Notes on the stele of Helianthus stem

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The outermost layer is the endodermis whose cells are rich in starch grains; hence also known as the starch sheath. It has the thickening* on radial walls called casparian thickenings. Next to the endodermis is the pericycle. It is represented by patches (cap like) of sclerenchyma present immedi­ately above each vascular bundle.

The vascular bundles are many in number, arranged in the form of a ring surrounding central pith. Each vascular bundle is conjoint (xylem and phloem combining to form a single bun­dle), collateral (xylem and pholem on the same radius with x/p arrangement), open (open to secondary growth i.e., cabium is present) with endarch (protoxylem pointing towards the pith) xylem.

Phloem consists of seive tubes companion cells and phloem parenchyma. In between phloem and xylem is present cambium which is said to be intrafascicular (present within a vascular bundle).

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The cambial cells are thin walled, rectangular and actively dividing. Next to the cambium, internally is the xylem, with the protoxylem facing the pith? Xy­lem consists of vessels, trachieds, wood fibres and xylem parenchyma . Between the vessels are found the wood parenchyma. Between the vascular bundles are present radially elongated parenchyma cells constituting the primary medullary rays. Occupying the central region of the stem is wide parenchymatous pith.

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