Download Tissue systems

Lecture #4 – Plant Structure,
Growth And Development
Image – the Angel Oak
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Key Concepts:
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What is a kingdom?
Why study plants?
What makes a plant a plant?
The hierarchy of structure – plant cells,
tissues and organs
Growth
Primary growth – elongation
Secondary growth – diameter expansion
Morphogenesis occurs during growth
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Carolus
Linnaeus
(1707-1778)
Image – Linnaeus
The founder of
modern taxonomy
defined kingdoms
by morphological
similarity
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Linnaeus’ Taxonomic Hierarchy
Taxonomic Category
Example (taxon)
Kingdom
Plantae, also Metaphyta = all plants
Division (phylum)
Magnoliophyta = all angiosperms
Class
Liliopsida = all monocots
Order
Asparagales = related families (Orchidaceae,
Iridaceae, etc)
Family
Orchidaceae = related genera (Platanthera,
Spiranthes, etc)
Genus
Platanthera = related species (P. ciliaris, P. integra,
etc)
Specific name/epithet
ciliaris = one species
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Linnaeus’ Taxonomic Hierarchy
Taxonomic Category
Example (taxon)
Kingdom
Plantae, also Metaphyta = all plants
Division (phylum)
Magnoliophyta = all angiosperms
Class
Liliopsida = all monocots
Order
Asparagales = related families (Orchidaceae,
Iridaceae, etc)
Family
Orchidaceae = related genera (Platanthera,
Spiranthes, etc)
Genus
Platanthera = related species (P. ciliaris, P. integra,
etc)
Specific name/epithet
ciliaris = one species
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Images – the yellow fringed orchid
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Platanthera ciliaris
Linnaeus recognized only 2 kingdoms
• If it moved – animal; if it didn’t – plant
• Fungi were lumped with plants
• The microscopic world was largely unknown
Images – the 3 multicellular kingdoms, animals, fungi and plants
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The 5 kingdom system – developed in the
1960’s and used until recently
Diagram – the 5 kingdom system
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Molecular data supports 3 domain
classification scheme
Diagram – 3 domain system of classification
Kingdoms are defined by monophyletic lineage
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Classification is Dynamic!
Diagram – transition from 5 kingdom to 3 domain
system indicating dynamic nature of classification
Multicellular eukaryotes remain fairly well defined –
the plants, fungi and animals. Classification of single
celled organisms is still underway.
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Current Taxonomic Hierarchy
Taxonomic Category
Example (taxon)
Domain
Eukarya = all eukaryotic organisms
Kingdom
Plantae, also Metaphyta = all plants
Division (phylum)
Magnoliophyta = all angiosperms
Class
Liliopsida = all monocots
Order
Asparagales = related families (Orchidaceae,
Iridaceae, etc)
Family
Orchidaceae = related genera (Platanthera,
Spiranthes, etc)
Genus
Platanthera = related species (P. ciliaris, P. integra,
etc)
Specific name/epithet
ciliaris = one species
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Why Plants?
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Why Plants?
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Food
Pharmaceuticals
Building materials
Furniture
Paper
Chemicals
Horticulture/Floriculture
etc…..
Image – shooting stars
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What makes a plant a plant???
• multicellular, eukaryotic organisms with extensive specialization
• almost all are photosynthetic, with chloroplasts (= green)
– some obtain additional nutrition through parasitism or carnivory
– some are saprophytic, entirely without chlorophyll (eat dead OM)
• excess carbohydrates stored as starch (coiled, branched polymer of
glucose)
• cell walls of cellulose = fibrous (not branched) polysaccharide =
accounts for the relative rigidity of the cell wall
• cell division by formation of cell plate
• most extant plant species are terrestrial (many characteristics that are
adapted for terrestrial life)
• separated from cyanobacteria by chloroplasts
• separated from green algae by various adaptations to terrestrial life
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Images and diagrams – characteristics that separate
plants from other kingdoms
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What makes a plant a plant???
