Stele (biology)

Stele refers to the central part of the root or stem in higher plants, containing the tissues derived from the procambium. These tissues include the xylem, phloem, and sometimes pericycle and cambium (in plants that undergo secondary growth). The stele is a critical structure for the transport of water, nutrients, and organic compounds, as well as for the structural support of the plant.

Types of Steles

The configuration of the stele varies among different plant groups, leading to the classification into several types:

• Protostele: The simplest type of stele, consisting of a solid column of vascular tissue, usually xylem surrounded by phloem. Protosteles are found in the earliest vascular plants and some modern ferns.
• Siphonostele: Characterized by a hollow cylinder of vascular tissue surrounding a pith. Siphonosteles are common in ferns and some gymnosperms.
• Eustele: Found in most dicotyledons and many monocotyledons, the eustele consists of discrete vascular bundles arranged in a ring around the pith. In dicots, this arrangement facilitates secondary growth.
• Atactostele: This type is found in monocots, where the vascular bundles are scattered throughout the ground tissue rather than being arranged in a ring.

Function

The primary function of the stele is the conduction of water and nutrients from the roots to the leaves and the transport of synthesized organic compounds from the leaves to other parts of the plant. Additionally, the stele provides structural support, with the lignified xylem helping to support the plant.

Evolution

The evolution of the stele is closely tied to the evolution of vascular plants. The transition from simple protosteles to more complex eusteles and atactosteles reflects the evolutionary trend towards more efficient water and nutrient transport mechanisms, supporting increased plant size and complexity.

Development

The development of the stele from the procambium during primary growth and the role of the cambium in secondary growth (in plants that exhibit this feature) are key aspects of plant development. The differentiation of procambial cells into xylem, phloem, and other tissues is regulated by a complex interplay of genetic and hormonal signals.

See Also

• Vascular tissue
• Xylem
• Phloem
• Root
• Stem (plant)