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Meristems and plant structure
The plant body
Apical Meristems:
how the plant grows
Cell walls and plasmodesmata
Some basic cell types not
mentioned yet
Apical meristems
Indeterminate growth
e.g., many tomato
varieties
Determinate growth
e.g., wheat
Biennial and determinate plants
Digitalis purpurea
Foxglove is a biennial
A rosette of leaves is
produced close to the
ground in the first year of
growth
Triticum aestivum
Like many grass species
wheat is determinate. It
produces a fixed number of
leaves and a terminal
inflorescence
Ricinus:
Shoot apical meristem
Castor bean
Meristems: how tissues are produced
Coleus
Longitudinal section through
the apical meristem
Apical meristem
Transverse section through the
apical meristem and newly forming
leaves
Scanning electron microscope picture
of Myriophyllum apical meristem
Myriophyllum SEM picture
The apical
dome is usually
convex or flat,
as in this
example, and
its surface is
smooth.
Developing
leaves cover
the apex
Water-milfoil
Most recently
budded leaves
Diagram of shoot apical growth
Cell division
Elongation
zone
Differentiation
of vascular
tissue
Coleus
Auxiliary bud meristem
The auxiliary meristem may
develop into a foliated branch.
Equisetum meristem
Equisetum shoot
apex with a single
apical cell
The organization of the meristem
is different from Coleus
Root apical
meristem
Zea mays root apex
Zea mays root apex
showing the junction
between root apex
and the root cap
Lateral root development in
Zea mays
A meristem develops from parenchyma
and the lateral root grows out through
the cortex
… structural carbohydrates?
Cellulose
Cell walls and plasmodesmata
Microfibers
Electron microscope
picture of cellulosemicrofibrils in the
wall of the green alga
Oozystis solitaria
Growing plant cells expand through
water uptake. In a growing cell
enzymes weaken cross-links
between the cellulose microfibres of
the cell wall allowing it to expand as
water flows in by osmosis.
Plasmodesmata
Plasmodesmata
Plasmodesmata seen in Section through the cell wall: They are not
simple openings but have a complex internal structure.
Some basic cell types
1. Parenchyma
2. Collenchyma
3. Sclerids
4. Bulliform cells
1. Parenchyma
Note the nucleus and
chloroplasts
Young parenchyma tissue cut parallel with the epidermis of
Euphorbia pulcherrima (poinsettia). Note the cell contents.
2. Collenchyma
Collenchyma is the typical
supporting tissue of the
primary plant body.
It develops from parenchyma.
The cell walls are unevenly
thickened.
It is common in organs like
stems, petioles, laminae or
roots.
Apium petiole, collenchyma
Apium is celery – and it is the petiole that you eat!
Fig 31.5 B and C
3. Sclerids
Thickening of the cell wall
Parenchyma Developing sclerid
Sclerenchyma cells are
the principal supporting
cells in plant parts that
have ceased elongation.
Sclerenchyma fibres are
the source material for
many fabrics, e.g., flax,
hemp and jute.
A sclerid with the cell completely occupied by wall
Contrary to
collenchyma mature
sclerenchyma is
composed of dead cells
with extremely thick
cell walls (secondary
walls) that make up to
90% of the whole.
Leaf of Podocarpus
Note the wall laminations and the Pits around 3 & 5
o'clock. Viewed with polarized light
Fig 31. D
turgor pressure?
4.
Bulliform cells
Transverse section of grass
leaf Poa praetense.
During drought water is
lost from the thin walled
bulliform cells and the two
sides of the leaf blade fold
up toward each other so
the leaf is less exposed to
sunlight and is heated less.
Once adequate water is
available, turgor increases,
and the leaves open again.
Sections you need to have read
4.19, 31.1 through 31.8
Courses that deal with this topic
Botany 441 Morphology and anatomy of land plants