(a) : Stele is a column containing vascular tissues which is surrounded by pericycle and separated from ground tissue by endodermis. siphonostele is medullated protostele or protostele with a central non-vascular pith. Leaf gaps are absent. siphonostele is of two types : In Ectophloic siphonostele, central pith is surrounded successively by xylem, phloem, pericycle and endodermis. In amphiphloic siphonostele there is a central pith and xylem is surrounded on either side by phloem, pericycle and endodermis. It is found in Osmunda and Equisetum.