In a vascular plant, the stele is the central part of the root or stem containing the tissues derived from the procambium. These include vascular tissue, in some cases ground tissue (pith) and a pericycle, which, if present, defines the outermost boundary of the stele. Outside the stele lies the endodermis, which is the innermost cell layer of the cortex. The concept of the stele was developed in the late 19th century by French botanists P. E. L. van Tieghem and H. Doultion as a model for understanding the relationship between the shoot and root, and for discussing the evolution of vascular plant morphology. Now, at the beginning of the 21st century, plant molecular biologists are coming to understand the genetics and developmental pathways that govern tissue patterns in the stele. Moreover, physiologists are examining how the anatomy (sizes and shapes) of different steles affect the function of organs. 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 usually three basic types of protostele: 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, thus appearing to form alternating bands. 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. Siphonostele Siphonosteles have a central region of ground tissue called the pith, with the vascular strand comprising a hollow cylinder surrounding the pith. Siphonosteles often have interruptions