Stele

The stele is the central cylinder of the stem (and roots) composed of:

  • Primary xylem

  • Primary phloem

  • Pith (if present)

Steles are commonly found in younger stems and some older stems and roots.

Types of Steles

Why Steles Matter:

  • Show evolution of vascular organization

  • Help identify plant groups

  • Explain how stems efficiently transport water and food

Protostele

  • Simplest and most primitive stele

  • Consists of a solid core of vascular tissue (xylem)

  • Phloem surrounds the xylem

  • No pith present

  • Common in primitive seed plants: Whisk ferns, Club mosses

  • Key Pattern:

✔ Solid vascular core
✔ No pith

Siphonostele

  • Tubular arrangement of vascular tissue

  • Pith is present at the center (parenchyma)

  • Xylem and phloem form a cylinder around the pith

  • Common in ferns

  • Key Pattern:

✔ Hollow center (pith)
✔ Vascular tissue forms a tube

Eustele

  • Found in present-day gymnosperms and flowering plants (dicots)

  • Primary xylem and phloem are arranged in discrete vascular bundles

  • Bundles are arranged around the pith

✔ Vascular tissue divided into bundles
✔ Advanced stele type

Origin of stems

  • Arises from the plumule of the embryo

  • Develops from the shoot apical meristem

  • Forms the primary shoot system (stem, branches, leaves)

Primary Development

  • Controlled by the apical meristem

  • Increases length of the stem

  • Produces nodes and internodes

  • Forms primary tissues: epidermis, cortex, pith, primary xylem and phloem

Secondary Development

  • Controlled by lateral meristems

  • Vascular cambium → secondary xylem (wood) and secondary phloem

  • Cork cambium → bark

  • Increases girth/diameter of the stem

  • Common in dicots and gymnosperms

Stem Modifications (Developmental Variations)

  • Underground: rhizome, tuber, bulb, corm

  • Sub-aerial: runner, stolon, sucker, offset

  • Aerial: tendrils, thorns, cladodes, phylloclades

Botanical Significance

  • Provides support and conduction

  • Enables photosynthesis and storage

  • Essential for plant growth and survival

Taxonomic Importance

  • Stem structure used in plant classification

  • Arrangement of vascular bundles

  • Presence or absence of secondary growth

  • Type of stem modification

  • Distinguishes monocots, dicots, and gymnosperms