• Multicellular, eukaryotic organisms with extensive
specialization
• Almost all are photosynthetic, with chloroplasts (= green)
 Some obtain additional nutrition through parasitism or carnivory
 Some are saprophytic, entirely without chlorophyll (absorb dead
OM)
• Excess carbohydrates stored as starch (coiled, branched
polymer of glucose)
• Cell walls of cellulose = fibrous (not branched)
polysaccharide = accounts for the relative rigidity of the cell
wall
• Cell division by formation of cell plate
• Most extant plant species are terrestrial (many
characteristics that are adapted for terrestrial life)
• Separated from cyanobacteria by chloroplasts
• Separated from green algae by various adaptations to 16
terrestrial life
Read this later….
Plants were the first organisms to
move onto land
• Occurred about 475mya
• Very different conditions from former
marine habitat
• Many new traits emerged in adaptation to
life on dry land
• Extensive adaptive radiation into many
new ecological niches
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Four major
groups of plants
have emerged
since plants took
to land
Diagram – phylogeny of land plants;
same on next slide
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We will focus
on
angiosperms
Next semester in
211 you will learn
more about the
transition from
water to land,
and the evolution
of reproductive
strategies in all
plants
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Angiosperms – the flowering plants:
90% of the Earth’s modern flora
Images – flowering plants
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Basic Structure of the Plant Cell –
what’s unique???
Diagram – plant cell; same on next slide
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Basic Structure of the Plant Cell
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Critical Thinking
• Do all plant cells have chloroplasts???
• How can you tell???
23
Critical Thinking
• Do all plant cells have chloroplasts???
• NO!!!
• How can you tell???
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Critical Thinking
• Do all plant cells have
chloroplasts???
• NO!!!
• How can you tell???
• Chlorophyll reflects
green light
Image – chloroplast free white
bracts on white-top sedge
Green tissues have
chloroplasts
Non-green tissues
don’t
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More on the cell wall:
• All cell walls are
produced by the cell
membrane, outside
• Primary wall is
produced first
Diagram – primary and secondary
cell walls; same on next slide
 Mostly cellulose
• Secondary walls are
produced later
 Lignified, so ???
• Secondary walls are
interior to primary
walls
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More on the cell wall:
• All cell walls are
produced by the cell
membrane
• Primary wall is
produced first
 Mostly cellulose
• Secondary walls are
produced later
 Lignified, so rigid!
• Secondary walls are
interior to primary
walls
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Five Major
Plant Cell
Types
Micrographs – plant cell types
• Parenchyma
• Collenchyma
• Sclerenchyma
• Xylem elements
• Phloem elements
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Parenchyma
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Thin primary wall
No secondary wall
Many metabolic and storage functions
Bulk of the plant body
Micrographs – parenchyma cells
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Collenchyma
• Thick primary
wall
• No secondary
wall
Micrograph – collenchyma cells;
same on next slide
Implications???
• Support growing
tissues
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Collenchyma
• Thick primary
wall
• No secondary
wall
Extensible – no
lignin means they
can elongate
• Support growing
tissues
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Sclerenchyma
• Thick secondary wall
• Secondary walls are
lignified
Micrograph – sclerenchma
cells; same on next slide
Implications???
• Support mature plant
parts
• Often dead at maturity
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Sclerenchyma
• Thick secondary wall
• Secondary walls are
lignified
Lignified cells are rigid
and fixed in size
• Support mature plant
parts
• Often dead at maturity
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Collenchyma vs. Sclerenchyma
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Both provide structural support
Both have thick walls
Collenchyma = thick primary wall, no lignin
Sclerenchyma = thick secondary wall, lignified
Micrographs – collenchyma and sclerenchyma cell comparison
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Xylem Elements
• Lignified secondary
walls
• Always dead at
maturity (open)
• Function to transport
water and dissolved
nutrients, and to
support the plant
• Tracheids and vessel
elements
Diagrams and
micrograph – tracheids
and vessel elements
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Critical Thinking
• Vessel elements and
the convergent
evolution of rings
• What else looks like
this????
• What is the
function????
