Stele
Stele is a term used in biology to refer to the central part of the root or stem containing the tissues derived from the procambium. These tissues include xylem, phloem, and pericycle. The arrangement of these tissues varies widely among plant species, depending on their evolutionary lineage.
Structure and Function
The stele is the central part of the root or stem, surrounded by the cortex. It contains the vascular tissues, xylem and phloem, as well as supporting tissues, such as pericycle and sometimes also medullary rays. The xylem is responsible for water transport from the roots to the rest of the plant, while the phloem transports nutrients, such as sugars.
The pericycle, a layer of cells surrounding the vascular tissues, plays a crucial role in secondary growth by giving rise to the vascular cambium, a meristematic tissue that produces additional xylem and phloem cells. In roots, the pericycle also produces branch roots.
Types of Stele
There are several types of stele, depending on the arrangement of the vascular tissues:
- Protostele: The simplest type of stele, found in some ferns and lycophytes. The xylem is in the center, surrounded by a ring of phloem.
- Siphonostele: A type of stele where the vascular tissue forms a cylinder around a central pith. It is found in most ferns and some gymnosperms.
- Eustele: A type of stele found in most flowering plants, where the vascular tissues are arranged in discrete vascular bundles around the pith.
Evolutionary Significance
The evolution of different types of stele is thought to be associated with the adaptation of plants to different environments and modes of life. For example, the protostele is associated with the earliest vascular plants, which were small and simple, while the eustele is associated with the more complex flowering plants, which have a higher demand for water and nutrient transport.
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