The Vascular Tissue System
The vascular tissue system consists of vascular bundles that distributed in the stele.
The stele is the central cylinder of stem & root delimited externally by cortex & enclosed pith or no at the center.
Functions: • Xylem functions in water conduction with mineral from roots to shoots.
• Phloem functions in carbohydrates translocation.
Origin: Originate from procambium meristem.
Constituents: • Phloem (bast elements).
• Vascular cambium.
• Xylem (wood elements).
Vascular Cambium
Locality: In between xylem & phloem (in stems).
Characters: The cambial cells are living, thin-walled, able to divide, elongated with tapered ends, uniseriate or multiseriate.
Position:
• Fascicular ( inside vascular bundle ).
• Interfascicular( in between vascular bundles ).
Behavior:
• In monocot stems:
the cambium is completely differentiated into pimary xylem & phloem so, the vascular bundle is closed
& secondary growth will not form inside the bundle.
• In dicot stems:
the cambium remains meristematic to give secondary
growth so, the vascular bundle is opened.
N.B. In roots:
the primary phloem & xylem originate from
pericycle side by side forming radial vascular bundle.
Phloem: Complex tissue consisting of:
In Angiosperms (Dicots & Monocots)
• Sieve tube cells.
• Companion cells.
• Phloem parenchyma.
In Gymnosperm (Conifers)
• Sieve cells.
• Albuminous cells.
• Phloem parenchyma.
Portions of phloem:
• Protophloem consists of (sieve tube cells & parenchyma) in angiosperms,
(sieve cells & parenchyma) in gymnosperms.
• Metaphloem (sieve tube cells, companion cells & parenchyma) in angiosperms, (sieve cells, albuminous cells & parenchyma) in gymnosperm.
Xylem: Complex tissue consisting of:.
In Angiosperms (Dicots & Monocots):
• Xylem vessels (function in water conduction).
• Xylem (wood) fibers (xylary fibers) (give mechanical support).
• Xylem (wood) parenchyma (aid in storage of food & water).
In Gymnosperms (Conifers):
• Tracheids with bordered pits (water conduction).
• Xylem (wood) fibers (mechanical support).
• Xylem (wood) parenchyma (storage of food & water).
Portions of Xylem:
• Protoxylem consists of (xylem vessels & xylem parenchyma) in angiosperms, (tracheides with bordered pits & xylem parenchyma) in gymnosperms.
• Metaxylem consists of (xylem vessels, xylem fibers & xylem parenchyma) in angiosperms, (tracheides with bordered pits, xylem fibers & xylem parenchyma) in gymnosperms.
Development:
• In Roots
The development of protoxylem is toward the pericycle & endodermis, the xylem strand is called centripetal or exarch.
• In Stems
The development of protoxylem is toward the pith, the xylem is cetrifugal or endarch.
• In hypocotyl of angiospermic seedlings
The development is both centripetal and centrifugal, the xylem is mesarch.
Xylem Vessel Lignification:
Lignification or perforation occurs on one end walls of vessel or on the lateral walls.
The perforated part is called perforated plate which may be simple or multiple.
Types:
• Annular • Spiral • Reticulate • Pitted •Scalariform
Types of Vascular Bundles
A. Radial: Xylem & phyloem lie radially side by side (in roots).
• In dicot roots: • In monocot roots:
2-12 xylem arches. more than 13 arches.
B. Conjoint: Xylem & phloem lie on the same radius (in stems).
• Collateral: Phloem lies outwards only.
1. In dicots: 2. In monocots:
contain cambium no cambium,
(Opened) (Closed).
• Bicollateral: Phloem present on
both sides of xylem.
C. Concentric: One type of vascular tissue surrounds the other. (Closed v. bundle).
• Amphicribral: • Amphivasal:
Phloem surrounds Xylem surrounds
the xylem . the phloem .
Secretory and Excretory Tissue System
Secretion: is the separation of certain substances from the protoplast.
• Secretory cell: the secretion formed is stored within the secretory cell.
• Excretory cell: the secretion formed is exuded from the secreting cell.
Nature of secreted substances:
• Products utilized by plants (resins, tannins& crystals).
• Products with special physiological function (enzymes&hormones).
Types of secretory tissues:
A. laticeferous tissue: In which latex is found.
• Non-articulate latex ducts (latex cells): not fuse together.
• Articulate latex ducts (latex vessels): forming a complex network.
B. Glandular tissue:
1. External glands: occur on the epidermis.
• Glandular epidermal hairs: Glandular trichomes as in dermal system.
• Nectarines: in entomophilous plants (insect-pollinated) to attract insects.
2. Internal glands: embedded in the interior tissues.
• Oil glands: contain essential oils (volatile and odoriferous), lysigenous in nature (formed by lyses of the cells forming large cavities of glands).
• Resin glands: in gymnosperms forming ducts of schizogenous nature (formed by separation of cells forming cavities with epithelial lining).
• Digestive glands: in insectivorous plants secreting protein-digesting enzymes.
• Hydathodes: called water stomata or water pores that exudates water under conditions of low transpiration and abundant soil moisture.
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