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Lecture #7
Angiosperms: Form & Function
Plant cells
• contain all the usual
eukaryotic “suspects”
– mitochondria, Golgi, ER, vacuoles
etc….
– double phospholipid plasma
membrane with embedded proteins
and carbohydrates
– BUT they also possess a cell wall
– cytoplasm and the organelles are
sometimes referred to as the
protoplasm
– nucleus is similar to the animal cell
nucleus
Plant cells
• unlike animal cells – plant
cells are capable of storing
large quantities of many
substances
• some substances are
stored within the
cytoplasm
– other materials are stored
within vacuoles
• primary central vacuole –
water and salts and wastes
• spherosomes or lipid
bodies - lipid
• plant cells also possess specialized structures
Plastids
– plastids – group of dynamic organelles that are able to perform many
functions
• made of an inner membrane and an outer membrane with a stroma in between
• several types within a plant cell
• 1. chloroplasts – forms from immature pro-plastids once the cell is exposed to
light
– contain the photosynthetic pigment chlorophyll and all the accessory proteins and
complexes required for photosynthesis
– inner membrane is elaborately folded into membrane sheets called thylakoid
membranes (large surface area for photosynthetic machinery)
– in certain areas the thylakoid membranes is folded into vesicle-like bags stacked
together – grana (transport of H+ ions required in photosynthesis)
chloroplast
Plastids
• 2. amyloplasts – in plants tissues that can’t photosynthesize
– roots, bark and wood cells
– accumulate sugar and store it as starch
• 3. chromoplasts – e.g. in tomatoes and yellow squash
– bright red, yellow and orange lipids accumulated here
• 4. leucoplasts – large and unpigmented plastids
– no chlorophyll or lipid pigments
– involved in the synthesis of fats and phospholipids
potato amyloplasts
tomato chromoplasts
The Cell Wall
• all cells of the plant have cell walls
(except the sperm)
• it is an active, dynamic organelle with
many metabolic functions
• contains large amounts of cellulose
– in the form of cellulose microfibrils
– made by rosette protein complexes
• all cells have a thin primary cell wall
– made of cellulose
– also contains pectin – complex
polysaccharide that allows for plant
growth
– pectin also forms the middle lamina
found between two plant cells
middle lamina – “cement”-like
layer of pectin found between
two plant cells
The Cell Wall
• in cells that require strength – there is a
thicker secondary wall that is
synthesized from the protoplasm
– laid down in between the primary wall
and the plasma membrane
– the secondary wall is almost always
impregnated with lignin
– lignin is strong and resists fungal and
bacterial attacks
both the primary and
secondary walls are
permanent and are not
degraded or depolymerized
• plant cells are in communication with one another – despite the presence of
the cell wall
• connections between cells = plasmodesma
(plural = plamsodesmata)
– the plasma membrane of one cell passes through the plasmodesma and is
continuous with the PM of the adjacent plant cell
– often plant cells have clusters of the PDs – called pit fields
• there exists a diffusional space from one cell wall to another =
apoplast
• the transport of materials from one cell to another through this
cell wall space = apoplastic
• the transport of materials from the inside of one cell through the
plasmodesma into the neighbouring cell = symplastic
• the presence of plasmodesmata means that the
individuality of the plant cell is eliminated
• the plant becomes one interconnected mass of cells
and protoplasm = symplast
Plant Cells
• three types of plant cells:
• classified based on the nature of their cell walls
• 1. parenchyma – only have thin primary walls
– undifferentiated cells
– differentiation leads to the other two cell types
• 2. collenchyma – primary cell walls thin in
some areas, thick in others
• 3. sclerenchyma – primary and secondary
walls containing lignin
1. Parenchyma
• only possess thin primary cell walls
• these cells comprise parenchyma tissue –
ground tissue that fills the gaps in between
other tissue types
• most common type of cell and tissue within