Micrograph – rings of lignin in
developing vessel element; same
on next slide
36
Critical Thinking
• Vessel elements and
the convergent
evolution of rings
• What else looks like
this????
• What is the
function????
• Stiff rings hold the
“tube” open
 Trachea in both
vertebrates and inverts
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Phloem Elements
• Sieve tube members +
companion cells
• STM lack nucleus,
ribosomes – their
metabolism is controlled
by the companion cells
Micrograph – phloem elements
• Function to transport the
products of metabolism
• Non-angiosperms have
more primitive phloem
elements
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Critical Thinking
• What might be the functional advantage of
a cell with no nucleus???
Diagram – phloem elements
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Critical Thinking
• What might be the functional advantage of
a cell with no nucleus???
• Sieve plates are very open
• Plus, function is to move large volumes of
sap around the plant
Nucleus and other organelles get in the way
• But, phloem transport requires ATP and
thus a living cell
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Plants are
Simple 
Only Five Major
Cell Types
Micrographs – plant cell types
• Parenchyma
• Collenchyma
• Sclerenchyma
• Xylem elements
• Phloem elements
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Hands On
• Use thin sections and stains to see
different plant cells (Page 13)
• Sections must be VERY thin to allow light
to pass through
• Use toluidine blue to increase contrast
• With a fresh section, use phloroglucinol to
see lignified areas of the tissues
• Follow instructions for staining in manual,
and take notes to answer questions on
handout – label and keep your samples 42
Five Major
Plant Cell
Types
Micrographs – plant cell types
• Parenchyma
• Collenchyma
• Sclerenchyma
• Xylem elements
• Phloem elements
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Tissue
Systems
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Diagram – plant tissue types
Epidermis
Vascular
Ground
Meristem
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Epidermis Tissue:
• Covers the outer surface of all
plant parts
• Shoot surfaces covered with
waxy cuticle
Micrograph
and diagram –
epidermis
Helps to protect the plant and
prevent desiccation
• Usually a single, transparent
cell layer
• Tight joints; stomata allow for
gas exchange
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Critical Thinking
• Do roots have a waxy cuticle???
• Why or why not???
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Critical Thinking
•
•
•
•
Do roots have a waxy cuticle???
No
Why or why not???
Wax is waterproof
Roots absorb water from the soil
A waxy coating would be a functional
DISadvantage
Never forget the importance of
natural selection!!!!!
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Hands On
• Look at your leaf cross sections
• Can you see the epidermis?
• Can you see the waxy cuticle?
Diagram of leaf tissue arrangement
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Vascular Tissue:
• Transports water, solutes,
and metabolic products
throughout the plant
• Confers structural support
• Includes xylem elements,
phloem elements,
parenchyma and
sclerenchyma fibers
Micrograph – vascular
bundle in cross section
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Critical Thinking
• Why does vascular tissue give structural
support to a plant???
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Critical Thinking
• Why does vascular tissue give structural
support to a plant???
• LIGNIN
• Xylem and sclerenchyma fibers are
lignified!
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Hands On
• Look at your cross sections – leaf and
stem
• Can you see the vascular tissues?
Diagram of leaf tissue arrangement
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Ground Tissue:
• Bulk of the plant
body – pith, cortex
and mesophyll
• Mostly parenchyma
• Most metabolic,
structural and
storage functions
Micrograph and diagram – ground
tissues in stems and leaves
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Hands On
• Look at the stem cross sections
• Can you see the ground tissues?
• The potatoes are mostly ground tissue
What characteristics do they share with other
stems?
What differences?
What function???
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Critical Thinking
• Is this what the inside of a tree looks
like???
Micrograph – herbaceous dicot stem
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Critical Thinking
• Is this what the inside of a tree looks
like???
