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.
- Are of 2 types:
Xylem & Phloem form separate bundles present on different radii.
Xylem is exarch i.e., protoxylem towards periphery & metaxylem towards the center.
Found in roots.
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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