Objectives
Cell Division
&
Meristems
• Chromosomal & non-chromosomal
events of cell division
• Types of primary meristems
• The tunica-corpus concept (shoots)
• “Open” & “Closed” meristems (roots)
February 1, 2006
• Distinguish between primary &
secondary growth
• Anatomy of the root surface
Mitotic Stages
in Onion Root
Roles During Plant Cell Division
Chromosomes & their replication during cell cycle
(M) Mitosis
Other Roles During Cell Division
• Microtubules – spindle; guiding
dictyosome vesicles to cell plate;
pre-prophase band.
• Microfilaments – guide new wall
materials into place on membranes.
• Phragmoplast – aggregation of
microtubules to cell plate; callose
often present.
• Cytokinesis – non-nuclear cell
division.
(G2) Gap 2
(G1) Gap 1
(S) Phase
DNA synthesis
Cells that
cease division
Concepts of Meristematic Shoot
Organization
•Wolff (1759) recognized plant apex as site
of primary plant growth.
•Hanstein (1870) developed “histogen”
theory (dermatogen, periblem,
plerome) in roots.
•Schmidt (1924) concept of tunica-corpus
structure & growth in shoots.
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Types of Primary Meristems
Apical Meristem: A complex of cells
composed of initials & their immediate
derivatives.
• Shoot apical
• Root apical
• Lateral (shoots & roots)
• Intercalary (rib/file)
• Calyptrogen
Intercalary Meristems
• Occur near tip of shoot or root
• Cell division is localized
• Derivatives also divide before
differentiation
• Cell division may also occur at
distances away from apex in
a “wave effect”
Pea Meristems
Derived from the apical
meristem; continuing
division from some distance.
May be at single site, or
multiple ones (waves).
Zone of Cell
Elongation
Zone of Cell
Division
Primary Meristems
(a.k.a. Histogens)
•Protoderm: epidermis & rhizodermis
•Ground
meristem:
pith, cortex, endodermis
Anticlinal vs. Periclinal
Divisions
•Procambium: vascular tissues, pericycle
•Calyptrogen: rootcap
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Root & Shoot Meristems
Shoot Apical Meristem (SAM)
1. Single Apical Cell – Found in non-seed plants, e.g.
Psilotum, Equisetum, & many ferns.
Promeristem – Initiating cells & their
recent derivatives
Primary meristems – Initial differentiation
into Protoderm, Procambium & Ground
Meristem (& Calyptrogen)
Interpretation of Tunica-Corpus
Concept
2. Non-stratified – Found in most gymnosperms
A. Initial layer
B. Central mother cell zone
C. Peripheral zone originating leaves
D. Rib meristem, forming pith
3. Stratified (Tunica-Corpus) – Found in most angiosperms
A. Tunica zone of 1-5 layers (mostly 2) with anticlinal
divisions
B. Corpus zone of random orientation
Gymnosperm Shoot Apex
(Non-Stratified)
Tunica = Protoderm plus additional
cells (Calyptrogen in root)
Corpus = Central mother cells, rib
meristem, flank meristem
Tunica-Corpus
Rib (File) Meristem
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Root Quiescent Centers -- Not
Found in Shoots
Root Meristem
1. Single Apical Cell – Equisetum & some ferns.
Apical cell forms derivatives from each side.
•
Closed meristem
Central cells in distinct layers
(stratified)
2. Multiple Initials – (closed) 2 or 3 distinct layers of
cells giving rise to vascular cylinder, cortex &
root cap. (Found in both monocots & dicots)
Open meristem
Cells ahead of central cells act as
columella mother cells
3. Common Initials (open meristems) – all root tissues
from a common group of unstratified initials.
(Found in both monocots & dicots)
Tobacco
Separate initials for
central cylinder, cortex
& rootcap. (Closed
meristem)
Lateral cells act as cortical &
protoderm initials
Corn Root Apical Meristem
Onion
Common initials.
(Open or transversal
meristem)
Soybean Root: Conventional SEM vs. Cryo
Terms to Know
• Growth: Increase in size by cell division
&/or cell enlargement
• Differentiation: Physiological &
Conventional
morphological change usually
associated with specialization
• Development: Change in form &
complexity from beginning to maturity
• Morphogenesis: Combination of
Cryo
development & differentiation to
generate new form
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