Short notes on the structure of stele in Helianthus root

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Occupying the central region of the root is the vascular cylinder. The out­ermost layer of the stele is the endodermis. It is made up of a single layer of barrel shaped cells, closely packed without any intercellular spaces. The endodermal cells have thickenings on their radial walls called Casparian thickenings. In between endodermal cells thin walled cells are present. They are called the passage cells. Usually they are present opposite the protoxylem. Internal to the endodermis is the pericycle. It is made up of a single layer of rough, thin walled parenchyma cells.

Surrounded by the pericycle are the vascular bundles. They are radial (Xylem and Phloem form separate alternating bundles) with exarch (protoxylem pointing towards the periphery) and tetrarch (four protoxylem points) xy­lem Xylem consists of annular and sprial vessels with metaxylem being made up of reticulate and pitted vessels.

Occupying the central region of the stele is a narrow parenchymatous pith. Sometimes the metaxylem groups join together so that pith is obliterated. Between the radial vascular bundles is present parenchyma constituting the conjuctive tissue.

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