Stele (biology) - Protostele

Protostele

The earliest vascular plants had stems with a central core of vascular tissue. This consisted of a cylindrical strand of xylem, surrounded by a region of phloem. Around the vascular tissue there might have been an endodermis that regulated the flow of water into and out of the vascular system. Such an arrangement is termed a protostele.

There are three basic types of protosteles:

• haplostele - consisting of a cylindrical core of xylem surrounded by a ring of phloem. An endodermis generally surrounds the stele. A centrarch (protoxylem in the center of a metaxylem cylinder) haplostele is prevalent in members of the rhyniophyte grade, such as Rhynia.
• actinostele - a variation of the protostele in which the core is lobed or fluted. This stele is found in many species of club moss (Lycopodium and related genera). Actinosteles are typically exarch (protoxylem external to the metaxylem) and consist of several to many patches of protoxylem at the tips of the lobes of the metaxylem. Exarch protosteles are a defining characteristic of the lycophyte lineage.
• plectostele - a protostele in which plate-like regions of xylem appear in transverse section surrounded by phloem tissue. In fact, these discrete plates are interconnected in longitudinal section. Some modern club mosses have plectosteles in their stems. The plectostele may be derived from the actinostele.