Download Meristem

Plant Body Organization
Meristems
There are several basic types of plant meristems.
These include the:
Apical meristem
Floral meristem
Lateral meristem
Leaf meristems (blastozones)
Intercalary meristem
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Plant Body Organization
Apical meristems
The main growing points in plants are the shoot and root apical meristems.
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Plant Body Organization
Shoot Apical Meristems
The shoot apical meristem (SAM)
can be separated into two basic
regions – the promeristem and the
meristematic zones.
The promeristem is at the very tip
of the apical meristem and consists
of the apical initials and adjacent
cells.
The meristematic zones lie behind
the promeristem and are separated
into the protoderm (leads to
epidermal tissue), procambium
(differentiates into vascular
tissue) and ground meristem (origin
of cortex and pith cells).
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Photomicrograph of a coleus shoot tip.
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Plant Body Organization
Shoot Apical Meristems
Meristematic cells are typically
small and densely cytoplasmic.
Apical dome
Leaf
primordium
As the shoot tip grows it lays
down tissue in a consistent
pattern.
Developing
leaf
The meristematic zones occupy
the apical dome. Adjacent to
the apical dome, leaf primordia
are initiated at regular
intervals.
Procambium produces vascular
tissue connecting the leaf to
the stem. Eventually an axillary
bud forms in the leaf axil.
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Ground
tissue
Axillary
bud
Procambium
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Plant Body Organization
Shoot Apical Meristems
Most angiosperm shoot apical meristems can be described by the tunica-corpus
theory of meristem organization, where outer cell layers or the tunica covers
the inner body or corpus layers of the meristem. The tunica layers are
characterized by having anticlinal (perpendicular to the surface) cell divisions,
while the corpus layers can have periclinal (parallel to the surface) cell divisions.
Tunica
Tunica
Corpus
Corpus
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Plant Body Organization
Shoot Apical Meristems
Angiosperm apical meristems usually have a two or three-layered tunica.
The tunica layers are designated as L-I, L-II, and L-III. The L-I gives
rise to the protoderm (epidermal layers). The L-II gives rise to the sexual
gametes, the procambium (vascular system) and ground meristem.
The L-III when present forms parts of the ground meristem.
Tunica
L-I
L-II
L-III
Corpus
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Plant Body Organization
Shoot Apical Meristems
Cryptomeria - Abies type
Gymnosperm meristems
do not have distinct tunica
layers and are better
described by apical
meristematic zones.
Central
mother cells
Apical initials
Surface
layer cells
The major zones include:
Surface layer meristem
Central mother cells
Peripheral zone
(flank meristem)
Peripheral
zone
Central rib meristem.
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Peripheral
zone
Rib meristem
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Plant Body Organization
Root Apical Meristem
The root meristem lies
under a protective root
cap.
Vascular cylinder
Cortex
Meristem
Root cap
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Plant Body Organization
Root Apical Meristem
The root tip can be divided into three regions. The cell division zone contains
the apical meristem. Most of the root growth occurs in the elongation zone as
cells expand. The maturation zone produces the root hairs and lateral roots.
Elongation zone
Maturation zone
Cell division
zone
Root cap
Root apical
meristem
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Plant Body Organization
Root Apical Meristem
During development:
The protoderm
becomes the epidermis.
Protoderm
Procambium
The procambium
becomes the vascular
system.
Ground meristem
Apical meristem
The ground meristem
becomes cortex.
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Root cap
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Plant Body Organization
Root Apical Meristem
The center of the root apical meristem contains a region with relatively few cell
divisions called the quiescent zone. Although cell division is slow in this region,
the quiescent zone plays an important role in establishing tissue organization in
the root meristem.
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Plant Body Organization
Floral Meristem
Under proper conditions, a vegetative shoot apical meristem can transition
into a floral meristem. Upon flower evocation, the meristem becomes
broader and less dome-shaped. The floral appendages initiate in whorls in
order starting with the outer sepals, petals, stamens and inner pistil.
