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.