Dicotyledonous Stem

• Stem functions?

  • Holds the leaves in good position for optimal light absorption.
  • Holds flowers in good position for pollination.
  • Holds fruit in good position for seed distribution.
  • Transports food from leaves to roots.
  • Transports water and minerals from roots to leaves.
  • Stores reserve nutrients.
• External Structure?

  • Consists of nodes & internodes
  • The node is where the leaves and side branches develop.
  • The internode is the region between nodes.
  • The terminal bud occurs at the tip of the stem and consists of meristematic tissue which consistently divides and forms new cells and results in the growth of the plant.
  • Auxiliary buds develop into lateral branches or flowers.
• Internal Structure?

  Epidermis

  • outer layer of stem.
  • some are modified to form stomata.
  • covered with a cuticle
  • protects underlying tissues

  Cortex

  • under the epidermis and has 3 layers.
  • Collenchyma
  • Parenchyma
  • Endodermis

  Central Cylinder

  • xylem and phloem occur in vascular bundle
  • cambium is meristematic and makes secondary growth possible
  • medulla rays transport substances between pith and cortex.
• Cortex functions?

  Collenchyma

  • Strengthens the stem and keeps it up right.
  • Sometimes contains chloroplasts for photosynthesis
  • can be replaced by sclerenchyma
  • sclerenchyma protect vascular bundles

  Parenchyma

  • Store organic substances
  • large intercellular spaces for gaseous exchange

  Endodermis

  • not easily distinguished as it is in the root and is absent in some stems.
  • stores starch.
• Secondary growth?

  • Mitosis takes place in the cambium ring.
  • Secondary xylem is formed on the inside and secondary phloem is formed on the outside.
  • Each year, the cambium ring forms a new ring of secondary vascular tissues.
  • Secondary tissue rings cause the circumference to increase.
  • Annual rings - secondary xylem.