Leaf
primordia
Sepals
Pistil
Petals
Stamens
Vegetative shoot
apical meristem
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Transition
meristem
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Floral meristem with flower
parts initiating
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Plant Body Organization
Floral Meristem
Floral meristem formation in Clarkia amoena. The floral meristem has a
broader, flatter appearance and floral appendages are being initiated to
the periphery of the meristem.
Vegetative meristem
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Floral meristem
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Plant Body Organization
Lateral Meristem
Lateral meristems are responsible for increase in axial
growth (diameter) of the stem or root. The two basic types
are the vascular cambium and the cork cambium (phellogen).
Vascular cambium
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Cork cambium
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Plant Body Organization
Lateral Meristem – Vascular Cambium
The cambium is a lateral
meristem that is responsible
for radial growth in stem
and root tissue.
The cambium is responsible
for producing primary and
secondary vascular tissue.
Xylem cells are produced to
one side and phloem cells to
the other side of the
cambium.
Xylem
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Cambium
Phloem
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Phloem
fibers
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Plant Body Organization
Lateral Meristem - Vascular Cambium
The meristematic cells in the
cambium are called fusiform
initials and periclinal divisions
in the fusiform initials create
cells to both sides within the
cambial zone.
Cambial cells
Cambium in a stem
cross-section.
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Plant Body Organization
Lateral Meristem - Vascular Cambium
A fusiform initial is longer
than it is wide and is wedgeshaped at either end.
Periclinal divisions lead to the
production of either xylem or
phloem cells.
Periclinal
plane of
division.
One set of divisions leads to a
xylem cell. An additional
division produces a phloem
cell.
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Plant Body Organization
Lateral Meristem Vascular Cambium
Storied cambium
Non-storied cambium
Fusiform initials may be
either formed as a storied
or non-storied cambium.
In a storied cambium, the
cells are shorter and line up
in a way that only the tips
overlap.
Cells in a non-storied
cambium are more elongated
and a greater portion of
each cell overlaps.
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Plant Body Organization
Lateral Meristem Vascular Cambium
Another type of initial cell
found in the cambium is the
ray initial. Ray initials are
smaller and less elongated
than fusiform initials. They
give rise to the ray cells.
Periclinal plane
of division.
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Fusiform Ray
initials initials
Diagram of a
transverse stem
section showing ray
cells within a nonstoried cambium.
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Ray
cells
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Plant Body Organization
Lateral Meristem - Vascular Cambium
Ray initials originally form from fusiform initials. They can form in a number of
ways. They may form from a cut-out portion of the fusiform initial or the entire
fusiform initial may form transverse sections that become ray initials.
Tier of ray initials
Fusiform initial
Ray initial
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Plant Body Organization
Lateral Meristem - Vascular Cambium
Rays form files of living
cells within the secondary
vascular system.
They function as
vegetative storage tissue.
Rays
Xylem
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Cambium
Phloem
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Plant Body Organization
Lateral Meristem - Vascular Cambium
In young stems, xylem and
phloem appear in separated
vascular bundles.
Cambium within the vascular
bundle is called fascicular
cambium, while the cambium
between bundles is called
interfasicular cambium.
Vascular
bundle
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Interfasicular
cambium
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Plant Body Organization
Lateral Meristem - Vascular Cambium
With age, the vascular
system and the cambium
forms a complete ring
within the stem and the
secondary vascular system
is formed.
Xylem
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Cambium
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Plant Body Organization
Lateral Meristem - Cork Cambium
In addition to the vascular
cambium, there is a cork
cambium responsible for
making bark (periderm).
The periderm must continue
to expand as the stem
expands and the cork
cambium is responsible for
that axial growth.
Periderm
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Cork
cambium
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Plant Body Organization
Lateral Meristem - Cork Cambium
The cork cambium is also called
the phellogen.
It gives rise to cells that form
the periderm, which is the term
for the protective bark tissue
that replaces the epidermis in a
woody stem.
