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Meristems

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Meristems

  1. 1. 1/13/2010 Retain the ability to divide indefinitely Thin-walled Very little differentiation Iso-diametric RESULT of divisions: NEW cells are added Richer protoplasm Position Devoid of reserve materials and crystals y APICAL MERISTEMS Plastids proplastids INTERCALARY MERISTEMS Vacuoles small, not obvious, scattered LATERAL MERISTEMS Origin PRIMARY SECONDARY Process of growth and morpho-physiological specialization of the cells 1
  2. 2. 1/13/2010 Promeristem = apical initials + derivatives Initial = cells which remain within the meristem Partly differentiatiated meristematic zone: Protoderm Procambium Ground meristem 2
  3. 3. 1/13/2010 Shoot apex- where new leaves and tissues of *Pteridophytes- apical cell (1 initial) or apical the stem arise initials– tetrahedral (pyramidal), base is directed towards the surface of the apex Models of tissue organization in the shoot apex *Gymnosperms- surface meristem; (apical A. Apical cell theory initials– periclinal) B. Histogen theory central mother cells C. Tunica-corpus (most accepted in angiosperms) Histogen theory (Hanstein, 1868) 1. dermatogen – outermost 2. plerome – central 3. periblem – between 1 and 2 p Each develop from independent group of initials (histogens) Meristems are destined from the beginning to produce certain tissues 3
  4. 4. 1/13/2010 1. All cells have basically equal potential of Two regions: TUNICA and CORPUS differentiation No constant relationship can be traced 2. One zone of apical meristem may between the particular initials of the contribute cells to another one promeristem and the inner tissues of the shoot h t 2 regions can be distinguished by their plane of cell division Outermost layer Inner cell mass Surrounds the inner cell mass (corpus) Divides in all directions Anticlinal division Enlarges in volume Enlarges in surface area g Layer: 1-9 TYPES A. Usual – 1. CMC 2. rib meristem 3. peripheral B. Opuntia -- + cambium-like transition zone 4
  5. 5. 1/13/2010 Central zone– (waiting meristem)- promeristem - corpus + portions of tunica - gives rise to: Rib zone or pith rib meristem - below central zone; center location - becomes the pith Peripheral 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, plants), endosperm Rib meristem - right angle to the longitudinal axis of the cell row - parallel longitudinal files 5
  6. 6. 1/13/2010 Plate meristem -anticlinal divisions - does no. of layers increase? NO! - plate-like structure is formed Example: leaf blade of angiosperm l l f bl d f Plate and Rib meristem Occur in ground meristem account for Spreading of lamina cylindrical elongated: root, stem, petiole Initiated by periclinal divisions at the side Dicots Below the surface layer of the apical meristem - corpus or tunica or both Inconsistent with tunica corpus :Mantle and Monocots superficial tunica core terms Gymnosperms y p peripheral zone p p Origin: tunica or corpus Vascular cryptogams single or group of superficial cells PLASTOCHRON - period bet. the initiation of two successive leaf primordia Periclinal before *Maximal area the pair of leaf primordia emerge Division leaf buttress (lateral *Minimal area as leaf primordia become elevated prominence) Leaf L f grows upward from buttress Affects Periodic changes in shape of shoot apex *For plants with decussate phyllotaxy 6
  7. 7. 1/13/2010 after plastochron 7; apex in maximal phase. early plastochron 8 Dichotomous branching (seedless vascular plants) leaves of pair 8 somewhat elongated original apical meristem undergoes a median division into two equal parts, each forms a shoot midphase of plastochron 8; apex in minimal early plastochron 9; the primordia of phase. pair 9 alternate with those of pair 8 Monopodial Branches occur laterally at the apex Where do branches originate? pockets of meristematic cells, which remain axil of 2nd or 3rd leaf from the apex spatially associated with the leaf axil Superficial layers --- exogenous Axillary buds—nascent state of branches y derived from the apical meristem but separated from it by vacuolated cells Originate from detached meristems Initiation of bud -- combination of anticlinal division (superficial layers) -- pericilinal (deeper layers) results to protrusions of bud above the surface 7
  8. 8. 1/13/2010 Replaces vegetative apex in reproductive stage Signified end of growth at that apical Shows a uniform, densely staining, small-celled meristem peripheral zone (1 or more layers) encloses lighter-staining, larger-celled core or end of growth of the plant and approach Axis elongation limited of death in annual plants rib meristem f b formation is discontinued d d Central tissue enlarges and becomes vacuolated Meristematic activity restricted at the peripheral mantle-like zone (tunica and part of corpus) concerned with production of floral organs flower parts appear in close sequence Bi-directional production of cells Protoderm Subterminal in position Meristem of the cortex Meristem of the vascular cylinder No lateral appendages (leaves, branches) Promeristem Branches occur beyond region of most active y g growth columella Endogenous branching CLOSED TYPE- the initials are already discrete Grows uniformly (no nodes and internodes) immediately adjacent to the central cells Calyptrogen- 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 8
  9. 9. 1/13/2010 Epidermis and cortex –common initials Root cap –calyptrogen; monocots Epidermis and root cap– common origin Dermatocalyptrogen; eudicots Columella - the central core of the rootcap is distinct from the peripheral p p p part in having g few or no longitudinal divisions. 9
  10. 10. 1/13/2010 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 meristematic region Inserted between differentiated tissue i regions Internodes mature basipetally Nodes mature first Stems of monocots: internodes and leaf sheaths; in Equisetum Parallel to the circumference of the organ Vascular cambium (VC) and cork cambium Involved in growth in thickness (VC) Dicotyledons and gymnosperms 10
  11. 11. 1/13/2010 Fascicular + interfascicular cambium Fascicular cambium – came from procambium Interfascicular cambium – interfascicular parenchyma Develops between primary xylem and phloem 2’ XYLEM- centripetal; 2’ PHLOEM- centrifugally 1. Fusiform initials Intense vacuolation -- elongated and tapered Walls -- 1’ pit fields with plasmodesmata -- tracheary elements, fibers, xylem and Periclinal division– radial wall are thicker phloem parenchyma, sieve elements 2. Ray initials Procambium – gabled ends; stain deeply; -- smaller; isodiametric Cambium – flat ends; long and short cells; -- vascular rays intense vacuolation 1. Storied or stratified cambium -- fusiform initials are arranged in horizontal rows so that their ends are at the same level 2. Non- storied -- fusiform initials partially overlap one another 11
  12. 12. 1/13/2010 Phellogen One type of initials Rectangular in xs; regular polygons in ls Vacuolated; may have chloroplasts and ; y p tannins No intercellular spaces Part of the periderm Origin: external to VC– epidermis, cortex, phloem parenchyma 12

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