Explain the evolution of steles in pteridophytes.
Answers
Stele (= Greek word meaning a column) can be defined as the unit of vascular system that is made up of xylem, phloem, interfascicular tissues, medullary rays, pericycle and pith (if present).
The term stele refers to the central core of plant axis and is restricted only to primary tissues. The term is used in case of pteridophytes and is seldom applied in case of angiosperms and gymnosperms. Endodermis delimits a stele on the peripheral side.
A protostele is composed of a solid core of xylem mass surrounded by phloem, which in turn remains encircled by pericycle. Endodermis delimits protostele on the peripheral side. In protostele pith is absent and the protoxylem is exarch. There is no leaf gap. Some dicotyledonous roots have radial stele where pith is completely absent. Such radial stele is also referred to as protostele.
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