Siphonostele

Siphonostele is a type of stele, which is the central vascular cylinder found in the stems and roots of plants. In a siphonostele, the vascular tissues (xylem and phloem) are arranged in a hollow cylinder surrounding a central pith. This contrasts with a protostele, where the vascular tissues form a solid core without a central pith. The term siphonostele literally means "tube stele."

Siphonosteles are characteristic of the stems of ferns and some primitive seed plants. They can be further classified based on the arrangement of vascular tissues and the presence or absence of leaf gaps. An important variation is the dictyostele, which is a highly dissected siphonostele found in many ferns, where numerous leaf gaps create a network of vascular bridges. Another related term is eustele, which is found in dicotyledonous stems and consists of vascular bundles arranged in a ring around a central pith. The evolution of the siphonostele from the protostele is considered a significant step in the development of plant vascular anatomy, allowing for more complex branching and support systems.