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Retain the ability to divide indefinitelyVery little differentiationRESULT of divisions: NEW cells are added

PositionPositionAPICAL MERISTEMSINTERCALARY MERISTEMSLATERAL MERISTEMS

OriginPRIMARY SECONDARY

Thin-walledIso-diametricRicher protoplasmDevoid of reserve materials and crystalsyPlastids proplastidsVacuoles small, not obvious, scattered

Process of growth and morpho-physiological specialization of the cells

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Promeristem = apical initials + derivatives

Initial = cells which remain within the meristem

Partly differentiatiated meristematic zone:ProtodermProcambiumGround meristem

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Shoot apex- where new leaves and tissues of the stem arise

Models of tissue organization in the shoot apexA. Apical cell theoryB. Histogen theoryC. Tunica-corpus (most accepted in angiosperms)

*Pteridophytes- apical cell (1 initial) or apical initials– tetrahedral (pyramidal), base is directed towards the surface of the apex

*Gymnosperms- surface meristem; (apical initials– periclinal)

central mother cells

Histogen theory (Hanstein, 1868)1. dermatogen – outermost2. plerome – central3. periblem – between 1 and 2p

Each develop from independent group of initials (histogens)Meristems are destined from the beginning to produce certain tissues

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1. All cells have basically equal potential of differentiation

2. One zone of apical meristem may contribute cells to another one

Two regions: TUNICA and CORPUSNo constant relationship can be traced between the particular initials of the promeristem and the inner tissues of the h tshoot

2 regions can be distinguished by their plane of cell division

Outermost layerSurrounds the inner cell mass (corpus)Anticlinal divisionEnlarges in surface areagLayer: 1-9

Inner cell massDivides in all directionsEnlarges in volume

TYPESA. Usual – 1. CMC

2. rib meristem3. peripheral

B. Opuntia -- + cambium-like transition zone

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Central zone– (waiting meristem)- promeristem- corpus + portions of tunica - gives rise to:Rib zone or pith rib meristem

below central zone; center location- below central zone; center location- becomes the pithPeripheral zone or peripheral meristem- encircles the other zones- most meristematic (eumeristem)- densest protoplast and smallest dimensions- gives rise to leaf primordia,procambium, cortical ground tissue

Mass meristem- division in all planes- RESULT: isodiamteric or spheroidal or definite shape cells

spores sperms (seedless plants) - spores, sperms (seedless plants), endospermRib meristem- right angle to the longitudinal axis of the cell row- parallel longitudinal files

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Plate meristem-anticlinal divisions- does no. of layers increase?- plate-like structure is formed

l l f bl d f

NO!

Example: leaf blade of angiosperm

Plate and Rib meristemOccur in ground meristemaccount for Spreading of lamina

cylindrical elongated: root, stem, petiole

Initiated by periclinal divisions at the side of the apical meristemInconsistent with tunica corpus :Mantle and core termsOrigin: tunica or corpus

Dicots Below the surface layer- corpus or tunica or bothMonocots superficial tunicaGymnosperms peripheral zoney p p pVascular cryptogams single or group of superficial cells

PericlinalDivision leaf buttress (lateral prominence)L f Leaf grows upward from buttressAffects Periodic changes in shape of shoot apex

PLASTOCHRON- period bet. the initiation of two successive leaf primordia*Maximal area

*Minimal area

beforethe pair of leaf primordia emerge

as leaf primordia become elevated

*For plants with decussate phyllotaxy

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after plastochron 7; apex in maximal phase.

early plastochron 8

leaves of pair 8 somewhat elongated

midphase of plastochron 8; apex in minimal phase.

early plastochron 9; the primordia of pair 9 alternate with those of pair 8

Dichotomous branching (seedless vascular plants)original apical meristem undergoes

a median division into two equal parts, each forms a shoot

MonopodialBranches occur laterally at the apex

Where do branches originate?axil of 2nd or 3rd leaf from the apex

Superficial layers --- exogenousAxillary buds—nascent state of branchesy

Originate from detached meristems

pockets of meristematic cells, which remain spatially associated with the leaf axil

derived from the apical meristem but separated from it by vacuolated cells

Initiation of bud-- combination of anticlinal division (superficial layers)-- pericilinal (deeper layers)

results to protrusions of bud above the surface

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Replaces vegetative apex in reproductive stageShows a uniform, densely staining, small-celled peripheral zone (1 or more layers)

encloses lighter-staining, larger-celled coreAxis elongation limited

b f d drib meristem formation is discontinuedCentral tissue enlarges and becomes vacuolated Meristematic activity restricted at the peripheral mantle-like zone (tunica and part of corpus)

concerned with production of floral organsflower parts appear in close sequence

Signified end of growth at that apical meristemor end of growth of the plant and approach of death in annual plants

Bi-directional production of cellsSubterminal in positionNo lateral appendages (leaves, branches)Branches occur beyond region of most active y ggrowthEndogenous branchingGrows uniformly (no nodes and internodes)

ProtodermMeristem of the cortexMeristem of the vascular cylinderPromeristem

columellaCLOSED TYPE- the initials are already discrete

immediately adjacent to the central cellsCalyptrogen- intials of the root cap

OPEN TYPE – tissue systems become distinct only some distance away from the central cells

Single apical cell or initials (vascular cryptogams)

Angiosperms : CLOSED and OPEN type (~ Histogen theory)

a. CLOSED – 3 tiers or layers of initials- apex of central cylinder- cortex- root cap

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Epidermis and root cap– common originDermatocalyptrogen; eudicots

Epidermis and cortex –common initialsRoot cap –calyptrogen; monocots

Columella- the central core of therootcap is distinct from the peripheral part in having p p p gfew or no longitudinal divisions.

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b. Open type- without any boundaries with reference to the

derivative regions of the root

Quiescent center- low mitotic activity- reservoir of cells

- may be due to hormones ( high levels of auxin) , pressure exerted by rapidly dividing neighbouring cells (antagonistic direction of cell growth)

Isolated meristematic regions that are disjunct from the subapical meristematicregionInserted between differentiated tissue

iregionsInternodes mature basipetallyNodes mature firstStems of monocots: internodes and leaf sheaths; in Equisetum

Parallel to the circumference of the organVascular cambium (VC) and cork cambiumInvolved in growth in thickness (VC)

Dicotyledons and gymnosperms

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Fascicular + interfascicular cambiumFascicular cambium – came from procambiumInterfascicular cambium – interfascicularparenchyma

Develops between primary xylem and phloem2’ XYLEM- centripetal; 2’ PHLOEM-centrifugally

1. Fusiform initials-- elongated and tapered -- tracheary elements, fibers, xylem and phloem parenchyma, sieve elements

2. Ray initials-- smaller; isodiametric-- vascular rays

Intense vacuolationWalls -- 1’ pit fields with plasmodesmataPericlinal division– radial wall are thicker

Procambium – gabled ends; stain deeply; Cambium – flat ends; long and short cells; intense vacuolation

1. Storied or stratified cambium-- fusiform initials are arranged in horizontal

rows so that their ends are at the same level2. Non- storied

-- fusiform initials partially overlap one another

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PhellogenOne type of initialsRectangular in xs; regular polygons in lsVacuolated; may have chloroplasts and ; y ptanninsNo intercellular spaces

Part of the peridermOrigin: external to VC– epidermis, cortex, phloem parenchyma