Types of Stele

The term stele refers to the central cylinder of vascular tissue running through a root or stem — xylem, phloem, pericycle and, in some forms, medullary rays and pith. Steles fall into two broad categories, protostele and siphonostele, each split into several named sub-types that trace a rough evolutionary sequence from simple to complex.

In a protostele, phloem simply surrounds a solid xylem core, with no pith. Four sub-types are recognised: haplostele — xylem surrounded by phloem with a smooth outline (Selaginella); actinostele — a star-shaped xylem core surrounded by phloem (Lycopodium serratum); plectostele — xylem plates alternating with phloem plates (Lycopodium clavatum); and mixed protostele — xylem groups scattered irregularly through the phloem (Lycopodium cernuum).

In a siphonostele, xylem surrounds a central pith, with phloem around the xylem. Recognised sub-types include: ectophloic siphonostele — phloem restricted to the outer side of the xylem, pith at the centre (Osmunda); amphiphloic siphonostele — phloem present on both the inner and outer sides of the xylem, pith at the centre (Marsilea); solenostele — a siphonostele perforated wherever a leaf trace departs, itself split into an ectophloic form (pith centred, xylem surrounded by phloem — Osmunda) and an amphiphloic form (pith centred, phloem on both sides of the xylem — Adiantum pedatum); dictyostele — the stele broken into several separate vascular strands, each called a meristele (Adiantum capillus-veneris); eustele — the stele split into distinct collateral vascular bundles arranged in a …