the plant
• relatively undifferentiated
• metabolically active
• most usually remain alive once they mature
• capable of dividing even after they mature =
known as meristematic
1. Parenchyma
• some parenchyma cell subtypes are
specialized for specific tasks
– a. chlorenchyma cells – parenchymal
cells containing chloroplasts
– b. glandular cells – secrete
– c. transfer cells – mediate short distance
transport of material
• parenchyma tissues can be classified
according to their function
palisade
– spongy parenchyma – round loosely
packed parenchyma cells in the leaf –
surround air spaces
– palisade parenchyma – columnar
shaped parenchyma cells found beneath
the epidermis of the leaf
spongy
2. Collenchyma
collenchyma
– thin primary cell wall in some
areas
• in other areas the cell wall thickens –
most often the corners of the cell
• cells exhibit plasticity – the ability to
become deformed by pressure or
tension and to retain this new shape
once this force is removed
• collenchyma tissue - usually only
produced in elongating shoot tips –
give the tips strength as it elongates
but it can be stretched
– found just underneath the epidermis
parenchyma sclerenchyma
3. Sclerenchyma
– has both primary and
secondary cell walls
• these cell walls are lignified
• have the property of elasticitythe ability to become deformed by
pressure or tension and to return
to normal shape once this force is
removed
• most sclerenchyma cells die once
they mature
• because they only need to provide
strength to the plant
• some can remain alive and
metabolically active
3. Sclerenchyma
• two types of sclerenchyma cells –
conducting and mechanical
– mechanical – comprised of long fibers
and short sclereids with thick
secondary walls
– conducting – makes up vascular tissues
• sclerenchyma tissue: develops
mainly in organs that have stopped
growing and have achieved their
final shape
• the absorption of water by
collenchyma and parenchyma
tissues can cause swelling –
sclerenchyma tissues prevent the
cell from expanding
stone cells in “pit” of a pear
Plant Tissues
• found throughout the plant
– i.e. in roots, stems and leaves
– so each forms a tissue system that is continuous through
all parts of the plant
• 1. dermal
• 2. vascular
• 3. ground
• 1. dermal tissue - plant’s outer protective covering
– forms the first line of defense
– usually a single tissue – epidermis
– epidermis = single layer of parenchyma cells –
cells are tightly joined together
Plant
Tissues
epidermis
epidermis
– epidermis
• forms specialized structures in roots, stems and leaves
– e.g. roots - root hairs
– e.g. stems – trichomes – hair like outgrowths from the stem
epidermis that make it difficult for insects to land on
Plant
Tissues
• in leaves and most stems – epidermis is covered with a
waxy cuticle to prevent water loss & protect against
pathogen damage (fungi & bacteria)
– cuticle is made of a fatty protein called cutin
– in very dry environments – there may be an additional layer
of wax
• in woody plants – the periderm replaces the epidermis in
older regions of the stem and roots
Trichromes
– epidermis:
• outer walls contact the environment – regulate the exchange of materials
• BUT the presence of cutin can inhibit the entry of CO2 needed for
photosynthesis
• epidermis contains pairs of cells (guard cells) that surround a hole in the
epidermis (stomatal pore)
• guard cells + stomatal pore = stoma (plural = stomata)
• guard cells control the size of the pore – control CO2 entry
– absorb water and swell – increased turgid pressure curves the cell and opens
the pore
2. Vascular tissues
• plant vascular system is NOT a circulatory system
• vascular tissue: xylem & phloem
– xylem: for the conduction of water & minerals
• conducting cells: tracheids & vessel elements
• water and minerals enter the xylem in the roots and are
conducted upward to the leaves and stems
– phloem: for the conduction of sugars
• conducting cells: sieve cells and sieve tube members
• phloem picks up sugar from where it is abundant in the plant
and transports it to where it is needed
PARENCHYMA CELLS
Xylem
WATER-CONDUCTING CELLS OF THE XYLEM
– two types of conducting cells: tracheids
and vessel elements
Vessel
Tracheids
100 µm
