Stele types

Stele types
The arrangement of xylem and
phloem in plant axes
Stele
• Anything from the endodermis inward
• The configuration of the vascular tissues
in transverse section (=cross section)
• The configuration of the xylem in
transverse section
• Stele terms are used for stems and roots
to describe the vascular system
• There are three basic stele types in
vascular plants
Stele types in vascular plants
1. Protostele = solid core of xylem, has no
pith, no leaf gaps
2. Siphonostele = hollow cylinder of xylem,
has pith, has leaf gaps
3. Eustele = xylem and phloem located in
vascular bundles, sympodia, has pith,
does not have leaf gaps
Stele types
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Protosteles
• Haplostele – circular in outline
• Actinostele – star-shaped stele
• Plectostele – plates of xylem
• Medullated protostele (fossils and
some lycopods) –has a pith but no leaf
gaps
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Protosteles *
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*
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Protosteles
• Haplostele – found in stems of some
ferns (fossil and living), also in roots of
higher plants (seed plants), early land
plants and Psilotum & Tmesipteris
• Actinostele – found in roots of higher
plants and stems of lycopods, Psilotum
Note: when in roots the phloem is located
in between the xylem arms
• Plectostele – found in stems of lycopods
Gleichenia – a fern stem
(=rhizome) with a haplostele
Phloem sieve cells
Mixed protostele
=has parenchyma
mixed with xylem
tracheids
Actinostele
Star-shaped
stele
Found in roots
of higher
plants
Ranunculus
root x.s.
Actinostele
Ranunculus (buttercup) root x.s.
Psilotum axis
Endodermis
pericycle
x
x
x
ph
ph
Suberized endodermal cell
Casparian strip (suberin)
Developing root
mature root
Plectostele
Lycopodium stem x.s.
Steles can change at different stem levels
Lycopodium stem x.s.
Siphonosteles
Siphonostele = hollow cylinder of xylem,
has pith, has leaf gaps
1. Solenostele = simple siphonostele with
few gaps (e.g., one at a time) stem
produces few leaves or one leaf at a time
2. Dictyostele = highly dissected
siphonostele (e.g., one producing several
leaves at a time, therefore, many gaps
This stele type is common in ferns (stems)
Fossil solenosteles
Solenostele
Leaf gap
pith
stele
cortex
Adiantum (fern) rhizome x.s.
Amphiphloic siphonostele
phloem
x
=phloem on both
sides of the xylem
pith
Dictyostele
Polypodium
(fern)
rhizome
(=stem)
Osmunda rhizome DICTYOSTELE
Osmunda ectophloic dictyostele
= phloem on the outside of the xylem only
Lycopodium
stem in l.s.
Giving off leaf
traces
plectostele
Vascular plant steles
Medullated
Protostele
***
*** Note: this is a mistake in your textbook p. 395
(should be as on this slide)
Eustele
Has pith, NO leaf gaps
Ring of vascular bundles
e.g., Helianthus (sunflower)
pith
Found in dicot stems (a group of flowering plants) &
Gymnosperms (seed plants such as conifers)
Eustele
Monocot
root
Atactostele
Corn
Zea mays
Stem
Stele type found in
Monocots=
Grasses, orchids, aroids,
Lilies, rushes, sedges
Scattered vascular bundles in ground tissue – no pith per se
A
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Coleus
Shoot
apex
T
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E
Leaf
primordia
G
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Plants grow in length by divisions of cells at apical meristems
sa
l.s. shoot apex
Primary tissues
Those tissues produced at an apical
meristem; responsible for growth in
length of a plant
Maturation of these cells and their
differentiation (differentiation =
becoming different)
All cells at the apex in a seed plant are
identical in appearance: small,
cuboidal, nucleate. Later they become
different
Primary tissues include: pith, cortex,
epidermis, primary xylem, primary
phloem, endodermis, pericycle
Shoot apex
Seed plant
Non-seed plant
Apical cell
Apical
cell
division
Equisetum
apical cell
Secondary growth
• Growth produced at a lateral (rather
than apical) meristem; responsible for
the growth in diameter of a plant
• E.g.s, wood=secondary xylem,
secondary phloem, periderm=bark
• Not all plant groups have secondary
growth; often herbaceous plants do not
• There are 2 lateral meristems in typical
seed plants
– vascular cambium
– phellogen (cork cambium)
Vascular cambium
• The lateral meristem that produces
secondary xylem (wood), secondary phloem,
and vascular rays
• A ring of meristematic cells that forms
between the xylem and the phloem (in
residual procambium) and in the ground
tissue (parenchyma) between the vascular
bundles
• Produces secondary xylem toward the inside
and secondary phloem toward the outside,
vascular rays in both directions
Secondary
growth
Tissues produced by
a lateral meristemgrowth in
diameter of the
stem/root
Lateral meristems
Vascular cambium –
produces secondary
xylem & phloem
(conducting cells) &
vascular rays
(parenchyma)
Secondary xylem = wood
Phellogen = cork cambium
– produces the bark
system (periderm)
Vascular
cambium
formation
in gymnosperm or
eudicot
eustele
Vascular cambium in the stem
Origin of the vascular cambium
• In woody shoots– the vascular cambium
arises from residual procambium between
the xylem and phloem of the vascular
bundles (=fascicular cambium) and from the
parenchyma cells between the bundles
(=interfascicular cambium)
• A ring of meristematic cells
Vascular cambium
• Produces secondary xylem toward the
inside
• Secondary phloem toward the outside
• Vascular rays (parenchyma) in these
two tissues
• Contains two types of initials that give
rise to secondary tissues
1. fusiform initials
2. ray initials
Vascular cambium t.l.s.
Give rise to vascular rays
Give rise to xylem and
phloem conducting cells
(tracheids and sieve cells or
sieve tube elements)
vc
Ph fibers
1x
1ph
Secondary tissues are produced in radial rows
Pinus
Pseudotsuga
Transition from earlywood to latewood
2x
1x
2ph
p
Vascular cambium
periderm
3 year old stem
Cell divisions in the vascular cambium
Multiplicative = anticlinal division-adds to the
width of the vascular cambium to keep up
with the growth in diameter
Additive = periclinal division-produces the
mother cells
Vascular rays are living parenchyma cells
x.s. width of rays seen
r.l.s. height of rays seen
Uniseriate,
biseriate, and
multiseriate
rays
Carpinus
t.l.s. height and
number of cells
wide seen
in rays
Periderm
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•
•
•
Protective tissue
Secondary origin
Called bark-or the bark system
Consists of 3 parts
– Phellogen = cork cambium = lateral
meristem
– Phellem = cork-produced to
outside-cells suberized and
sometimes also lignified
– Phelloderm = parenchyma-likeproduced toward the insideusually thin-walled cells
Stages in periderm formation
pl
ph
pd
ep
pl
ph
ph=phellog
pd=phellod
Fate of the epidermis and cuticle is to be
sloughed off the outside
of the stem as phellem
produced
Cork cells = PHELLEM
Cork cambium = PHELLOGEN
Cork parenchyma = PHELLODERM
PERIDERM