• No – wood is xylem tissue
The bulk of a tree is wood, not ground tissue
Micrograph of herbaceous eudicot stem; image of woody
stem; diagram of woody stem tissue organization
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Meristem Tissue:
• How the plant grows
• Cells divide constantly during the growing
season to make new tissues
• More details later
Image – new growth at tip of stem
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Plants are
Simple 
Only Four Major
Tissue Types
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•
•
•
Diagram – plant tissue systems
Epidermis
Vascular
Ground
Meristem
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Tissues Make Organs:
• Roots – anchor the plant, absorb water and
nutrients
• Stems – support the leaves
• Leaves – main site of photosynthesis
• Reproductive organs (flowers, cones, etc –
more later)
All organs have additional functions –
hormone synthesis, transport, etc…
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Plant Organ Systems
Diagram – root and shoot systems
60
Hands On
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Show ‘n’ Tell
What plant parts did you bring???
Discuss your plants with your team
Focus on visible tissues and organs
Be prepared to demonstrate your findings
to the whole class
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Modern molecular evidence indicates
four classes of angiosperms
paleoherbs
magnoliids
eudicots
monocots
ancestral
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Not all plants have the same tissue organization in their organs
Paleoherbs and Magnoliids comprise
about 3% of angiosperms
Paleoherbs
• Aristolochiaceae,
Nymphaeaceae, etc
Magnoliids
• Magnoliaceae,
Lauraceae, nutmeg,
black pepper, etc
Images – water lily and magnolia
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Modern evidence indicates 4 classes of
angiosperms
paleoherbs
magnoliids
eudicots
monocots
~ 97% of
angiosperms
ancestral
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Monocots include grasses, sedges,
iris, orchids, lilies, palms, etc…..
Images – monocots
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Critical Thinking
• Grasses are arguably the most important
plant family
• Why???
66
Critical Thinking
• Grasses are arguably the most important
plant family
• Why???
• They feed the world
Direct nutrition for most of the world – grains
such as rice, wheat and corn
Indirect nutrition by feeding the animals we
eat
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Eudicots include 70+% of all
angiosperms:
• Most broadleaf trees and shrubs
• Most fruit and vegetable crops
• Most herbaceous flowering plants
Images – eudicots
68
Monocots vs. Eudicots
Monocots
• Flower parts in multiples
of 3
• Parallel leaf venation
• Single cotyledon
• Vascular bundles in a
ring in the roots
• Vascular bundles in
complex arrangement
in the stem
• ~90,000 species
Eudicots
• Flower parts in multiples
of 4 or 5
• Netted leaf venation
• Two cotyledons
• Vascular tissues in a
solid core in the roots
• Vascular bundles in a
ring around the stem
• Modern classification
indicates 2 small primitive
groups + eudicots
• 200,000+ species
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Root System Tissue Organization
Eudicots
Monocots
Micrographs – cross sections of eudicot and moncot
roots; same on next 3 slides
Epidermis, ground, endodermis, pericycle, vascular tissues
70
Eudicot root – closeup
Epidermis
Cortex
Endodermis
Pericycle
Vascular
tissues – in
solid core
71
Monocot root – closeup
Epidermis
Cortex
Endodermis
Pericycle
Vascular tissues – in
ring
Pith in the very center
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Critical Thinking
• Where do branch roots form???
73
Critical Thinking
• Where do branch roots form???
• The pericycle is the meristem tissue
• Roots branch from the inside and push
their way out
Micrograph – root emerging from pericycle
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Stem System Tissue Organization
Eudicots
Monocots
Micrograph – eudicot and monocot stem tissue
organization; same on next 4 slides
Epidermis, ground, vascular tissues
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Eudicot stem – closeup
Epidermis
Cortex
Vascular
tissues –
bundles in
a ring
Pith
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Monocot stem – closeup
Epidermis
Cortex
Vascular
tissues –
bundles are
scattered
77
Wood forms from a meristem that
links the vascular bundles:
78
Stem System Tissue Organization
Eudicots
Monocots
Monocots cannot make wood
More on wood formation later
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Monocots, Palmetto Trees,
Ft. Moultrie and the SC State Flag
Various images and a micrograph of a monocot stem – an
example of one influence of plants on American history
80
Hands On
• Examine the micrographs and discuss with
your team (switch PowerPoints)
• What is the tissue organization in each
slide, and how does that tell you what
plant part is represented?
