Grade 11 Biology: Dicotyledon Root and Stem Structure and Function

Dicotyledon Root

Parts

  • Area of cell division
  • Area of elongation
  • Area of root hairs
  • Area of differentiation
  • Area of mature primary tissue
  • Area of secondary growth

Functions

  • Absorbs water and mineral salts: Essential for plant nutrition.
  • Anchors the plant: Provides stability.
  • Stores food: Examples include potatoes, where roots store starch.
  • Can be used for reproduction: Some plants can reproduce vegetatively from roots.

Cross Section

  • Epidermis: The outermost layer.
    • Root hair: Extensions of epidermal cells to increase surface area for water and nutrient absorption.
  • Cortex: The region beneath the epidermis.
  • Endodermis: The innermost layer of the cortex, containing the Casparian strip.
  • Vascular Cylinder/Stele: The central core.
    • Pericycle: A layer of cells just inside the endodermis.
    • Xylem: Transports water and mineral salts. Arranged in an X formation in dicot roots.
    • Phloem: Transports food (sugars). Located between the arms of the xylem.

Dicotyledon Root Structure

  • Epidermis
    • Root hairs are present.
    • Single-cell layer thickness.
    • Takes up water and mineral salts.
  • Cortex
    • Located just beneath the epidermis.
    • Starts under the epidermis and ends with the endodermis.
    • Endodermis contains the Casparian strip.
      • Casparian Strip: A band of suberin around endodermal cells that regulates water flow to the stele.
    • Functions:
      • Transports water and dissolved mineral salts to the vascular cylinder/stele.
      • Stores food, e.g., starch.
      • Regulates the flow of water with the help of the Casparian strip.
  • Vascular cylinder/Stele
    • Starts at the pericycle just beneath the endodermis.
    • Central cylinder:
      • The center is made of primary xylem (X formation).
      • Primary phloem is between the arms of xylem.
      • Cambium: Divides xylem and phloem; responsible for secondary growth.

Dicotyledon Stem

General Information

  • Function: Carries flowers and leaves.
  • Divided into:
    • Node: Points where leaves grow from/are attached to.
    • Internode: Lengths on the stem between nodes.
  • Functions:
    • Lends support to leaves and flowers.
    • Transports water and mineral salts from roots to leaves.
    • Transports food from leaves to the rest of the plant.
    • Can store food.

Dicotyledon Stem Structures

  • Consist of:
    • Secondary tissue on the outside
    • Epidermis: The outermost layer.
    • Collenchyma: Provides support, found just under the epidermis.
    • Cortex: Ground tissue surrounding vascular bundles.
    • Vascular bundles: Discrete bundles containing xylem, phloem, and cambium.
    • Sclerenchyma: Provides support and protection to vascular bundles.
    • Phloem: Transports food (sugars).
    • Vascular cambium: Meristematic tissue between xylem and phloem.
    • Xylem: Transports water and mineral salts.
    • Pith: Central part of the stem composed of parenchyma cells.

Arrangement of Tissues

  • Epidermis forms the outside layer.
  • Collenchyma is just under the epidermis.
  • Cortex (parenchyma cells) surrounds vascular bundles.
  • Vascular bundles are between the cortex and pith.
  • Pith (parenchyma cells) is in the central part of the stem.

Vascular Bundle

  • Sclerenchyma: On the outside (dark tissue), providing support.
  • Phloem: Just under the sclerenchyma.
  • Xylem: On the inside of the bundle.
  • Vascular cambium: Between phloem and xylem.
    • Found in a ring in the stem.
    • Meristematic tissue.
    • Forms secondary phloem and xylem.

Secondary Growth in Stem

  • Apical meristem: Found on the ends of plants → Primary growth (length).
  • Secondary growth: Lateral growth in the plant = stem increases in thickness/girth.
  • Vascular cambium in the bundle divides into new xylem and phloem.
  • New xylem forms concentric circles → tree rings.
  • Cambium forms new xylem inwards → the forming of new xylem pushes the cambium outwards.
  • New phloem on the outer side pushed outward → squeezed together with the cortex.
  • Springwood: When cambium forms xylem in Spring → enough nutrients and water; thus, wide, thin-walled xylem grows = Springwood → light ring.
  • Summerwood: When cambium forms xylem in late Summer/Autumn → not enough nutrients and water; thus, narrow, thick-walled xylem grows = Summerwood → dark ring.