• either type of cell can be called a “tracheary
element”
– both are types of conducting
sclerenchyma cells
– tracheids and vessel elements develop
first as immature parenchyma cells with
thin primary walls
– cell elongates and deposits and secondary
wall
– cell then dies and the protoplasm
degenerates – leaving a hollow dead cell
comprised of two cell walls
– now a type of sclerenchyma cell
Pits
Tracheids and vessels
(colorized SEM)
Vessel
element
Vessel elements with
perforated end walls
pit-pair
aligned
perforations
Tracheids
• xylem:
– tracheids: long cells with tapered ends
• tracheids obtain water from those under them and
pass the water on to those above them – connection
is known as a pit-pair
• pit-pair has a pit membrane – provides some friction
to water flow
• xylem:
– vessel elements: shorter and wider
cells with flat ends
• posses a large hole at either end of the
cell = perforation
• the vessel elements align their
perforations to form a vessel
• very little friction to water movement
(vs. tracheid)
• while the vessel elements are short in
length – the vessels they create can run
from root tip to shoot tip
vessel element
vessel element
tracheid
hardwoods of angiosperms are
mainly vessel elements
softwoods of gymnosperms are
mostly tracheids
PARENCHYMA CELLS
• xylem:
– many tracheids or vessel elements
can be identified by their
secondary wall
– secondary wall is organized as
rings = annular thickenings
• the majority of the primary wall is
uncovered - for water entry and exit
• large surface area for water uptake and
movement
– the strongest type of xylem cell is
called a pitted tracheary element
• the majority of the primary wall is covered
with secondary wall except for small
regions called circular bordered pits
• water uptake and movement through
these is slow
aligned
perforations
tracheids
vessel
elements
Phloem
– two cell types: sieve cells
and sieve tube members
– either are referred to as
sieve elements
– are parenchyma cells
with only a primary cell
wall
– as sieve elements
develop – the
plasmodesmata enlarge
and become sieve pores
• sieve areas – clusters of
sieve pores
• sieve plates are two
stacked sieve areas
SUGAR-CONDUCTING CELLS OF THE PHLOEM
Sieve-tube members:
longitudinal view
(LM)
Companion
cell
Sieve-tube
member
Plasmodesma
Sieve
plate
Nucleus
Cytoplasm
Companion
cell
Sieve-tube members:
longitudinal view
30 µm
15 µm
Sieve plate with pores (LM)
Phloem
– sieve cell is long and spindleshaped
– sieve tube members are shorter
and wider with flat ends
– these cells must remain alive
for function – yet their nuclei
degenerate
– sieve tube members are
associated with companion
cells
• still retain a nucleus and provide the
sieve cells with the necessary nuclear
control over their metabolism
– sieve cells are associated with
albuminous cells
Vascular Bundles
• xylem and phloem occur
together in roots and stems
as vascular bundles
• bundles are located just
interior to the cortex
• the arrangement of the
vascular bundles helps
define a monocot stem
from a eudicot stem
• also arranged differently
according to whether they
are in a root or a stem
– e.g. in angiosperms there is a
solid central vascular cylinder
called a stele
3. Ground Tissue
• tissues that are neither dermal or vascular
– includes cells specialized for storage, photosynthesis,
and support
– made up of:
• 1. parenchyma – made up of parenchyma cells
– many parenchyma cells contain chloroplasts
• 2. sclerenchyma – made up of sclerenchyma cells
– provides support to the plant
• 3. collenchyma- made up of collenchyma cells
– found in the cortex and pith of stem, the cortex of the
root, the mesophyll of leaves and the endosperm of
seeds
3 Plant organs
• plants respond to changes in their environment by
altering their growth
• plants have organs each comprised of specific tissues
– organ = made up of multiple tissues
• 3 organs – form a root system and a shoot system
– 1. stems – for transport & support of leaves
– 2. leaves – for photosynthesis
– 3. roots – for absorption
Reproductive shoot (flower)
Terminal bud
Node
Internode
Terminal
bud
Vegetable
shoot
Leaf
Shoot
system
Blade
Petiole
Axillary
bud
Stem
Taproot
Lateral roots
Root
system
Stems
• 1. organ that raises or separates leaves, exposing them
