Stelar Architecture

Arrangement of xylem and phloem in cross section

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

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)

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

Equisetum stele

    • 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