Define Siphonostele. Explain its types.

In siphonostele xylem is surrounded by phloem with pith at the centre. It includes Ectophloic siphonostele, Amphiphlojc siphonostele, Solenostele, Eustele, Atactostele and Polycylic stele.
(i) Eetophloic siphonostele: The phloem is restricted only on the external side of the xylem. Pith is in centre. E.g. Osmunda.
(ii) Amphiphloic siphonostele: The phloem is present on both the sides of xylem. The pith is in the centre. E.g. Marsilea.
Solenostele: The stele is perforated at a place or places corresponding the origin or the leaf trace.
(a) Eetophloic solenostele-Pith is in the centre and the xylem is surrounded by phloem. E.g. Osmunda.
(6) Amphiphloic solenostele - Pith is in the centre and the phloem is present on both sides of the xylem. E.g. Adiantum pedatum.
(c) Dictyostele - The stele is separated into several vascular strands and each one is called meristele. E.g. Adiantum capillus-veneris
(iv) Eustele: The stele is split into distinct collateral vascular bundles around the pith. E.g. Dicot stem.
(v) Atactostele: The stele is split into distinct collateral vascular bundles and are scattered in the ground tissue. E.g. Monocot stem.
(vi) Polycyclic stele: The vascular tissues are present in the form of two or more concentri1C cylinders. E.g. Pteridium.