to sunlight
• 2. also raises reproductive structures – facilitating the
dispersal of pollen and fruit
• stem – is the main axis
• shoot – stem plus any leaves, flowers or buds off of the
stem
• stems or shoots consist of:
Terminal bud
Bud scale
Axillary buds
This year’s growth
(one year old)
Leaf scar
Node
Stem
Internode
One-year-old s
shoot formed
from axillary bud
near shoot apex
– a. an alternating system of nodes – where leaves are
attached
– b. internodes – between the nodes
Last year’s growth
(two years old)
Leaf scar
Scars left by termina
bud scales of previou
winters
Growth of twoLeaf scar
years ago (three
years old)
Stems
• the arrangement of leaves on the stem = phyllotaxy
– so that two leaves don’t lie over each other and shade
one another
– 1. opposite - two leaves on opposite sides of the stem at
the node
– 2. whorled – three or more leaves per node
– 3. alternate – leaves alternate up the stem; one leaf per
node
– 4. spiral – the nodes themselves “spiral” up the plant; the
leaves can be opposite, alternate or whorled at the nodes
Evolutionary Adaptations of Stems
• Rhizomes – a horizontal shoot that grows
just below the surface
– can give rise to vertical shoots
• Bulbs – are vertical underground shoots
– consist mainly of enlarged, fleshy leaves
that store food
• Stolons – horizontal shoots that grow
along the surface
– often called “runners”
– asexual reproduction – plantlets can form
along the runner where it encounters a
suitable habitat
• Tubers – enlarged ends of rhizomes or
stolons
– specialized for storing food
– “eyes” – are clusters of axillary buds that
mark the nodes
rhizome
• the angle formed by a stem and leaf – called an axil
– the location of the axillary bud – structure that can form a
lateral shoot (e.g. branch stem or a leaf petiole) or a flower
– vegetative bud if it forms a shoot
– floral bud if it forms a flower
– can be covered with thick modified leaves = bud scales
– new growth from these buds results in “scars”
Apical bud
Axillary buds
Stems
• most stem growth is concentrated near the stem tip –
consists of an apical or terminal bud
– if the apical bud is near an axillary bud can prevent the growth
of a new shoot
– apical dominance = inhibition of axillary bud growth by the
nearby apical bud
– removal of the apical bud can stimulate growth of axillary buds
and new shoots
• axillary and apical buds contain an undifferentiated tissue
called meristem
Stems
• stems grow longer by creating new
cells at their tips
• growth is at regions known as shoot
apical meristems (SAMs)
• growth is via mitosis
• as the small daughter cells grow to
the size of the parent – they push the
meristem upward – lower and older
cells mature and become part of the
growing stem
• region below the apical meristem =
subapical meristem
– site of differentiation
• apical meristem is flanked by small,
developing leaf primordia which
protect the AM
Stem growth:
Apical meristems
the apical meristem and leaf
primordia = bud
apical meristem
developing
vascular tissue
subapical
meristem
developing leaf primordia
axillary bud
Stem growth:
Primary and Secondary growth
• primary growth in the AM leads to the formation of
the subapical meristem
• Subapical Meristem is composed of three types of
subapical cells
– 1. protoderm – gives rise to the epidermis
– 2. provascular tissue – gives rise to primary xylem
and primary phloem
– 3. ground meristem – gives rise to pith and cortex
• primary growth is followed by secondary
growth – continued differentiation
– 1. epidermis becomes the cork cambium
(in some plants)
– 2. provascular tissue gives rise to the
vascular cambium - becomes the
secondary xylem and phloem (woody
tissues)
– 3. pith becomes the interfasicular
cambium and the cortex forms the cork
cambium (which forms cork)
Stem growth:
Primary and
Secondary growth
• subapical meristem
– region where division and
differentiation takes place
– some subapical meristem cells stop
dividing and form the first tracheids
and vessel elements  protoxylem
or primary xylem (first xylem)
• the remaining meristem cells
keep dividing but eventually stop
and differentiate into larger
tracheids and elements – called
metaxylem
Stem growth:
Subapical meristem
