1. vascular plant structure - los angeles mission college...plant organs evolved to obtain...
TRANSCRIPT
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1
Chapter 35:
Plant Structure, Growth &
Development
2. Vascular Plant Growth
1. Vascular Plant Structure
3. Vascular Plant Development
1. Vascular Plant Structure
Lateral
(branch)
roots
Taproot
Stem
Axillary bud
LeafBlade
Petiole
Vegetative
shoot
Apical bud
Internode
Node
Apical bud
Reproductive shoot (flower)
Shoot
system
Root
system
“Roots & Shoots”
• stems
• leaves
• flowers, fruits
• taproot (if present)
• lateral roots
SHOOT System(above ground)
ROOT System(below ground)
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Overall Organization of
Vascular Plants
Plants have a hierarchical organization
consisting of organs, tissues, and cells:
ORGANS
• distinct functional structure consisting of
multiple types of tissues
TISSUES
• collection of 1 or more cell types that
performs a specific function within an organ
3 Basic Plant Organs
ROOTS• absorb water, minerals and other nutrients from
the soil
• anchor & support plant in the ground
STEMS• structural support of plant above ground
• transport of water & nutrients throughout the
plant
LEAVES• harvesting light & CO2 for photosynthesis
Plant organs evolved to obtain nutrients, water
and energy on land – below & above ground
CO2
O2
H2O
Minerals
Root Function
• anchorage in the soil
Roots supply the plant
with:
• water
• mineral nutrients
• carbohydrate storage
• roots rely on shoot
system for carbohydrates
*over-watering can suffocate a plant!
• roots also need access
to O2
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Root Structures
The first root to emerge during plant development,
the primary root, will then give rise to:
• lateral roots to increase absorption and anchorage
• tiny root hairs to maximize
surface area for absorption
In may plants, the primary
root develops into a
prominent tap root which
provides:
• support for a large,
vertical (tall) shoot system
• storage for carbohydrates
Fibrous Root Structures
In some plants, usually monocots, the primary
root disappears and a fibrous root system
forms which:
• retains topsoil
• increases survival from
grazing animals since
plant can grow back from
remaining roots
Prop roots Buttress roots Pneumatophores
In other plants, adventitious roots develop from
unusual sources (stems, leaves) which may provide:
• greater structural
support
• greater O2 access in watery
environments
Evolutionary Adaptations of Roots
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Stems support and position the photosynthetic
structures (leaves) and reproductive structures (e.g.,
flowers, cones) to maximize their success.
• points of leaf attachment called
nodes
Stem Structure and Function
Stem structures include:
• internodes – the stems
between each node
• apical buds at the shoot tips
where growth occurs
• axillary buds which give rise to
lateral branches, thorns or
flowers
Lateral
(branch)
roots
Taproot
Stem
Axillary bud
LeafBlade
Petiole
Vegetative
shoot
Apical bud
Internode
Node
Apical bud
Reproductive shoot (flower)
Shoot
system
Root
system
Evolutionary Adaptations of Stems
Stems can be modified to serve a variety of
functions:
• rhizomes which grow just beneath
the soil surface and give rise to
vertical shoots from axillary buds
• stolons that function as “runners”
along the soil surface giving rise to
new plantlets
• tubers that serve
as storage “sinks”
for carbohydrates
Root
Rhizome
Rhizomes
Stolons
Stolon
Tubers
Leaf Structure & Function
Leaves are the primary photosynthetic organs.
PetioleAxillary
bud
Leaflet
Compound leaf
PetioleAxillary
bud
Simple leaf
Leaf structures include:
• one or more blades
• a stalk called a petiole that
connects the leaf to a stem
• simple leaves have 1 blade
• compound leaves have
multiple blades called leaflets
• veins that have a branched
(dicots) or parallel (monocots)
arrangement
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Storage leaves
Stem
Evolutionary Adaptations of Leaves
Leaves can be modified for a variety of functions:
• tendrils which cling
to larger support
structures
• spines to repel
herbivores
• bulbs that store
nutrients
• reproductive leaves
that detach and give
rise to a new plant
(asexual)
Spines
Tendrils
Reproductive leaves
3 Basic Plant Tissue TypesDermal tissue
• outer, protective covering
of the plant
Vascular tissue
• transports water, nutrients
& gives structural support
Ground tissue
• everything else!
Dermal
tissueGround
tissue Vascular
tissue
each of these tissues forms
a continuous tissue system
throughout the plant
There is also a type of
undifferentiated tissue called
meristem which we will
address later in this chapter.
