siphonosteles

A 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 cylinder surrounding a central pith. This arrangement is a modification of the simpler protostele, where the vascular tissues are solid. A key characteristic of the siphonostele is the presence of a central pith, which is composed of parenchyma cells. Leaf gaps, which are areas where vascular tissue is absent in the stele to supply a leaf, are also a defining feature of siphonosteles. These gaps are typically located above the point where the vascular bundles enter the leaf. Siphonosteles can be further classified into two main types: ectophloic siphonosteles, where the phloem is located only on the outer side of the xylem, and amphiphloic siphonosteles, where phloem is present on both the inner and outer sides of the xylem. This latter type often has a central pith and an inner cylinder of phloem. Siphonosteles are commonly found in the stems of ferns and some early seed plants. Their evolution represents a significant development in plant vascular organization, allowing for greater complexity and the development of larger plant bodies.