MERISTEMS

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

CYTOLOGICAL CHARACTERISTICS Thin-walled Iso-diametric Richer protoplasm Devoid of reserve materials and crystals Plastids proplastids Vacuoles small, not obvious, scattered

DIFFERENTIATION Process of growth and morpho-

physiological specialization of the cells

STAGES OF DEVELOPMENT OF PRIMARY MERISTEMS Promeristem = apical initials + derivatives

Initial = cells which remain within the meristem

Partly differentiatiated meristematic zone:ProtodermProcambiumGround meristem

APICAL MERISTEMA. SHOOT APEXB. ROOT APEX

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)

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 – 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

COMMENTS 1. All cells have basically equal potential

of differentiation 2. One zone of apical meristem may

contribute cells to another one

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

TUNICA Outermost layer Surrounds the inner cell mass (corpus) Anticlinal division Enlarges in surface area Layer: 1-9

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

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

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

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)

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

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

ANGIOSPERM ROOT APEX Epidermis and root cap– common origin Dermatocalyptrogen; eudicots

Epidermis and cortex –common initials Root cap –calyptrogen; monocots

Columella- the central core of the rootcap 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 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

LATERAL MERISTEM Parallel to the circumference of the

organ Vascular cambium (VC) and cork

cambium Involved in growth in thickness (VC)

Dicotyledons and gymnosperms

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

VASCULAR CAMBIUM1. Fusiform initials

-- elongated and tapered -- tracheary elements, fibers, xylem and phloem parenchyma, sieve elements

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

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

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

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

PRIMARY THICKENING IN MONOCOTS