81
Leaf Tissue Arrangement
Micrograph – cross-section of leaf tissue arrangement
Epidermis, ground, vascular tissues
82
Leaf closeup
Epidermis
Diagram – leaf tissue arrangement
Cortex –
palisade
mesophyll
Cortex – spongy
mesophyll
Vascular tissues
83
Stomata – pores to allow for gas
exchange and transpiration
Micrograph – epidermis tissue
showing stomata
84
Hands On
• Make a cross section of both monocot and
eudicot leaves
• Stain with T-blue
• Position both on the slide for side-by-side
comparison
• Note the similarities and differences in
tissue organization
85
See, plants really are simple 
• 5 cell types
• 4 tissue types
• 4 organ types
Diagram – shoot and root systems
86
Plant Growth
• Remember, most plants are anchored by
roots
• They can’t move to escape or take
advantage of changes in their environment
• Plants adjust to their environment
• Simple structure + lots of developmental
flexibility allow plants to alter when and
how they grow
Developmental flexibility comes from
meristems
87
Meristem Tissues
• Actively dividing cells that generate all
other cells in the plant body
• Cause indeterminate growth
Stems and roots elongate throughout the
plant’s life (indeterminate primary growth)
Trees continually expand in diameter
(indeterminate secondary growth)
Branches form in roots and stems
88
Not all plant parts have
indeterminate growth patterns
Indeterminate:
Roots
and
Stems
Determinate:
Leaves
Flowers
Fruits
These parts grow
throughout the life of
the plant, exploring
new environments or
responding to
damage
These parts grow to a
genetically +/predetermined size
and shape and then
stop – cannot repair
89
damage
Some mature cells can
de-differentiate to become
meristematic once more!!!
• Primarily occurs in the indeterminate parts
Stems and roots
• A process that very seldom occurs in other
kingdoms
• Allows stems and roots to repair damage
and form branches and sprouts
90
Critical Thinking
• Not all stem and root cells can dedifferentiate….
• What would control this???
91
Critical Thinking
• Not all stem and root cells can dedifferentiate…
• What would control this???
• Lignin!!!
Lignin is strong and rigid
Once a cell is lignified, it cannot expand or
divide
92
Growth in Plants:
an irreversible increase in size due to
metabolic processes
(processes that use ATP energy)
• Cell division produces new cells = function
of meristem
• Cell expansion increases the size of the
new cells = up to 80% of size increase
• Cell differentiation occurs during and after
expansion
93
The plane of cell division contributes to morphogenesis
Diagram – planes of cell division and the effect on morphogenesis
94
Division in one plane results in
files of cells
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Division in two planes results in
sheets of cells
96
Division in three planes results
in 3-D masses of cells
97
Critical Thinking
• What tissues are files of cells???
• What tissues are sheets of cells???
• What tissues are 3-D bulky???
98
Critical Thinking
• What tissues are files of cells???
Primary vascular tissues, sclerenchyma fibers
• What tissues are sheets of cells???
Epidermis, secondary vascular tissues
• What tissues are 3-D bulky???