More on Dermal Tissue…
In nonwoody plants and structures (e.g., leaves) the
dermal tissue is epidermis.
• epidermis is frequently covered with a waxy cuticle to
minimize water loss
• some plants also have trichomes in epidermal tissue
which provide protection from water loss, intense light
and insects
In woody plants the epidermis develops into a
protective laver called periderm (part of the bark).
Trichomes
30
0 μ
m
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More on Vascular Tissue…
Plant vascular tissue consists of phloem & xylem.
Xylem
• transports water & minerals upward from the root
system to the organs and tissues of the shoot system
Phloem
• transports photosynthetic
products (e.g., sugars)
downward to the roots and
other parts of the plant
Phloem & xylem are organized
into vascular bundles or
cylinders called steles.
More on Ground Tissue…
Tissues that are not dermal or vascular are ground
tissue which come in 2 general types.
Pith
• ground tissue found
internal to the
vascular tissue
Cortex
• ground tissue found
between the dermal
and vascular tissue
Ground tissues include cells involved in storage,
transport, structural support and photosynthesis.
Basic Plant Cell Types
• Parenchyma
• Collenchyma
• Sclerenchyma
• Water-conducting cells of xylem
• Water-conducting cells of xylem
Plant cells fall into one of 5 general types:
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Parenchyma cells in a privet(Ligustrum) leaf (LM)
25 μm
Parenchyma Cells
• have thin primary (1o) cell walls without a
secondary (2o) cell wall
• the least differentiated plant cell type
• the most
metabolically active
plant cell type
• are capable of
undergoing cell
division and further
differentiation
Collenchyma Cells
• provide flexible support in newly formed
shoot structures without restraining
growth
• flexible 1o cell
walls with
irregular
2o wall
thickening
Collenchyma cells
(in Helianthus stem) (LM) 5 μm
Sclerenchyma Cells
• provide rigid support due to thick 2o cell walls
containing lignin that are dead at maturity
5 μm
Sclereid cells in pear (LM)
Cell wall
Fiber cells (cross section from ash tree) (LM)
25 μm
• 2 types of
sclerenchyma
cells:
• sclereid cells
with very thick
2o cell walls
• long and
slender fiber
cells arranged
in threads
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Water-Conducting Xylem Cells
2 types of xylem
cells, both of
which are dead at
maturity:
100 μm
Tracheids and vessels
(colorized SEM)
TracheidsVessel
Perforation
plate
Vessel
element
Vessel elements, with
perforated end wallsTracheids
Pits
TRACHEIDS
• found in all xylem
vessels
• long, thin with
tapered ends
VESSEL ELEMENTS
• wider, less tapered
• perforated ends
Sugar-Conducting Phloem Cells
2 types of phloem cells, both of which are
alive at maturity:
SIEVE CELLS
• found in seedless
vascular plants &
gymnosperms
SIEVE-TUBE ELEMENTS
• cells that form sieve
tubes in angiosperms
• have sieve plates
between elements &
supporting
companion cells
2. Vascular Plant Growth
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Meristem Tissue
Unlike animals, plants are capable of indeterminate
growth – growth throughout the life of the plant.
This unlimited growth potential is due to meristem
tissue – a special, undifferentiated tissue with
unlimited replicative potential.
• in contrast, animals and some plant structures (e.g.,
flowers, thorns) exhibit determinate growth in which
they stop growing when they reach a certain size
There are 2 types of meristems:
• APICAL MERISTEM • LATERAL MERISTEM
Root apical
meristems
Axillary bud
meristem
Shoot tip
(shoot apical
meristem and
young leaves)
Apical Meristem
Apical meristem is located at
the tips of roots and shoots
and is responsible for growth
in length – what is called
primary growth.
• in non-woody (herbaceous)
plants, most if not all growth is
due to apical meristem
• in woody plants (e.g., trees),
there is also growth in width,
what is referred to as
secondary growth…
Vascular
cambium
Cork cambium
Lateral
meristems
Primary
xylem
Secondary
xylem
Pith
Periderm
Vascular
cambium
Secondary
phloem
Primary
phloem
Cortex
Cork cambium
Secondary growth in stems
Pith
Primary xylem
Primary phloem
Cortex
Epidermis
Primary growth in stems
Lateral MeristemSecondary growth in width is due to
2 types of lateral meristem:
• vascular cambium which adds new
layers of phloem & xylem
• cork cambium which replaces the
epidermis with protective periderm
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Primary Growth of Roots
100 μm
Mitotic
cells
Root cap
Zone of cell
division
(including
apical
meristem)
Zone of
elongation
Zone of
differentiation
Dermal
Ground
Vascular
Vascular cylinderCortex
Epidermis
Root hair
Root tips have a protective, non-dividing root cap.
