protostele

In the centre of the axes is a protostele with an elliptical cross-section.

Members of Lycopodiophyta have a protostele, and the sporophyte generation is dominant.

The stems and branches have protostele, with a triangular-shaped core of xylem.

Such an arrangement is termed a protostele.

Their rhizomes have a "vitalized" protostele or in some taxa a solenostele.

An interesting case is that of Psilotum, which bears a protostele, and enations devoid of vascular tissue.

Both the horizontal rhizomes and the vertical stems had vascular tissue which formed a central core (protostele), and is described as having centrarch development.

The spines of Asteroxylon had a primitive vasuclar supply - at the very least, leaf traces could be seen departing from the central protostele towards each individual "leaf".

The Mississippian-aged lyginopteridaleans tended to have a simple protostele usually surrounded by secondary wood, but in later forms there was a eustele with a central core of pith or mixed-pith.

Outgrowths of the protostele later emerged towards the enations (as in Asteroxylon), and eventually continued to grow fully into the leaf to form the mid-vein (such as in Baragwanathia).

One theory, the "enation theory", holds that the leaves developed by outgrowths of the protostele connecting with existing enations, but it is also possible that microphylls evolved by a branching axis forming "webbing".

The young trunk began as a protostele in which the outer xylem matured first (exarch), but the later and higher portion of the trunk developed as an ectophloic siphonostele in which the xylem was flanked by phloem tissue on both its inner and outer side.