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Define Siphonostele. Explain its types.

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In siphonostele xylem is surrounded by phloem with pith at the centre. It includes Ectophloic siphonostele, Amphiphloic siphonostele, Solenostele, Eustele, Atactostele and Polycylic stele.

1. Ectophloic siphonostele: The phloem is restricted only on the external side of the xylem. Pith is in centre. E.g. Osmunda. 

2. Amphiphloic siphonostele: The phloem is present on both the sides of xylem. The pith is in the centre. E.g. Marsilea. 

3. Solenostele: The stele is perforated at a place or places corresponding the origin of the leaf trace.

  • Ectophloic solenostele – Pith is in the centre and the xylem is surrounded by phloem. E.g. Osmunda.
  • Amphiphloic solenostele – Pith is in the centre and the phloem is present on both sides of the xylem. E.g. Adiantum pedatum.
  • Dictyostele – The stele is separated into several vascular strands and each one is called meristele. E.g. Adiantum capillus – veneris.

4. Eustele: The stele is split into distinct collateral vascular bundles around the pith. E.g. Dicot stem. 

5. Atactostele: The stele is split into distinct collateral vascular bundles and are scattered in the ground tissue. E.g. Monocot stem. 

6. Polycyclic stele: The vascular tissues are present in the form of two or more concentric cylinders. E.g. Pteridium.

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