T
metaxylem
V
protoxylem
Stem growth: Subapical meristem
– on the outer edge of the developing
vascular bundle the exterior cells
are called protophloem or primary
phloem
• those cells closest to the
metaxylem form metaphloem
cells
• metaphloem is bigger than
primary phloem and lasts longer
– known often as just phloem
• metaphloem cells are much
smaller than metaxylem
T
metaxylem
V
protoxylem
Stems: Internal organization
• epidermis – layer of parenchyma cells covered with a cuticle
• covered with a waxy cuticle to prevent water loss & protect against
pathogen damage (fungi & bacteria)
• cortex – interior to the epidermis
– simple and homogenous in most stems
– composed of photosynthetic parenchyma and sometimes collenchyma
Stems: Internal organization
• vascular bundles – xylem and phloem
– unique arrangement depending on whether the plant is a eudicot or a
monocot
• pith – most interior portion of the stem
– region of parenchyma
– similar to the parenchyma of the cortex
Stems: Internal organization
• vascular bundles
– each bundle has xylem and phloem strands running parallel to each other
– vascular bundles first contain both primary xylem and primary phloem
• then develop metaxylem and metaphloem
– monocots – distributed as a complex network throughout the inner part of
the stem
• frequently described as “scattered” in arrangement
– eudicots and gynmnosperms – vascular bundles are arranged in the
periphery
• surrounding an inner ground tissue of parenchyma called the pith
Phloem
Xylem
Sclerenchyma
(fiber cells)
Ground
tissue
Ground tissue
connecting
pith to cortex
Pith
Epidermis
Key
Cortex
Epidermis
Vascular
bundles
Dermal
Ground
1 mm
A eudicot (sunflower) stem. Vascular bundles form a ring.
Ground tissue toward the inside is called pith, and ground
tissue toward the outside is called cortex. (LM of transverse
section)
Vascular
Vascular
bundles
1 mm
A monocot (maize) stem. Vascular bundles are scattered throughout
the ground tissue. In such an arrangement, ground tissue is not
partitioned into pith and cortex. (LM of transverse section)
Medullary
Rays
• vascular bundles of monocots
– between the bundles is parenchyma
– frequently described as “scattered” in arrangement
– more complex than a random arrangement
• vascular bundles of eudicots and gymnosperms
– vascular bundles are arranged in the periphery
surrounding an inner tissue of parenchyma called the
pith (ground tissue)
Dicot
Monocot Stem Vascular Bundles
• cap of sclerenchyma on
top of primary phloem
and metaphloem
– called phloem in the
figure
• below this is a region of
large tracheids & vessel
elements = metaxylem
– called xylem in the figure
• smaller tracheids and
vessel elements below
this is primary xylem
• most interior layer is
another layer of
sclerenchyma
Eudicot Stem Vascular Bundles
• cap of sclerenchyma
• region of phloem (mostly
metaphloem)
• below this is a region of
fascicular/vascular cambium
– play a role in secondary growth
of a dicot stem
– produces secondary xylem and
phloem in the “woody” stem
– no VC is mature monocot stems
(no secondary growth)
Eudicot Stem Vascular Bundles
• below this is a region of
primary xylem &
metaxylem
• below these bundles is
the parenchyma of the
pith
• parenchyma between
vascular bundles are
medullary rays
Leaves
• the main photosynthetic organ
• consist of:
– 1. a flattened blade
– 2. stalk called the petiole - joins
the leaf to the stem at the node
• contains vascular tissue in the
form of veins
– monocots – parallel veins
– eudicots – branched network
Blade
Vein
Petiole
Axillary bud
Leaves
• blade morphology:
– simple – single undivided blade
– compound – blade consists of
multiple leaflets
– double compound – each leaflet
divides into smaller leaflets
Simple leaf
Petiole
Axillary bud
Leaflet
Compound leaf
• like compound leaves – this
morphology may resist tearing by
strong winds
Petiole
Axillary bud
Doubly compound leaf
Leaflet
Petiole
Axillary bud
Evolutionary Adaptations of Leaves
Tendrils.
• tendrils –modified leaves or lateral branches
capable of wrapping around small objects
– e.g. pea plants, ivy
• spines – non-photosynthetic
– e.g. cacti
– photosynthesis carried out by the stem
• needles – capable of photosynthesis
Spines.
