Leaf structure

• Waxy cuticle:

  • the upper surface of a leaf is covered by a waxy cuticle
  • this is waterproof to reduce water loss significantly
• Upper epidermis:

  • a single layer of cells
  • cells are transparent since they contain no chloroplasts. This means light can pass straight through to the tissues below
  • there are not many stomata in the upper epidermis since the heat from direct sunlight would cause excessive evaporation
• Palisade mesophyll:

  • the main photosynthetic tissue
  • the cells are deep and packed full of a large number of chloroplasts
  • the chloroplasts are able to move and so arrange themselves in a position that gives maximum light absorption
  • Many cells can fit in the layer to maximise light absorption
• Spongy mesophyll:

  • cells containing fewer chloroplasts so it is less important for photosynthesis than the palisade layer
  • the cells are loosely packed and have a large number of air spaces between them
  • gas exchange occurs on the moist cell surfaces
  • the cell surface provide a large surface area for gas exchange
  • There is no ventilation mechanism so it relies on diffusion
• Lower epidermis:

  • no waxy cuticle
  • has large numbers of stomata which open to allow the diffusion of gases into and out of the leaf
  • water evaporates from the surfaces of the spongy mesophyll cells into the air space and diffuses out through the stomata as water vapour
• Leaf structure:

  A) upper epidermis

  B) palisade mesophyll

  C) xylem

  D) phloem

  E) spongy mesophyll

  F) lower epidermis

  
• Dicotyledonous leaf structure:

  A) waxy cuticle

  B) palisade mesophyll

  C) air space

  D) bundle sheath parenchyma

  E) xylem

  F) phloem

  G) spongy mesophyll

  H) stoma

  I) guard cells

  
• stomatal pores:

  • sites of gas exchange with the external environment
  • Water vapour is lost from stomata when these are open
• Guard cells:

  • control the opening and closing of stomata
• Xylem:

  • Transport of water and ions from roots to all other parts of the plant
• Phloem:

  • Transport of carbon compounds (sucrose and amino acids) from the site of photosynthesis and storage organs to the rest of the plant
• Bundle of sheath cells:

  • Provides support and protection to the vascular bundles (in some plants the cells enable more efficient photosynthesis)