How does a siphonostele differ from a protostele?

In the study of lower vascular plants, the stele—the vascular cylinder of a stem or root—exhibits different structural patterns. The primary distinction between a protostele and a siphonostele lies in the arrangement of their tissues around the center of the cylinder.1

• Protostele: Considered the most primitive type, the protostele consists of a solid central rod of xylem surrounded by a narrow band of phloem.12
• Siphonostele: This structure features a central core of pith parenchyma, with the vascular tissue arranged as a cylinder surrounding this pith.31

In siphonosteles, the presence of leaf attachments can create "leaf gaps" in the vascular cylinder where tissue branches off into the leaf. While a protostele is a solid core, the siphonostele is characterized by its hollowed, pith-centered organization.13