Chlorophyll is broken down

  3. A protective layer of cork cells develop on stem side of abscission zone

  4. Enzymes dissolve middle lamella in abscission zone

  5. Protective layer of cork seals off area after leaf detach (forms leaf scar)
• Economic Importance of Leaves

  • Food, spices, drinks
  • Dyes
  • Fibres
  • Fuel
  • Drugs