Plant Anatomy: Steles and Their Types

Study Notes

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

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).

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.

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.

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.