Dicotyledonous Stem

Main Functions of the Stem

• Positions Leaves: Ensures leaves are in a favorable position for sunlight absorption, aiding photosynthesis.
• Positions Flowers: Holds flowers properly for effective pollination.
• Positions Fruit: Places fruits favorably for seed distribution.
• Transports Water/Minerals: Moves water and minerals from roots to other plant parts.
• Transports Organic Substances: Carries organic substances from leaves to other plant parts.

External Structure of the Stem

• Components:
  • Nodes: Locations where leaves and side branches develop.
  • Internodes: Sections between two nodes.
  • Apical/Terminal Bud: Located at the stem tip, containing meristematic tissue for primary growth (increases length).
  • Axillary Bud: Found in the axils between the petiole and stem, potentially developing into lateral branches or flowers.

Internal Structure of a Dicotyledonous Stem

• Composed of Three Regions:
  1. Epidermis
  2. Cortex
  3. Central Cylinder

Cross Section of a Dicotyledonous Stem Showing Tissues

1. Epidermis

   • Outer stem layer with single-layered thin-walled, brick-shaped cells.
   • Can form stomata and is covered with a waterproof cuticle.
   • May have multicellular hairy outgrowths.

2. Cortex

   • Located directly after the epidermis.
   • Consists of four layers: Collenchyma, chlorenchyma, parenchyma, and endodermis.
   • Collenchyma: Under epidermis with thickened corner cell walls and small intercellular spaces.
   • Parenchyma: Thin-walled with large intercellular spaces.
   • Endodermis: Inner cortex layer with brick-shaped cells, sometimes absent or indistinct, stores starch.

3. Central Cylinder

   • Contains vascular tissue (xylem and phloem) in vascular bundles arranged in an oval shape.
   • Xylem: Located inside.
   • Phloem: Located outside, under the schlerenchyma cap.
   • Schlerenchyma Cap: Outer phloem covering with dead cells and thickened walls.
   • Cambium: Meristematic tissue between xylem and phloem.
   • Pith: Central stem region.

Functions of Tissues in a Dicotyledonous Stem

• Epidermis: Protects underlying tissues.
• Cuticle: Prevents moisture loss.
• Stomata: Facilitates gaseous exchange.
• Collenchyma: Provides strength and upright structure, may contain chloroplasts for photosynthesis.
• Parenchyma: Stores organic substances, assists in gaseous exchange through intercellular spaces.
• Endodermis: Stores starch.
• Schlerenchyma Fibres: Protect vascular bundles, strengthen and support the stem.

Classwork and Homework Assignments

• Classwork:
  1. List five main stem functions.
  2. Draw and label the external structure of a dicot stem.
  3. Describe the four external stem parts.
• Homework:
  1. Draw and label a cross-section of a dicotyledonous stem.
  2. List and describe the internal stem regions.
  3. List six functions of stem tissues.

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Source: Page 12, Mind Action Series, January 2023