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Vascular tissue system

by Ramneet Kaur
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Vascular tissue system

  • Constitute the vascular bundles.
  • Procambial strands of plerome of apical meristem forms the vascular tissue.
  • It consists of xylem & phloem.

Xylem(wood) is made of tracheids, vessels, xylem parenchyma & xylem fibres.

Phoem(bast) is made of sieve elements, companion cells, phloem parenchyma & phloem fibres.

  • Early formed xylem & phloem are referred as protoxylem & protophloem.
  • Later formed are referred as metaxylem & metaphloem.

VASCULAR BUNDLES:

  • Are of 2 types:
  1. RADIAL:

Xylem & Phloem form separate bundles present on different radii.

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Xylem is exarch i.e., protoxylem towards periphery & metaxylem towards the center.

Found in roots.

  1. CONJOINT:  

Xylem & Phloem together form a bundle.

Xylem is endarch i.e., protoxylem towards center & metaxylem towards the periphery.

Found in stem & leaves.

Conjoint vascular bundles are of 3 types:

  • Collateral: xylem towards inner side & phloem towards outer side.

It’s of 2 types: open & closed.

Open: cambium is present in between xylem & phloem. Seen in dicot stem.

Closed: cambium is not present between xylem & phloem.

Seen in monocot stem.

  • Bicollateral: cambium & phloem are present on both sides of xylem. Seen in family Cucurbitaceae, Solanaceae.
  • Concentric: in this one surrounds the other.

It’s of 2 types:

Amphivasal: xylem surrounds phloem. E.g., Yucca, Dracena, Aloe.

Amphicribal: phloem surrounds xylem. E.g., ferns.

STELE: it is the region inner to the endodermis which includes the vascular tissue, xylem & phloem. Stele originated & evolved in Pteridophytes.

Different types of stele are:

  • Protostele: most primitive stele. It has an inner core of xylem surrounded by phloem.
  • Siphonostele: it is protostele with central pith.
  • Dictyostele: the circular stele breaks into smaller parts due to the formation of leaf gaps.
  • Eustele: vascular tissue form vascular bundles, and they are arranged in rings. E.g., dicot stem.
  • Atactostele: vascular tissue exists as vascular bundles that lie scattered. E.g., monocot stem.

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