Secondary Xylem Structure

Secondary Xylem Structure

• Secondary xylem is divided into two systems: horizontal (ray system) and vertical (axial system).
• Horizontal system originates from ray initials, forming xylem rays.
• Vertical system originates from fusiform initials, comprising tracheary elements, xylem fibers and axial parenchyma.
• Xylem parenchyma consists of two types:
  • Ray parenchyma originate from cambial ray initials.
  • Axial parenchyma originate from cambial fusiform initials.
  • Ray parenchyma stores starch, fats, tannins, and other substances.
• Ray length/width are determined by cross section, longitudinal sections (radial and tangential).
• Ray height is determined in both radial and tangential longitudinal sections.
• Ray parenchyma can extend into phloem and pith, becoming medullary rays.

Axial Parenchyma Types

• Axial parenchyma are categorized by distribution:
  • Apotracheal: Independent of vessel distribution, further subdivided into:
    • Diffuse: Axial parenchyma forms uniseriate strands or single cells, scattered irregularly.
    • Banded/metatracheal: Parenchyma is in concentric bands.
    • Terminal: Parenchyma cells formed at the end of the growth ring (single cells or layers).
    • Initial: Parenchyma cells formed at the beginning of the growth ring (single cells or layers).
  • Paratracheal: Associated with vessels, further divided into:
    • Scanty: Parenchyma forms incomplete sheath around the vessel.
    • Unilateral: Parenchyma occurs on one side of the vessel (external/internal).
    • Vasicentric: Parenchyma forms complete sheath (circular or elliptical) around the vessel.
    • Aliform: Parenchyma forms complete sheath with lateral wing-like extensions.
    • Confluent: Aliform parenchyma forms diagonal or tangential bands.

Tyloses

• Xylem parenchyma is a significant part of woody plant xylem.
• It stores materials and facilitates radial water transfer between xylem and phloem.
• Ray and axial parenchyma play a role in defending against fungal pathogens.
• Parenchyma cells are often connected to xylem vessels/tracheids via pits.
• In some plants, parenchyma cells can form growths penetrating the vessel lumen.

Secondary Phloem

• Secondary phloem structure is similar to secondary xylem.
• Vertical system originates from fusiform initials and includes sieve elements, phloem parenchyma, and phloem fibers.
• Horizontal system originates from ray initials and consists of phloem ray parenchyma.
• Secondary phloem, like secondary xylem, can be either storied or non-storied, and often exhibits annual growth rings.
• In conifers, the vertical system includes sieve cells, phloem parenchyma, albuminous cells, and phloem fibers. Phloem rays are largely composed of parenchyma, sometimes with albuminous cells.
• In dicots, the vertical system includes sieve-tube members, companion cells, axial parenchyma, and xylem fibers. The horizontal system consists of uniseriate, biseriate, or multiseriate rays, composed of parenchyma cells.