Chapter 35:
Plant Structure, Growth &
Development
1. Vascular Plant Structure
2. Vascular Plant Growth
3. Vascular Plant Development
1. Vascular Plant Structure
“Roots & Shoots”
Reproductive shoot (flower)
Apical bud
SHOOT System
Node
(above ground)
Internode
Apical bud
Shoot
system
Vegetative
shoot
Leaf
• stems
• leaves
• flowers, fruits
Blade
Petiole
Axillary bud
Stem
ROOT System
Taproot
(below ground)
Lateral
(branch)
roots
Root
system
• taproot (if present)
• lateral roots
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
Plant organs evolved to obtain nutrients, water
and energy on land – below & above ground
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
Root Function
Roots supply the plant
with:
CO2
• anchorage in the soil
• water
• mineral nutrients
• carbohydrate storage
• roots rely on shoot
system for carbohydrates
• roots also need access
to O2
*over-watering can suffocate a plant!
Minerals
H 2O
O2
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:
• increases survival from
grazing animals since
plant can grow back from
remaining roots
• retains topsoil
Evolutionary Adaptations of Roots
In other plants, adventitious roots develop from
unusual sources (stems, leaves) which may provide:
• greater O2 access in watery
environments
• greater structural
support
Prop roots
Buttress roots
Pneumatophores
Stem Structure and Function
Stems support and position the photosynthetic
structures (leaves) and reproductive structures (e.g.,
flowers, cones) to maximize their success.
Reproductive shoot (flower)
Apical bud
Stem structures include:
Node
• points of leaf attachment called
nodes
• internodes – the stems
between each node
Internode
Apical bud
Leaf
Vegetative
Shoot
shoot
system
Blade
Petiole
Axillary bud
• apical buds at the shoot tips
where growth occurs
• axillary buds which give rise to
lateral branches, thorns or
flowers
Stem
Taproot
Lateral
Root
(branch)
system
roots
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
Rhizome
Root
Rhizomes
Stolo
• tubers that serve
as storage “sinks”
for carbohydrates
Tubers
Stolons
Leaf Structure & Function
Leaves are the primary photosynthetic organs.
Leaf structures include:
Simple leaf
• one or more blades
• simple leaves have 1 blade
• compound leaves have
multiple blades called leaflets
Axillary
bud
Petiole
Compound leaf
• a stalk called a petiole that
connects the leaf to a stem
• veins that have a branched
(dicots) or parallel (monocots)
arrangement
Leaflet
Axillary
bud
Petiole
Evolutionary Adaptations of Leaves
Leaves can be modified for a variety of functions:
• reproductive leaves
that detach and give
rise to a new plant
(asexual)
• tendrils which cling
to larger support
structures
Tendrils
Reproductive leaves
• spines to repel
herbivores
• bulbs that store
nutrients
Storage leaves
Stem
Spines
3 Basic Plant Tissue Types
Dermal tissue
• outer, protective covering
of the plant
Vascular tissue
• transports water, nutrients
& gives structural support
each of these tissues forms
a continuous tissue system
throughout the plant
Dermal
tissue
Ground
tissue
Vascular
tissue
Ground tissue
• everything else!
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
Trichomes
In woody plants the epidermis develops into a
protective laver called periderm (part of the bark).