Just underneath the
root cap is the
Zone of Cell Division
which contains the
apical meristem cells.
Beyond the Zone of
Cell Division are 2
zones in successive
developmental stages:
Zone of Elongation• pushes root into soil
Zone of Differentiation• cells adopt specific fates
100 μm
(a) Root with xylem and phloem in
the center (typical of eudicots)
Xylem
Phloem
Ground
Vascular
Dermal
Pericycle
Core ofparenchymacells
Vascular cylinder
Endodermis
Cortex
Epidermis
Endodermis
Pericycle
Xylem
Phloem
70 μm
In most eudicot
roots, there is a
central vascular
cylinder (stele)
with a “X-shaped”
arrangement of
xylem as seen in
cross section with
phloem filling in
between the
“arms” of the X.
Eudicot
Roots
100 μm
Xylem
Phloem
Ground
Vascular
Dermal
Pericycle
Core of
parenchyma
cells
Vascular cylinder
Endodermis
Cortex
Epidermis
(b) Root with parenchyma in the
center (typical of monocots)
In most monocot
roots, there is a
core of parenchyma
cells surrounded by
a ring of alternating
phloem and xylem
vessels.
Monocot
Roots
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Lateral Root Growth100 μm Epidermis
Lateral root
Emerginglateralroot
Cortex
Vascularcylinder Pericycle
1 2 3
Lateral root growth occurs from the meristematic
pericycle, the outermost layer of cells in the
vascular cylinder just inside the endodermis, the
innermost layer of cortex.
Primary Growth of Shoots
Leaf primordia
Young leaf
Shoot apical
meristem
Developing
vascular
strand
Axillary bud
meristems
0.25 mm
Primary growth of shoot structures occurs from:
• apical meristem
which lengthens
the stem and
gives rise to leaf
primordia
• axial meristem
which gives rise
to new branches
from the main
stem
Organization of Eudicot Stems
1 mm
Vascular
Ground
Dermal(a) Cross section of stem with
vascular bundles forming a
ring (typical of eudicots) (LM)
Cortex
Pith
Vascular
bundle
Epidermis
Xylem
Phloem
Sclerenchyma
(fiber cells) Ground tissueconnectingpith to cortex
In most eudicot
stems, the vascular
tissue consists of
bundles of phloem
and xylem arranged
in a ring around the
central pith tissue.
• the xylem is always
located inside the
phloem adjacent to
the pith
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1 mm
Vascular
Ground
Dermal(b) Cross section of stem with
scattered vascular bundles
(typical of monocots) (LM)
Epidermis
Vascular
bundles
Ground
tissue
Organization of Monocot Stems
In most monocot
stems, the vascular
tissue consists of
bundles of phloem
and xylem scattered
throughout the
ground tissue.
Leaf Structure
50 μ
m100 μ
m
Guard
cells
VeinCuticle
Dermal
Ground
Vascular
Lower
epidermis
Spongy
mesophyll
Palisade
mesophyll
Upper
epidermis
Phloem
Xylem
Bundle-
sheath
cell
(a) Cutaway drawing of leaf tissues
Stoma
Sclerenchyma
fibersCuticle
Guard
cells
Epidermal
cell
Stomatal
pore
(b) Surface view of a spiderwort
(Tradescantia) leaf (LM)
(c) Cross section of a lilac
(Syringa) leaf (LM)
Vein Air spaces Guard cells
Epidermis
• outer cell layer on both sides of leaf
• secrete waxy cuticle to waterproof the leaf
Mesophyll (ground tissue of leaf)
• loosely packed photosynthetic parenchyma cells
• palisade or spongy arrangement
Vascular Bundles
• phloem & xylem
• surrounded by bundle sheath cells
Stomata (singular = “stoma”)
• openings for gas exchange, transpiration
• regulated by guard cells
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Epidermis
Pith
Primary xylem
Vascular cambium
Primary phloem
Cortex
Pith
Primary
xylem
Vascular
cambium
Primary
phloem
Cortex
Epidermis
(a) Primary and secondary growth
in a two-year-old woody stem
Periderm
(mainly
corkcambia
and cork)
Secondary
phloem
Secondary
xylem
Vascular ray
Secondary xylem
Secondary phloem
First cork cambium
Cork
Cork
Most recent
cork cambium
Bark
Layers of
periderm
Secondary phloem
Vascular cambium Bark
Cork
cambium
Cork
PeridermLate wood
Early wood
Secondary
xylem
Growth ringVascular ray
1.4 mm1 m
m
(b) Cross section of a three-year-
old Tilia (linden) stem (LM)
Secondary
(2o) Growth
of Stems &
Roots
All gymnosperms and most eudicots undergo
growth in diameter or width – 2o growth.