Ground tissues – pith and cortex
99
Hands On
• Use pasta wheels to build all three tissue
types
• Each wheel = one cell
100
Growth in Plants:
an irreversible increase in size due to
metabolic processes
(processes that use ATP energy)
• Cell division produces new cells = function
of meristem
• Cell expansion increases the size of the
new cells = up to 80% of size increase
• Cell differentiation occurs during and after
expansion
101
Auxin-mediated cell expansion
Diagram – how auxin works to promote cell expansion
ATP is used
Use the index to find the figure on the
acid growth hypothesis
102
The direction of cell expansion depends on cellulose
orientation, and contributes to morphogenesis
Diagram – cellulose orientation in primary
wall and the effects on morphogenesis
103
Growth in Plants:
an irreversible increase in size due to
metabolic processes
(processes that use ATP energy)
• Cell division produces new cells = function
of meristem
• Cell expansion increases the size of the
new cells = up to 80% of size increase
• Cell differentiation occurs during and after
expansion
104
Expansion and
differentiation
occur in an
overlapping
zone in all plant
parts
Diagram – patterns of
growth in roots
105
REVIEW: Growth in Plants:
an irreversible increase in size due to
metabolic processes
(processes that use ATP energy)
• Cell division produces new cells = function
of meristem
• Cell expansion increases the size of the
new cells = up to 80% of size increase
• Cell differentiation occurs during and after
expansion
106
Location of the
meristems
determines the
pattern of plant
growth
Diagram – location of
meristems on the plant
body; next slide also
Most common
meristems:
apical, axillary
and lateral
107
Apical
meristems
cause
elongation of
roots and
stems
108
Micrograph – longitudinal section showing distribution of tissues in root
109
Root Cap
• Protects the meristem
• Determines geotropism
• Secretes mucigel
Eases movement of roots through soil
Secretes chemicals that enhance nutrient
uptake
• Constantly shedding cells
Mechanical abrasion as roots grow through
soil
• Constantly being replenished by meristem
110
Images – root cap and mucigel
111
Primary Growth in Roots
Diagram – longitudinal section of root showing
zones of growth; same on next 2 slides
112
Primary Growth in Roots
113
Primary Growth in Roots
114
Root Hairs
• Form as the epidermis
fully differentiates
• Extensions off epidermal
cells
 NOT files of cells
 Part of an epidermal cell
Micrograph – root hairs
extending from epidermis;
same on next few slides
• Hugely increase the
surface area of the
epidermis
• 10 cubic cm (double
handful) of soil might
contain 1 m of plant roots
 Mostly root hairs
115
Critical Thinking
• What is the selective advantage of root
hairs???
116
Critical Thinking
• What is the selective advantage of root
hairs???
• Increased surface area allows for more
absorption of water and nutrients
• Fine diameter allows roots to ramify
throughout the soil environment
117
Root Hairs
• By contrast, 10 cc of soil
may contain up to 1000 m
of fungal hyphae (1km!)
 These serve a similar
function for the fungus
 Ramify throughout the
substrate for maximum
absorption
 Some fungi form symbiotic
associations with plant
roots and both organisms
benefit from this huge
absorptive surface area!
 More in 211…..
118
Apical
meristems
cause
elongation of
roots and
stems
Diagram – location of apical meristems
119
Apical Meristems in Shoots
Micrograph – longitudinal
section of stem showing
apical and axillary meristems
120
Critical Thinking
• There is no “shoot cap” – why not???
121
Critical Thinking
• There is no “shoot cap” – why not???
• No selective advantage! Shoots “push”
through air – essentially no friction
122
Axillary
meristems
allow for
branching –
similar in
structure and
function to
apical
meristems
Diagram – meristem locations
Remember, pericycle in
roots has same function
123
Axillary Meristems in Shoots
Micrograph – longitudinal
section of stem showing
apical and axillary meristems;
same on next two slides
124
Primary Growth in Shoots
• Apical meristem
• Leaf primordia
• Axillary buds
125
As with roots – cell
division occurs first;
zones of expansion
and differentiation
overlap
Axillary buds may
activate to make
branches, or may
remain dormant
126
Primary growth of a shoot – elongation from the tip
Diagram – how stems elongate during primary growth
127
Hands On
• Start some seeds
Dampen a paper towel
Add seeds
Keep lightly covered – why???