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Phellogen
Periderm
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Plant Body Organization
Lateral Meristem - Cork Cambium
The periderm is made of
three layers.
The outer phellem (or cork)
layer.
The middle phellogen
(cork cambium).
And the inner phelloderm,
which is living parenchyma
cells formed to the inner side
of the phellogen.
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Plant Body Organization
Leaf Meristems
Unlike shoots and roots, leaves are produced in an enormous diversity of shapes and
sizes. Differential development of marginal meristems along the edge of the leaf
are responsible the various shapes including leaf edges and compound leaves.
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Plant Body Organization
Leaf Meristems - Marginal Meristems
In the marginal meristem
concept, a developing leaf
primordium there are two
regions of growth
associated with the apical
and marginal meristems.
Apical
meristem
The leaf apical meristem
is responsible for leaf
length.
Marginal
meristem
Marginal meristems occur
along the edge of the
developing leaf giving the
size and shape of the leaf
blade (lamina).
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Midvein
Petiole
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Plant Body Organization
Leaf Meristems - Blastozones
Evidence for initial cells in meristematic layers similar to those found in the
apical meristem have not been found to support specific marginal meristems. It is
currently thought that the leaf primordium produces topological meristematic
regions along the leaf margin termed blastozones. Segmentation and differential
development within the blastozone leads to the observed variations in leaf shape.
Leaf
primordium
Meristematic
blastozone
regions
Apical
dome
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Plant Body Organization
Leaf Meristems - Blastozones
Leaf formation proceeds in three basic stages. Initially, a leaf primordium initiates
on either side of the apical dome. This is followed by primary morphogenesis where
meristematic blastozones form along the leaf margin. Finally, during the secondary
morphogenesis stage, the cells expand and differentiate into specific tissue types.
The image to the right
shows the formation
of blastozone regions
leading to development
of a compound leaf
with opposite leaflets.
Meristematic
blastozone
regions
Leaf
primordium
Leaf
primordium
Each leaflet margin
would also have
blastozone regions
leading to the
serrated leaf edge.
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Future
leaflet
Apical
dome
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Plant Body Organization
Marginal Meristems – Plate Meristem
The lateral growth of a leaf occurs with a relatively uniform
thickness. This type of growth where parallel tissue layers
divide anticlinally is termed a plate meristem.
Parallel cell layers in the mesophyll of a developing coleus leaf.
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Plant Body Organization
Intercalary Meristem
Intercalary meristems are
produced in shoots at the
nodes especially in monocots.
It is typical for monocots to
produce numerous compact
nodes behind the apical
meristem that elongate later
in development using
intercalary meristems.
Apical
meristem
The corn embryo to the right
has initiated at least 6 nodes
in the apical region prior to
germination.
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Plant Body Organization
Intercalary Meristem
Intercalary is defined as
growth and cell division
that occurs away from
where the cells were
initially formed in the
original apical meristem.
In the tall monocots like
palms and bamboo, the
extension growth from
nodal intercalary
meristems can be quite
dramatic.
Emerging bamboo shoots
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Bamboo stems at full height
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Plant Body Organization
Intercalary Meristem
The apical meristem produces numerous nodes early in development near the base of the
plant. Stem extension growth proceeds in a telescopic fashion with cells in each node
dividing and elongating from the intercalary meristem.
Bamboo shoot section
Intact bamboo shoot with
outer coverings removed
Apical
meristem
Nodes
each with
a leaf
primordia
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Intercalary
meristems
at each
node
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Plant Body Organization
Intercalary Meristem
Some monocot leaves also
produce an intercalary
meristem in the form of a
narrow band in the collar region
near the base of the leaf.
The intercalary meristem
accounts for much of the leaf
blade extension growth and
this meristem can resume
growth if a portion of the
immature leaf is removed.
Continued leaf growth in mowed
turf grass is from the
intercalary meristem.
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Leaf blade
Collar region
(intercalary
meristem)
Leaf
sheath
Fescue
(Festuca)
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