3.2:STEMS

• Main difference between monocotyledon stem and dicotyledon stem


  • Arrangement of the vascular bundle
  • Presence or absence of secondary growth
• Stems


  • Support plants
  • Transport water
  • Some stems store food
  • Some stems are used for climbing
• Epidermis of stems


  • Single layer of cells that helps keep the shape of the stem and cuts down the loss of water vapour
  • Contains stomata that allow diffusion of gases
• Bark in woody stems


  • Composed of many layers of dead phloem cells
  • No stomata
  • Diffusion of gases occurs through small openings called lenticels
• Dicotyledon stem


  1. Vascular bundle composed of xylem, cambium and phloem arranged in a ring

  2. Cambium cells divide to form new cells, increasing stem width
• Pith


  Present in dicot stems but absent in dicot roots
• Secondary growth in woody dicot stems


  1. Cambium cells divide to form a complete ring around the stem

  2. Cells on inside of ring develop into xylem, cells on outside develop into phloem

  3. Old phloem and cortex pushed outwards as stem grows wider
• Functional xylem and phloem in older woody dicots


  On either side of the cambium, just before the bark
• Large old trees can be hollow inside and still survive
• Ring barking a tree will kill it as it cuts off the transport system
• Annual rings


  Represent growth in one year
• Cork cambium


  Second layer of meristem cells in the outer cortex that produce cork cells to form protective bark
• Non-woody dicot plants


  • Do not develop wood and bark as they are annual plants
  • Depend on lignified xylem, thick-walled phloem fibres, and turgor pressure in packing cells for support
• Monocotyledon stem


  • Vascular bundles scattered throughout cortex and pith
  • Xylem on inside, phloem on outside
  • No cambium between xylem and phloem
  • No secondary growth
• Monocots


  Depend on fibres and turgor pressure in packing cells for support