300 μm
•
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
Plant cells fall into one of 5 general types:
• Parenchyma
• Collenchyma
• Sclerenchyma
• Water-conducting cells of xylem
• Water-conducting cells of xylem
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
Parenchyma cells in a privet
(Ligustrum) leaf (LM)
25 μm
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
• 2 types of
sclerenchyma
cells:
•
5 μm
Sclereid cells in pear (LM)
sclereid cells
with very thick
2o cell walls
25 μm
Cell wall
•
long and
slender fiber
cells arranged
in threads
Fiber cells (cross section from ash tree) (LM)
Water-Conducting Xylem Cells
Vessel Tracheids 100 μm
2 types of xylem
cells, both of
which are dead at
maturity:
TRACHEIDS
• found in all xylem
vessels
• long, thin with
tapered ends
VESSEL ELEMENTS
• wider, less tapered
• perforated ends
Pits
Tracheids and vessels
(colorized SEM)
Perforation
plate
Vessel
element
Vessel elements, with
perforated end walls
Tracheids
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
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
Apical Meristem
Shoot tip
(shoot apical
meristem and
young leaves)
Axillary bud
meristem
Root apical
meristems
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…
Lateral Meristem
Secondary
growth in width is due to
Primary growth in stems
2 types of lateral meristem:
Epidermis
Cortex adds new
• vascular cambium which
Primary phloem
layers of phloem & xylem
xylem
• cork cambium which Primary
replaces
the
Pith
epidermis with protective periderm
Vascular
cambium
Cork cambium
Lateral
Secondary growth in stems
meristems
Cork cambium
Periderm
Pith
Primary
xylem
Secondary
xylem
Cortex
Primary
phloem
Secondary
phloem
Vascular
cambium
Primary Growth of Roots
Root tips have a protective, non-dividing root cap.
Cortex
Vascular cylinder
Epidermis
Root hair
Dermal
Ground
Vascular
Just underneath the
root cap is the
Zone of Cell Division
which contains the
apical meristem cells.
Zone of
differentiation
Beyond the Zone of
Cell Division are 2
zones in successive
developmental stages:
Zone of
elongation
Zone of cell
division
(including
apical
meristem)
Root cap
Mitotic
cells
Zone of Elongation
•
pushes root into soil
Zone of Differentiation
100 μm
•
cells adopt specific fates
Epidermis
Cortex
Endodermis
Vascular cylinder
Pericycle
Core of
parenchyma
cells
100 μm
(a) Root with xylem and phloem in
the center (typical of eudicots)
Xylem
Phloem
Endodermis
Pericycle
Xylem
Phloem
70 μm
Dermal
Ground
Vascular
Eudicot
Roots
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.
Monocot
Roots
Epidermis
Cortex
Endodermis
Vascular cylinder
Pericycle
In most monocot
roots, there is a
core of parenchyma
cells surrounded by
a ring of alternating
phloem and xylem
vessels.
Core of
parenchyma
cells
Xylem
Phloem
100 μm
(b) Root with parenchyma in the
center (typical of monocots)
Dermal
Ground
Vascular
Lateral Root Growth
Emerging
lateral
root
100 μm
Epidermis
Lateral root
Cortex
Vascular
cylinder
1
Pericycle
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
Primary growth of shoot structures occurs from:
• apical meristem
which lengthens
the stem and
gives rise to leaf
primordia
Leaf primordia
Young leaf
Shoot apical
meristem
Developing
vascular
strand
• axial meristem
which gives rise
to new branches
from the main
stem
Axillary bud
meristems
0.25 mm
Organization of Eudicot Stems
Phloem
Sclerenchyma
(fiber cells)
Xylem
Ground tissue
connecting
pith to cortex
Pith
Cortex
Epidermis
Vascular
bundle
(a) Cross section of stem with
vascular bundles forming a
ring (typical of eudicots) (LM)
1 mm
Dermal
Ground
Vascular
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
Organization of Monocot Stems
Ground
tissue
In most monocot
stems, the vascular
tissue consists of
bundles of phloem
and xylem scattered
throughout the
ground tissue.
