What is a protostele in lower vascular plants?

In botany, the stele refers to the central vascular cylinder of a plant's stem or root. A protostele is considered the simplest and most primitive type of stele found in lower vascular plants.1

Structurally, a protostele forms a solid rod of vascular tissue in which the xylem is located at the very center, surrounded by a narrow band of phloem. This central core is further enclosed by a pericycle—typically one or two cell layers thick—and a single layer of [endodermis]. The endodermis plays a critical role in regulating the movement of water and dissolved substances between the vascular tissue and the surrounding cortex.12