The term stele refers to the central cylinder of vascular tissues consisting of xylem, phloem; pericycle and sometimes medullary rays with pith.
There are two types of steles
(i) Protostele,
(ii) Siphonostele.
Protostele In protostele xylem surrounds phloem. The type includes Haplostele, Actinostele, Plectostele, and mixed protostele.
- Haplostele Xylem surrounded by phloem is known as haplostele. eg. Selaginella.
- Actinostele Star-shaped xylem core is surrounded by phloem is known as actinostele.
eg. Lycopodium serratum.
- Plectostele Xylem plates alternate with phloem plates, eg. Lycopodium clavatum.
- Mixed protostele Xylem groups uniformly scattered in the phloem, eg. Lycopodium cernuum.
Siphonostele: In siphonostele, xylem is surrounded by phloem with pith at the centre. It includes Ectophloic siphonostele, Amphiphloic siphonostele, Solenostele, Eustele, Atactostele and Polycyclic stele.
- Ectophloic siphonostele: The phloem is restricted only on the external side of the xylem. Pith is in centre, eg. Osmunda.
- Amphiphloic siphonostele: The phloem is present on both the sides of xylem. The pith is in the centre, eg. Marsilea.
- Solenostele: The stele is perforated at a place or places corresponding the origin of the leaf trace.
(a) Ectophloic solenostele: Pith is in the centre and the xylem is surrounded by phloem, eg. Osmunda.
(b) Amphiphloic solenostele: Pith is in the centre and the phloem is present on both sides of the xylem. eg. Adiantum pedatum
(c) Dictyostele: The stele is separated into several vascular strands and each one is called meristele. eg. Adiantum capillus-veneris.
- Eustele: The stele is split into distinct collateral vascular bundles around the pith, eg. Dicot stem.
- Atactostele: The stele is split into distinct collateral vascular bundles and are scattered in the ground tissue, eg. the Monocot stem.
- Polycyclicstele: The vascular tissues are present in the form of two or more concentric cylinders, eg. Pteridium.
