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Understanding Secondary Growth in Plants

Apr 24, 2025

Secondary Growth in Plants

Key Concepts

  • Primary Growth: Involves growth in length through apical meristems.
  • Secondary Growth: Involves an increase in girth, commonly seen in dicotyledonous plants, facilitated by lateral meristems.

Lateral Meristems

  1. Vascular Cambium

    • Responsible for cutting off vascular tissues (xylem and phloem).
    • Initially present as patches between xylem and phloem, eventually forming a complete ring.
  2. Cork Cambium

    • Not detailed in the transcript, but generally involved in forming the periderm, replacing the epidermis during secondary growth.

Formation and Activity of Cambium Ring

  • Interface Ocular Cambium: Formed between primary xylem and phloem.
  • Intra Vascular Cambium: Medullary cells become meristematic to form a continuous cambium ring.
  • Activity of Cambium Ring
    • Produces new cells towards the inner (secondary xylem) and outer (secondary phloem) sides.
    • More active on the inner side; results in more secondary xylem than phloem.
    • Secondary medullary rays: Narrow bands of parenchyma passing through xylem and phloem.

Wood Types

  • Springwood (Early Wood)

    • Formed during active cambium phase in spring.
    • Contains large xylem elements with wide vessels.
    • Lighter in color, lower density.
  • Autumn Wood (Late Wood)

    • Formed during less active cambium phase in winter.
    • Contains narrow vessels.
    • Darker in color, higher density.
  • Annual Rings

    • Combination of springwood and autumn wood visible as concentric rings.
    • Used to estimate the age of the tree.

Heartwood and Sapwood

  • Heartwood

    • Composed of dead elements, dark brown due to organic compound deposition (tannins, resins, oils).
    • Non-conductive, but provides structural support.
  • Sapwood

    • Lighter in color.
    • Functions in the conduction of water and minerals from roots to leaves.

Environmental Influence

  • Cambium activity is influenced by physiological conditions and environmental factors such as seasonal variations.