– usually seen in gymnosperms
• storage leaves – adapted to storing water
– e.g. succulents
• reproductive – leaves that produce adventitious
plantlets
– e.g. succulents
Storage
leaves.
• bracts – often mistaken for petals; modified
leaves that surround a group of flowers
– e.g. pointsettia
Bracts.
Leaf Tissues
-leaves are comprised of three tissue types
1. epidermis
2. mesophyll
3. vascular tissue - veins
Key
Guard
cells
to labels
Dermal
Ground
Vascular
Cuticle
Sclerenchyma
fibers
Stomatal pore
Epidermal
cells
50 µm
Surface view of a spiderwort
(Tradescantia) leaf (LM)
Stoma
Upper
epidermis
Palisade
mesophyll
Bundlesheath
cell
Spongy
mesophyll
Lower
epidermis
Guard
cells
Cuticle
Xylem
Phloem
Cutaway drawing of leaf tissues
Vein
Guard
cells
Vein
Air spaces
Guard cells
100 µm
Transverse section of a lilac
(Syringa) leaf (LM)
Leaf Tissues
– 1. epidermis: comprised of a cuticle, guard cells, stomata and trichomes
• water loss through the epidermis = transpiration
• similar to the stem epidermis – large, flat epidermal cells with guard cells and
trichomes interspersed
• epidermal cells: contain a coating of cutin and wax
• guard cells & stomata: upper and lower epidermis have different characteristics –
upper surface has fewer stomata if any at all
• trichomes – provide shade to the upper surface (deflects sunlight) and slow water
loss through the stomata on the lower surface
• also make it difficult to eat the leaf by insects
Leaf Tissues
– 2. mesophyll: ground tissues of the leaf located just below the
epidermis
• upper surface is a layer of cells called the palisade parenchyma
– main photosynthetic tissue of the plant- usually one layer thick
– BUT can be several layers thick in regions of intense sunlight
• layer below is made of spongy mesophyll
– loosely packed parenchyma cells with many intercellular air spaces to permit
diffusion of CO2 toward the palisade cells – for photosynthesis
vascular
bundles
• 3. vascular tissues or veins
– located between the palisade and mesophyll layers
– in dicots – branched venation
– large midrib (or midvein) and several smaller lateral veins with narrower
branches called minor veins
– primary xylem on the upper side and primary phloem on the lower side
– vascular tissue of larger veins is surrounded by a bundle sheath (made of
bundle sheath cells)
collenchyma
DICOT LEAF
• 3. vascular tissues or veins
– monocot – parallel venation
• distinct pattern of phloem and xylem
• xylem with a few very large vessel
elements (3 of them)
bundle sheath
Roots
• organ that bears no leaves or node
• have multiple functions:
– 1. anchors the vascular plant in the soil – done by the
lateral roots
• lateral root – plant organ that functions in increasing anchorage
– 2. absorbs minerals and water – mostly done at the tip of
the root by root hairs
• root hair = thin tubular extension of a root epidermal cell
– 3. stores carbohydrates
– 4. can undergo vegetative reproduction
– 5. production of hormones – e.g. cytokinin and gibberellin
(stem growth)
• numerous types of roots
• but two types of root systems: fibrous and taproot
Roots
• numerous types of roots
– adventitious – form from unusual
locations – e.g. leaves or stems
– aerial – growth above ground
Prop roots.
Storage roots.
• e.g. orchids
– aerating – grow up above the
ground or water
– coarse – undergo secondary
thickening and can be woody
– haustorial - seen in parasitic
plants; substrate is the body of
another plant
• roots known as haustoria
Aerating
“Strangling” aerial
roots.
Roots
• numerous types of roots
– prop – exposed adventitious roots
produced near the base of the stem
– stilt – adventitious roots that grow
down from lateral branches off of a
stem
– storage – for storage of food and
water – includes taproots
– structural – large roots with
secondary thickening, gives support to
large woody plants and trees
Prop roots.
Storage roots.
Aerating
“Strangling” aerial
roots.