Epidermis
Vascular
bundles
1 mm
Dermal
Ground
Vascular
(b) Cross section of stem with
scattered vascular bundles
(typical of monocots) (LM)
Leaf Structure
Guard
cells
Cuticle
50 μm
Stomatal
pore
Epidermal
cell
Sclerenchyma
fibers
Stoma
(b) Surface view of a spiderwort
(Tradescantia) leaf (LM)
Upper
epidermis
Palisade
mesophyll
100 μm
Spongy
mesophyll
Lower
epidermis
Bundlesheath
cell
Cuticle
Vein
Xylem
Phloem
(a) Cutaway drawing of leaf tissues
Guard
cells
Dermal
Ground
Vascular
Vein
Air spaces
Guard cells
(c) Cross section of a lilac
(Syringa) leaf (LM)
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
(a) Primary and secondary growth
in a two-year-old woody stem
Pith
Primary xylem
Vascular cambium
Primary phloem
Epidermis
Cortex
Cortex
Secondary
(2o) Growth
of Stems &
Roots
Epidermis
Primary
phloem
Vascular
cambium
Vascular ray
Primary
xylem
Secondary xylem
Pith
Secondary phloem
First cork cambium
Cork
Most recent
cork cambium
Periderm
(mainly
cork
cambia
and cork)
Secondary phloem
Vascular cambium
Bark
Cork
Cork
cambium
Late wood
Early wood
Layers of
periderm
Cork
1 mm
Secondary
phloem
Secondary
xylem
Bark
Vascular ray
Secondary
xylem
1.4 mm
Growth ring
(b) Cross section of a three-yearold Tilia (linden) stem (LM)
Periderm
All gymnosperms and most eudicots undergo
growth in diameter or width – 2o growth.
• most monocots undergo primary growth only
VASCULAR 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
CORK CAMBIUM
•
produces cork cells periderm in place of the original
epidermis to produce a protective outer layer
More on Vascular Cambium…
Vascular
cambium
Growth
X X C P P
X X C P
Secondary
xylem
Vascular
cambium
Secondary
phloem
X C P
X C
C
After one year
of growth
After two years
of growth
2o phloem and xylem cells form adjacent to
the vascular cambium cells, pushing earlier
layers further away from the vascular
cambium.
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
Ring-width
indexes
2
1.5
1
0.5
0
1600
1700
1800
Year
1900
2000
More on Woody Stems…
• older xylem that no longer transports fluid = hardwood
• newer, active xylem = sapwood
• 2o phloem + periderm = bark
Growth
ring
Vascular
ray
Heartwood
Secondary
xylem
Sapwood
Vascular cambium
Secondary phloem
Bark
Layers of periderm
3. Vascular Plant Development
Arabidopsis – A model Plant
Much of what we know about plant
development comes from studying
a tiny weed – Arabidopsis thaliana.
Arabidopsis has several advantages
that make it very practical to use as
a model plant organism:
•
small size
•
grows fast
•
small genome size
•
easy to genetically modify
Genetic Modification of Arabidopsis
Agrobacterium tumefaciens, a plant pathogen, is the key:
Agrobacterium tumefaciens
•
•
contains the
Ti plasmid with a
“T DNA” region
that is integrated
randomly into the
host plant cell
genome
1
DNA of interest
can be “cloned”
into the T DNA
region and then
introduced into
the host plant
genome
2
3
Ti
plasmid
Site where
restriction
enzyme cuts
T DNA
DNA with
the gene
of interest
Recombinant
Ti plasmid
Plant with
new trait
Asymmetrical Cell Division
& Cell Fate in Plants
Asymmetrical
cell division
Unspecialized
epidermal cell
Guard cell
“mother cell”
Developing
guard cells
Asymmetrical
or “uneven”
cell division
has been
shown to
precede the
adoption of
distinct cell
fates in plants
as shown in
this example.
An Arabidopsis mutant
called gnom demonstrates
the importance of
asymmetric cell division in
early plant development:
normal
•
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
gnom
mutant
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
Ca
St
Pe
Se
Pe
Pe
Se
Pe
Se
Normal Arabidopsis flower
Mutant
Arabidopsis flower
The ABC Hypothesis of Flowering
(a) A schematic diagram of the
ABC hypothesis
Sepals
Petals
A
Stamens
Carpels
B
C
C gene
activity
B+C
gene
activity
Carpel
Petal
A+B
gene
activity
Stamen
A gene
activity
Sepal
Active
BB
BB
genes: A A C C C C A A
Whorls:
BB
BB
C CC CCC C C
A A C CC C A A
AA
AA
AB BA AB BA
Mutant lacking A
Mutant lacking B
Mutant lacking C
Stamen Carpel
Petal
Sepal
Wild type
(b) Side view of flowers with organ identity mutations