It consists of a ring of endarch collateral bundles1 surrounding a hollow pith.
A leaf-trace, as it passes through the cortex, has a collateral structure, the protoxylem being situated at the inner edge of the xylem; when it reaches the leaf-base the position of the spiral tracheids is gradually altered, and the endarch arrangement (protoxylem internal) gives place to a mesarch structure (protoxylem more or less central and not on the edge of the xylem strand).
Similarly in the sporophylls of some cycads the bundles are endarch near the base and mesarch near the distal end of the stamen or carpel.
The leaves of the more primitive members of this series were entirely fern-like and possessed a fern-like vascular strand; while in the later members, including the modern Cycads, the leaf bundles, remaining unaffected by secondary thickening, are mesarch, while those of the stem-stele have become endarch.
When there is a single protoxylem strand in the centre of the stele, or when, as is more commonly the case, there are several protoxylem strands situated at the internal limit of the xylem,, the centre of the stem being occupied by parenchyma, the stele is endarch.
The constitution of the stele of a flowering plant entirely from endarch collateral bundles, which are either themselves leaf-traces or will form leaf-traces after junction with other similar bundles, is the great characteristic of the stem-stele of flowering plants.
The word usage examples above have been gathered from various sources to reflect current and historial usage. They do not represent the opinions of YourDictionary.com.