Plant Anatomy: Steles and Their Types

Steles

• Stele comprises pericycle (non-vascular tissue between phloem and cortex), primary vascular tissues, and pith (when present).
• Two main types: protostele and siphonostele.

Protostele

• Solid central cylinder of xylem, no pith.
• Three subtypes:
  • Haplostele: xylem more or less circular in cross-section.
  • Actinostele: xylem stellate in cross-section.
  • Plectostele: xylem split into longitudinal plates (some joined, some separate).

Siphonostele

• Cylinder of pith within the xylem.
• Thought to have evolved from protostele by the development of a pith inside the xylem.
• Two subtypes:
  • Amphiphloic siphonostele: phloem surrounds xylem both internally and externally. This type may consist of a continuous cylinder of vascular tissue. Leaf gaps, when present, result in a solenostele.
  • When leaf gaps are overlapped, the amphiphloic siphonostele appears as a network of vascular bundles separated with parenchyma cells, called dictyostele. Each bundle is an amphicribral vascular bundle (known as a meristele). Dictyostele can be monocyclic, dicyclic or polycyclic.
• In some plants, two or more vascular cylinders are present, termed distelic or polystelic plants respectively.

2-Ectophloic Siphonostele

• Phloem only surrounds xylem externally.
• More advanced than amphiphloic siphonostele.
• May consist of a continuous cylinder of vascular tissues, or a network of vascular bundles, separated by parenchyma cells.
• May be cyclic (eustele) or scattered (atactostele) in arrangement.

Root-Stem Transition Region

• Primary vascular tissues of roots and stems differ.

• Protoxylem is exarch in roots, endarch in stems.

• Xylem and phloem are arranged alternately in roots, while collateral in stems.

• Region where root and stem vascular systems meet, they adapt to each other in four ways:

  • Fumaria type: Root xylem strands fork radially, joining with phloem; same number of vascular bundles as stem.
  • Cucurbia type: Phloem and xylem strands fork, branches twist laterally and meet in pairs; stem vascular bundles twice that of root.
  • Lathyrus type: Xylem strands do not divide, while phloem does; every two phloem branches join one xylem strand.
  • Triticum type: Half of xylem strands fork, phloem strands unite in pairs; root bundles twice those of stem.