Plant Evolution & Paleobotany

Stelar Architecture

Arrangement of xylem and phloem in cross section

Stele: the core or column of xylem (and phloem) running through a plant's axes or stems (stḗlē means “column”).

There are three basic types of vascular arrangements in vascular plants: protosteles, siphonosteles, and eusteles.

Protostele

Xylem occupies the center-most portion of the stem cross-section, and the phloem surrounds the xylem.
These plants tend to have a large cortex with parenchyma cells.
This pattern is observed in many early vascular plants, lycophytes, and the whisk ferns (see below)
Some early woody plants in the Paleozoic displayed a protostele with a cambium and woody growth (e.g. progymnosperms). Modern woody plants are eusteles (see below)

Siphonostele

Vascular arrangement with a ring of xylem around a pith of parenchyma cells in the center. The phloem surrounds the xylem.
A break in the ring, called a "leaf gap", is caused by a vascular strand connecting a stem to a leaf.
This is a pattern observed in many ferns and some other spore-bearing plants. (see below)

Eustele

Vascular arrangement with discrete xylem bundles around a pith of parenchyma cells in the center. The phloem is outside (centripetally) of the xylem.
There is no leaf gap in a eustele, when a vascular strand goes from stem to leaf.
In woody plants, a cambium forms between these two tissues producing secondary xylem centrifugally, and secondary phloem centripetally.
This pattern is observed in seed plants (see below)

Above: Three major patterns of xylem and phloem arrangement in plant stems

Protostele

Protosteles are found in the stems of rhyniophytes, zosterophyllophytes, some clubmosses, trimerophytes, some monilophytes, and aneurophyte progymnosperms
Protosteles are found in the roots of eudicot angiosperms
There are subtypes of protosteles, including haplosteles, actinosteles, and plectosteles

Haplostele

- Xylem cross-section is circular (or cylindrical), with phloem surrounding the xylem (e.g. rhyniophytes)

Actinostele

- Xylem cross-section is lobed or star-shaped. Phloem usually occupies the space in between the lobes (e.g. basal clubmosses such as Asteroxylon)

Plectostele

- Xylem appears to be strip-like or in plates in the center of the cross-section. Phloem occupies the spaces in between the strips (e.g. Selaginellales)

Above: Subcategories of the protostele and siphonostele patterns

Siphonostele

Siphonsteles are found in stems of most monilophytes, especially the leptosporangiate or true ferns
There are a few sub-types of siphonosteles, including dictyosteles, polycyclic dictyosteles, and solenosteles

Dictyostele

- A single ring of xylem is broken up by several leaf gaps. Phloem surrounds this xylem ring

Polycyclic dictyostele

- Several rings of xylem, each with several leaf gaps. Phloem is found to the centripetal side of each ring

Solenostele

- Appears similar to a dictyostele, but phloem is found both centrifugally and centripetally.

Eustele

Eusteles are mostly found in seed plants, such as gymnosperms and angiosperms
There are a few sub-types of eusteles, including the equisetum stele and atactostele

Equisetostele

- The horsetail, Equisetum, has a stelar pattern that is unique among spore-bearing plants, which resembles a eustele with several air canals. This is probably a separate evolution of the eustele, different from the lignophyte clade.

Atactostele

- A pattern found in monocots, in which many small discrete vascular bundles occupy the entire cross-section of the plant

Above: "Equisetum stele" found in the horsetail, Equisetum

Above: Atactostele found in the stems of monocots, such as Zea

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