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Meristems

Meristems are undifferentiated tissues that retain the ability to divide indefinitely and add new cells. There are three main types - apical meristems at tips that add new cells, intercalary meristems between tissues that allow elongation, and lateral meristems like vascular cambium that add girth. Apical meristems follow the tunica-corpus model with outer layers adding surface area and inner layers adding volume. Root apices have initial cells that form distinct tissue layers close to the tip or more open organization farther out.

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MERISTEMS
mERISTEMSRetain the ability to divide indefinitelyVery little differentiationRESULT of divisions: NEW cells are addedPositionAPICAL MERISTEMSINTERCALARY MERISTEMSLATERAL MERISTEMSOriginPRIMARY SECONDARY
Cytological characteristicsThin-walledIso-diametricRicher protoplasmDevoid of reserve materials and crystalsPlastids  proplastidsVacuoles small, not obvious, scattered
DIFFERENTIATIONProcess of growth and morpho-physiological specialization of the cells
STAGES Of development of Primary meristemsPromeristem = apical initials + derivativesInitial = cells which remain within the meristemPartly differentiatiatedmeristematic zone:ProtodermProcambiumGround meristem
Apical meristema. shoot apexb. root apex
A. Shoot apexShoot apex- where new leaves and tissues of the stem ariseModels of tissue organization in the shoot apex A. Apical cell theory B. Histogen theory C. Tunica-corpus (most accepted in angiosperms)
Apical cell theory*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)Histogen theory (Hanstein, 1868) 1. dermatogen – outermost 2. plerome – central 3. periblem – between 1 and 2Each develop from independent group of initials (histogens)Meristems are destined from the beginning to produce certain tissues
Comments All cells have basically equal potential of differentiation One zone of apical meristem may contribute cells to another one
Tunica- corpus theory (Schmidt, 1924)Two regions: TUNICA and CORPUSNo constant relationship can be traced between the particular initials of the promeristem and the inner tissues of the shoot2 regions can be distinguished by their plane of cell division
TUNIcaOutermost layerSurrounds the inner cell mass (corpus)Anticlinal divisionEnlarges in surface areaLayer: 1-9
corpusInner cell massDivides in all directionsEnlarges in volumeTYPESA. Usual – 1. CMC 2. rib meristem 3. peripheralB. Opuntia -- + cambium-like transition zone
Zonation in shoot apexCentral zone– (waiting meristem)- promeristem - corpus + portions of tunica - gives rise to:Rib zone or pith rib meristem - 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
Origin of leavesInitiated by periclinal divisions at the side of the apical meristemOrigin: tunica or corpusDivision  leaf buttressAffects Periodic changes in shape of shoot apexBRANCHINGWhere do branches originate?Superficial layers --- exogenousAxillary buds
B. Root apexBi-directional production of cellsSubterminal in positionNo lateral appendages (leaves, branches)Branches occur beyond region of most active growthEndogenous branchingGrows uniformly (no nodes and internodes)
Root apexProtodermMeristem of the cortexMeristem of the vascular cylinderPromeristemcolumellaCLOSED TYPE- the initials are already discrete immediately adjacent to the central cellsCalyptrogen- intials of the root capOPEN TYPE – tissue systems become distinct only some distance away from the central cells
Root apicesSingle 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
Angiosperm root apexEpidermis and root cap– common originDermatocalyptrogen; eudicots
Epidermis and cortex –common initialsRoot cap –calyptrogen; monocots
Columellathe central core of therootcap is distinct from the peripheral part in having few or no longitudinal divisions.
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)
Intercalary meristemsIsolated meristematic regions that are disjunct from the subapicalmeristematic regionInserted between differentiated tissue regionsInternodes mature basipetallyNodes mature firstStems of monocots: internodes and leaf sheaths; in Equisetum
Lateral meristemParallel to the circumference of the organVascular cambium (VC) and cork cambiumInvolved in growth in thickness (VC)Dicotyledons and gymnosperms
Vascular cambiumFascicular + interfascicular cambiumFascicular cambium – came from procambiumInterfascicular cambium – interfascicular parenchymaDevelops between primary xylem and phloem2’ XYLEM- centripetal; 2’ PHLOEM- centrifugally
Vascular cambium1. Fusiform initials -- elongated and tapered -- tracheary elements, fibers, xylem and phloem parenchyma, sieve elements2. Ray initials -- smaller; isodiametric -- vascular rays
Vascular cambiumIntense vacuolationWalls -- 1’ pit fields with plasmodesmataPericlinal division– radial wall are thickerProcambium – 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
Cork cambiumPhellogenOne type of initialsRectangular in xs; regular polygons in lsVacuolated; may have chloroplasts and tanninsNo intercellular spacesPart of the peridermOrigin: external to VC– epidermis, cortex, phloem parenchyma
Primary thickening in monocots