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Lecture 6: Stem (External structure of stems (Scars (Bud scale scars: each…

- - - - Found at the growing tips or apex of stems, roots and in lateral or axillary buds
      - Responsible for the sustained increase in length of the plant body (Primary growth)
    - - Found towards the periphery of the roots and stems
      - consists of vascular cambium and cork cambium
      - responsible for the sustained increase in diameter/ secondary growth
- - - - Lateral meristem: add girth by producing secondary vascular tissue and periderm
      - Secondary Plant body: Tissue produced by meristems involved in secondary growth
      - Vascular Cambium: Secondary growth meristem which produces xylem and phloem. A layer of cells b/w xylem and phloem. In dicots only.
      - Cork Cambium: Secondary growth meristem which produces cork, a tough substance that replaces the epidermis. A layer of cells below epidermis region, also known as Phellogen.
      - Cork cambium + phelloderm + cork = periderm
        
        Where cork cells are impermeable to gases and water
- - - - Secondary xylem (wood)
      - Secondary phloem (Inner bark)
        
        Transport water, minerals and carbohydrates vertically throughout the plant
        
        Materials move horizontally through rays, chains of parenchyma cells radiating out from the center of the stem
    - - Cork cells + phelloderm (cork parenchyma) = periderm (Outer bark)
      - Produces periderm, replacing epidermis
      - Cork cells
        
        Outside, dead, heavily suberized walls
        
        Protects the stem against mechanical injury, mild fires, temperature extremes and water loss
        
        Stomata replaced by lenticels, areas where cork cells are loosely arranged for gas exchange
      - Cork parenchyma (Phelloderm)
        
        inside, stores water and food
        
        One to several cels thick, thinner than cork cell layer