• found in eudicots and
gymnosperms
• develops from the embryonic
root (known as the radicle)
• taproot gives rise to multiple
lateral roots
Taproot
systems
– lateral roots can also produce
smaller, lateral roots
– lateral roots can become swollen
like the main taproot
• e.g. sweet potatoes and cassava
• generally penetrate deeply
• are well adapted to deep soils
where groundwater is not close
to the surface
carrot
turnip
cassava
• most monocots – e.g. grasses
• mat of generally thin roots that spread out
below the soil surface
• most of the roots are similarly sized
• the embryonic root (radicle) dies early on
and doesn’t form a taproot
• many small roots emerge from primordial
tissues found in the stem
• each small root forms multiple lateral roots
• does not penetrate the soil deeply
• excellent at holding topsoil in place
Fibrous
Root
Systems
Root Structure
• fairly simple – no leaves, leaf
axils, axillary buds etc…
• root tip:
– tip of the root is where growth in
length occurs
– root apical meristem (RAM)
present in the root tip
– Root AM – more orderly than the
Shoot AM
• there is a quiescent region in the RAM
called the quiescent center
Root Apical
meristem
Root cap
Root Structure
• root cap:
– the RAM is protected by a root
cap as the root pushes
through soil
– golgi of the root cap cells
secrete a mucilage or slime to
help the root push through the
soil
– cells are small and
meristematic
Root Apical
meristem
Root cap
• Growth occurs just behind the
root tip, in three zones of
cells:
Root growth
– Zone of cell division
• region of mitotic division
• can also be called the Meristematic zone
– Zone of elongation – located
just behind the root apical
meristem
• cells begin differentiation in this region
• 1. outermost cells called the protoderm form the epidermis
• 2. center cells called provascular tissue form the primary xylem and phloem
• as these cells move up and away from the
root tip – primary X and P becomes
metaxylem and metaphloem
• permeable to water
Cortex
Vascular cylinder
Epidermis
Root hair
Zone of
maturation
Zone of
elongation
Apical
meristem
Root cap
Zone of cell
division
• Growth occurs just behind the
root tip, in three zones of
cells:
– Zone of maturation – also called
the root hair zone
• vascular bundles contain metaxylem and
metaphloem
• many of the epidermal cells extend out to
form root hairs
• innermost cells of the cortex
differentiates and form a cylinder called
the endodermis
– waterproof region that forms around
the developing vascular bundle
Root growth
Cortex
Vascular cylinder
Epidermis
Root hair
Zone of
maturation
Zone of
elongation
Apical
meristem
Root cap
Zone of cell
division
Roots: Internal organization
•
•
primary growth of roots produces the epidermis, ground tissue, and vascular tissue
from outermost to innermost:
– 1. epidermis – from the protoderm of the developing root
– 2. cortex – ground tissue
– 3. endodermis – encircles the vascular cylinder
– 4. vascular cylinder – contains xylem and phloem
• also contains a tissue called pericycle – layer of parenchyma or sclerenchyma cells
• just inside the endodermis
• in dicots – can form lateral roots
endodermis
Monocot Roots:
Vascular Bundle
Epidermis
Cortex
Vascular
cylinder
Endodermis
-Monocot roots:
-vascular bundle consists of a
core of parenchyma (similar to the
pith of a stem)
-core is surrounded by a ring of
alternating xylem and phloem tissue
Pericycle
Core of
parenchyma
cells
Xylem
Phloem
100 µm
Transverse section of a monocot root with
parenchyma in the center. The stele of many
monocot roots is a vascular cylinder with a
core of parenchyma surrounded by a ring of
alternating xylem and phloem.
Key
Dermal
Ground
Vascular
Pericycle
Endodermis
Xylem
Phloem
Core of
parenchyma
cells
Dicot Roots:
Vascular Bundle
Epidermis
Cortex
Vascular
cylinder
Endodermis
Pericycle
-Eudicots and gymnosperms:
-vascular bundle is a single
vascular cylinder called a stele
-surrounded by a pericycle and
endodermis
Core of
parenchyma
cells
Xylem
100 µm
Phloem
Transverse section of a typical root. In the
roots of typical gymnosperms and eudicots,
as well as some monocots, the stele is a
vascular cylinder consisting of a lobed core
of xylem with phloem between the lobes.
