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Transcript of Meristems
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MERISTEMS
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MERISTEMS Retain the ability to divide indefinitely Very little differentiation RESULT of divisions: NEW cells are added
Position APICAL MERISTEMS INTERCALARY MERISTEMS LATERAL MERISTEMS
Origin PRIMARY SECONDARY
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CYTOLOGICAL CHARACTERISTICS Thin-walled Iso-diametric Richer protoplasm Devoid of reserve materials and crystals Plastids proplastids Vacuoles small, not obvious, scattered
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DIFFERENTIATION Process of growth and morpho-
physiological specialization of the cells
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STAGES OF DEVELOPMENT OF PRIMARY MERISTEMS Promeristem = apical initials + derivatives
Initial = cells which remain within the meristem
Partly differentiatiated meristematic zone:ProtodermProcambiumGround meristem
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APICAL MERISTEMA. SHOOT APEXB. ROOT APEX
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A. SHOOT APEX 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)
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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
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HISTOGEN THEORY (HANSTEIN, 1868)
Histogen theory (Hanstein, 1868)1. dermatogen – outermost2. plerome – central3. periblem – between 1 and 2
Each develop from independent group of initials (histogens)
Meristems are destined from the beginning to produce certain tissues
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COMMENTS 1. All cells have basically equal potential
of differentiation 2. One zone of apical meristem may
contribute cells to another one
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TUNICA- CORPUS THEORY (SCHMIDT, 1924) Two regions: TUNICA and CORPUS No constant relationship can be traced
between the particular initials of the promeristem and the inner tissues of the shoot
2 regions can be distinguished by their plane of cell division
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TUNICA Outermost layer Surrounds the inner cell mass (corpus) Anticlinal division Enlarges in surface area Layer: 1-9
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CORPUS Inner cell mass Divides in all directions Enlarges in volume
TYPESA. Usual – 1. CMC 2. rib meristem 3. peripheral
B. Opuntia -- + cambium-like transition zone
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ZONATION IN SHOOT APEX 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
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ORIGIN OF LEAVES Initiated by periclinal divisions at the side
of the apical meristem Origin: tunica or corpus Division leaf buttress Affects Periodic changes in shape of
shoot apex
BRANCHING Where do branches originate? Superficial layers --- exogenous Axillary buds
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B. ROOT APEX Bi-directional production of cells Subterminal in position No lateral appendages (leaves,
branches) Branches occur beyond region of most
active growth Endogenous branching Grows uniformly (no nodes and
internodes)
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ROOT APEX Protoderm Meristem of the cortex Meristem of the vascular cylinder Promeristem
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
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ROOT APICES 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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ANGIOSPERM ROOT APEX Epidermis and root cap– common origin Dermatocalyptrogen; eudicots
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Epidermis and cortex –common initials Root cap –calyptrogen; monocots
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Columella- the central core of the rootcap is distinct from the peripheral part in having few or no longitudinal divisions.
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b. Open type- without any boundaries with reference
to the derivative regions of the root
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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)
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INTERCALARY MERISTEMS Isolated meristematic regions that are
disjunct from the subapical meristematic region
Inserted between differentiated tissue regions
Internodes mature basipetally Nodes mature first Stems of monocots: internodes and leaf
sheaths; in Equisetum
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LATERAL MERISTEM Parallel to the circumference of the
organ Vascular cambium (VC) and cork
cambium Involved in growth in thickness (VC)
Dicotyledons and gymnosperms
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VASCULAR CAMBIUM Fascicular + interfascicular cambium Fascicular cambium – came from
procambium Interfascicular cambium – interfascicular
parenchyma
Develops between primary xylem and phloem
2’ XYLEM- centripetal; 2’ PHLOEM- centrifugally
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VASCULAR CAMBIUM1. Fusiform initials
-- elongated and tapered -- tracheary elements, fibers, xylem and phloem parenchyma, sieve elements
2. Ray initials-- smaller; isodiametric-- vascular rays
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VASCULAR CAMBIUM Intense vacuolation Walls -- 1’ pit fields with plasmodesmata Periclinal division– radial wall are thicker
Procambium – gabled ends; stain deeply;
Cambium – flat ends; long and short cells; intense vacuolation
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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
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CORK CAMBIUM Phellogen One type of initials Rectangular in xs; regular polygons in ls Vacuolated; may have chloroplasts and
tannins No intercellular spaces
Part of the periderm Origin: external to VC– epidermis,
cortex, phloem parenchyma
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PRIMARY THICKENING IN MONOCOTS