Pterophyta: The location of the Stele varies from the Center to the Periphery of the Stem. Stele complexity varies from a Haplostele to a Dictyostele to a Polystele.

Megaphylls are present and more than one leaf trace may diverge to each Megaphyll. There can be many overlapping Leafy Gaps which result in a divided stele (Dictyostele). The Vascular Bundles of a Dictyostele are located in the Outer Cortex, rather than in the center of the stem. Rhizomes with no leaves tend to have simple, central Steles (Haplo, Actino, Plecto)

Coniferophyta & Anthophyta: In Herbaceous Plants & the New Growth of Woody Plants, the Stele is found in the Outer Cortex. This is a Eustele.

Monocots have an Atactostele in which most of the Vascular Bundles are in the outer Cortex but some are present throughout the rest of the stem.

Strengthening Tissue (Sclerenchyma) is associated with the Vascular Bundles rather than the Cortex.

Megaphylls rule and there are many Leaf Gaps in Dicot Steles.

Monocot leaves have many major veins which are directly connected to a complex of highly branched vascular bundles in the stem.

The Bottom Line is that increased Stelar Complexity and its more peripheral Location correlate with the presence of Megaphylls which have large, complex Leaf Traces & many Leaf Gaps in the stem.

Strengthening Tissues are now associated with Vascular Bundles.

The shift from a Eustele to Siphonostele is probably due to a functional shift for each stem segment as new ones are produced by the Shoot Apical Meristem & as Branches get larger.

The Eustele & Atactostele are associated with soft stems that have actively photosynthesizing leaves. These are relatively light-weight and require little structural support.

Furthermore, there is a need for more efficient radial transport out of and into vascular bundles of the leaf. Smaller Vascular Bundles are more efficient in terms of local radial transport than a large Stele located at the center of the stem, or minute Leaf Traces.

The development of a Siphonostele from a Eustele occurs in parts of the stem which have shed their leaves & are far removed from the shoot apex. There is a greater need for structural support in these areas, due to the increased mass that accrues from Primary Growth (new Leaves & Stem tissues). There is also a greater need for Longitudinal Transport to and from the metabolically active parts of the stem tip that is getting further and further away from the Roots.

Structural support is now provided by Secondary Xylem which is located near the periphery of the stem and extends to its center. Most of Xylem conduction occurs in its outer zone ("Sapwood") near the Vascular Cambium & Secondary Phloem.

Maranta  (Prayer Plant) is Monocot Megaphyll with Net Venation.

Dicot Stem with a Eustele


Typical Dicot Vascular Bundle: Note the Sclerenchyma Fibers associated with the Vascular Tissues.

Cross-section of Makaloa (Cyperus) Stem: This is a Monocot with an Atactostele.

Vascular Bundles of Makaloa. The blue stain indicates Lignin. Note the great amount of Sclerenchyma which surrounds each bundle!

Tilia Stem at the end of Primary Growth: There is little Strengthening Tissue present.

Tilia Stem after one year of growth: Strengthening Tissue (Sclerenchyma) is present in the Secondary Xylem & Secondary Phloem

Secondary Xylem provides most of  the support for woody plants.

There is a Correlation between the type of Stele and Plant Height. Small Mosses have the most simple Stele. These become more complex and lead to a Eustele for large Herbaceous plants or the growing tips of Woody plants. The Siphonostele develops from a Eustele with Mature Woody stems.