Endodermis
Pericycle
Xylem
Phloem
50 µm
Key
Dermal
Ground
Vascular
LE 35-14
100 µm
Emerging
lateral
root
Cortex
Vascular
cylinder
Epidermis
Lateral root
Lateral roots arise from within the pericycle, the outermost cell layer in the
vascular cylinder
Plants
• two designations: herbaceous (herb – nonwoody) and woody plants
• wood: considered to be made of secondary
xylem
Vascular Cambium
• meristematic tissue that forms during primary growth
–
–
–
–
also called a lateral meristem
source of secondary xylem and phloem
VC located between the xylem and phloem of the vascular bundle
in non-woody plants – the cambium ultimately stops dividing and
differentiates into more xylem and phloem
Vascular Cambium
– BUT in woody plants – the VC
never undergoes cell arrest
and continue to divide and
differentiate
• ultimately forms secondary
xylem and phloem
– made up of two types of cells
– fusiform initials and ray
initials
• fusiform – long and tapered cells
that can differentiate into
secondary xylem or phloem
• ray – shorter and more cuboidal
cells that forms more
parenchyma cells
Secondary vascular tissues
• found in woody plants
• vascular cambium differentiates into secondary xylem and
secondary phloem
• secondary phloem – no arrangement of rings or early and late
wood
• secondary xylem contains all the type of cells that are found in
primary xylem – tracheids, vessel elements, sclereid cells and
parenchyma
Secondary vascular tissues form from secondary
growth
Cork Cambium
• another lateral meristem
– forms from the outermost layer of secondary phloem
bark
(cork)
Cork Cambium
– cork cambium divides only for a few
weeks then differentiates into cork
cells (i.e. cork)
– cork cambium + cork = periderm
tissue
• also known as bark
– every few years a new CC must form
–so several layers of cork and bark build up
over the years
– the cork and bark are impermeable
to water – problem!
– specific cork cells round up as they
mature and form lenticels
secondary
phloem
LE 35-10
Primary growth in stems
Shoot apical
meristems
(in buds)
Epidermis
Cortex
Primary phloem
Primary xylem
Vascular
cambium
Cork
cambium
Lateral
meristems
Pith
Secondary growth in stems
Periderm
Cork
cambium
Pith
Cortex
Primary
phloem
Primary
xylem
Root apical
meristems
Secondary
xylem
Secondary
phloem
Vascular cambium
SO what the heck is bark???
– non-technical term
– all tissues outside of the
vascular cambium
– includes the:
• secondary phloem
• cork cambium
• cork (phellum)
– may also be called periderm
• outer covering of non-woody stems
and small woody stems
non woody plant?
small woody plant?
PERIDERM
big, honkin’ tree??
BARK
Secondary growth: Wood
• most commercial dicot woods contain a large
amount of vessel elements and fibers – strong
and tough (hardwoods)
• wood from conifers – fewer vessel elements
and fibers and have a softer consistency –
softwoods
• hardwoods:
–
–
–
–
maple
oak
walnut
birch
• softwoods:
–
–
–
–
–
pine
cedar
fir
spruce
redwood
Secondary growth: Wood
• vascular cambium becomes quiescent
during times of stress (extreme hot
and cold) and stops forming new cells
• when the VC resumes its growth and
starts to form new secondary X and P –
forms new rings of secondary xylem
or wood
– first wood that forms – early wood or
spring wood
– later in the season – late wood or
summer wood
• late wood + early wood = annual ring
• early years of a tree are characterized by vigorous
growth – innermost annual rings are wider – more
secondary xylem
Secondary growth: Wood
• oldest regions of the wood found at
the center = heartwood (darker)
– as the tree grows older the components of
the xylem in the inner rings stop
conducting water
– gradually converted to inactive wood and
then heartwood (dead?)
• youngest region of the tree is found at
the periphery= sapwood
– most actively conducting wood
Some websites to check out
• http://www.slideshare.net/apurvanagvenker/
anatomy-of-dicot-monocot-leaf
• http://waynesword.palomar.edu/index.htm
• https://www.youtube.com/watch?v=33H93Rlz
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