• most monocots undergo primary growth only
VASCULAR CAMBIUM
CORK CAMBIUM
• a single-celled ring of meristem between primary xylem
and phloem
• produces new (secondary) xylem toward the inside and
new (secondary) phloem toward the outside
• produces cork cells periderm in place of the original
epidermis to produce a protective outer layer
More on Vascular Cambium…
C
After one yearof growth
After two yearsof growth
Vascularcambium Growth
Secondaryxylem
Secondaryphloem
Vascularcambium
CX
CX P
CX PX
CX PX P
2o phloem and xylem cells form adjacent to
the vascular cambium cells, pushing earlier
layers further away from the vascular
cambium.
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Year
1600 1700 1800 1900 2000
Rin
g-w
idth
ind
exes
0
0.5
1
1.5
2
Growth Rings Reveal Past Climates
In woody stems, spring 2o xylem (spring wood)
differs from summer 2o xylem (summer wood),
giving the appearance of annual growth rings.
• warm & wet = wider ring • cold & dry = narrower ring
More on Woody Stems…
Growth
ring
Vascular
ray
Heartwood
Sapwood
Secondary
xylem
Secondary phloem
Vascular cambium
Layers of periderm
Bark
• older xylem that no longer transports fluid = hardwood
• newer, active xylem = sapwood
• 2o phloem + periderm = bark
3. Vascular Plant Development
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Arabidopsis – A model Plant
Much of what we know about plant
development comes from studying
a tiny weed – Arabidopsis thaliana.
• small size
• grows fast
• small genome size
Arabidopsis has several advantages
that make it very practical to use as
a model plant organism:
• easy to genetically modify
Genetic Modification of Arabidopsis
Plant with
new trait
Agrobacterium tumefaciens
Recombinant
Ti plasmid
DNA with
the gene
of interest
Site where
restriction
enzyme cuts
Ti
plasmid
1
2
3
T DNA
Agrobacterium tumefaciens, a plant pathogen, is the key:
• contains the
Ti plasmid with a
“T DNA” region
that is integrated
randomly into the
host plant cell
genome
• DNA of interest
can be “cloned”
into the T DNA
region and then
introduced into
the host plant
genome
Asymmetrical Cell Division
& Cell Fate in Plants
Unspecialized
epidermal cell
Developing
guard cells
Asymmetrical
cell division
Guard cell
“mother cell”
Asymmetrical
or “uneven”
cell division
has been
shown to
precede the
adoption of
distinct cell
fates in plants
as shown in
this example.
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An Arabidopsis mutant
called gnom demonstrates
the importance of
asymmetric cell division in
early plant development:
normal
gnom
mutant
• the 1st division of normal plant
zygotes is asymmetric and
determines the polarity of the
plant (i.e., root vs shoot
systems)
• the 1st division of gnom mutant
zygotes is symmetrical and the
embryo develops without any
polarity – no roots or shoots
Pe
Se
Normal Arabidopsis flower
StCa
MutantArabidopsis flower
Pe
Pe
Pe
Se
Se
Genetic Control of Flowering
Environmental cues such as day
length and temperature trigger flower
development in plants such as
Arabidopsis.
Mutants such as the one shown here
have led to the ABC hypothesis of
flower development:
• the inner whorls of
the mutant flower
develop into petals
and sepals instead
of stamens and a
carpel
The ABC Hypothesis of Flowering
(b) Side view of flowers with organ identity mutations
Wild type Mutant lacking A Mutant lacking B Mutant lacking C
Sepal
Petal
CarpelStamen
Whorls:
Activegenes: A A A AC C C C
B B B BC C C CC C C C
B B B BA A A AC CC C A B B AB A A B
A A A A
(a) A schematic diagram of the
ABC hypothesis
Sepals
Petals
Stamens
CarpelsAB
C
Sepal
Stamen
Petal
CarpelC geneactivity
A geneactivity
B + Cgene
activityA + Bgene
activity