• Keep a “journal”
128
Remember:
Diagram – primary vs. secondary growth
Elongation is
primary growth
Diameter
expansion is
secondary growth
129
Lateral
meristems
cause
diameter
expansion
Diagram – meristem locations
Roots also expand
in diameter, but it’s
more complicated –
we’ll save that for
BIOL 300
130
Lateral Meristems = Cambiums
Diagram – lateral meristems
131
Secondary
growth –
diameter
expansion
Images – cross section of wood and whole tree
132
Eudicot Stem – recall the
arrangement of vascular bundles
Micrograph – cross section of a
eudicot stem; same on next 2 slides
133
Eudicot Stem – recall the
arrangement of vascular bundles
Vascular
cambium
forms here:
134
Eudicot Stem – recall the
arrangement of vascular bundles
Vascular
cambium
forms here:
a cylinder of
meristem
tissue between
the xylem to
the interior and
the phloem to
the exterior
135
Secondary xylem and phloem form through
cell division by the vascular cambium
Diagram – location of the vascular
cambium relative to other tree tissues
136
During primary growth
the vascular tissues
form in bundles from
the apical meristem
Diagram – transition from primary
growth to secondary growth;
same on next slide
During secondary
growth the vascular
tissues form in
cylinders from the
vascular cambium
2o xylem to the
inside
2o phloem to the
outside
137
Secondary
xylem
accumulates
138
Secondary Xylem = Wood!
Micrograph – cross section of woody plant
showing secondary tissues; same on next slide
139
Annual growth rings are accumulating
rings of secondary xylem
Vascular cambium divides
essentially in two planes and
remains only a single cell layer
thick
Divisions make 2o xylem and 2o
phloem and also increase the
diameter of the cambium itself
One layer of cambium,
continuously increasing in
diameter
140
Year 1
Year 2
Year 3
Year 4
Wood accumulates with each year’s elongation
Step 1: Primary growth elongates the tip
Step 2: Vascular cambium forms connecting the bundles
Step 3: Secondary growth builds diameter
141
Critical Thinking
• Why do eudicot trees taper???
Diagram –
pattern of
accumulation of
secondary xylem
as a tree grows;
same on next
slide
142
Critical Thinking
•
•
•
•
Why do eudicot trees taper???
Elongation occurs from the tip
Every year adds height to the stem
Each new section of stem has just
one layer of secondary growth
The section below that has +1 layers
The section below that has +2 layers
The section below that has +3 layers
etc, etc, etc
The bottom of the tree has as many
rings as the tree is old
143
Bark
• All tissues external to the vascular
cambium
• Diameter expansion splits original
epidermis
Bark structurally and functionally replaces
epidermis
• Inner bark
Functional secondary phloem
• Outer bark
Composition varies as tree matures
144
Bark Formation
Micrograph – cross section of a tree showing bark formation
145
Cork Cambium
• Meristematic tissue
• Forms in a cylinder during 2o growth
• Divides to produce cork cells
Cells filled with waxy, waterproof suberin
• Eventually cork cambium becomes cork
itself
146
More on cork cambium
• First layer develops from cortex
De-differentiation!!!
• Second layer forms from cortex – same
process
• Third layer forms from cortex…..
• Cortex eventually runs out
• Then what???
147
More on cork cambium
• First layer develops from cortex
De-differentiation!!!
• Second layer forms from cortex – same
process
• Third layer forms from cortex…..
• Cortex eventually runs out
• Then what???
148
More on cork cambium
• First layer develops from cortex
De-differentiation!!!
• Second layer forms from cortex – same
process
• Third layer forms from cortex…..
• Cortex eventually runs out
• Then what???
149
More on cork cambium
• First layer develops from cortex
De-differentiation!!!
• Second layer forms from cortex – same
process
• Third layer forms from cortex…..
• Cortex eventually runs out
• Then what???
150
Critical Thinking
• What is the next available layer of
tissue???
Diagram – lateral meristems and
the secondary tissues in a tree;
same on next slide
151
Critical Thinking
• What is the next available layer of
tissue???
• Secondary phloem!
152
More on cork cambium
• First layer develops from cortex
De-differentiation!!!
• Second layer forms from cortex – same
process
• Third layer forms from cortex…..
• Cortex eventually runs out
• Then what???
• Cork cambium forms from 2o phloem once
all the cortex is used up
153
More on cork cambium
• First layer develops from cortex
De-differentiation!!!
• Second layer forms from cortex – same
process
• Third layer forms from cortex…..
• Cortex eventually runs out
• Then what???
• Cork cambium forms from 2o phloem
154
2o phloem does NOT accumulate like 2o xylem
Stem Tissue Derivations and
Fates:
Diagram – how undifferentiated cells develop into the tissues of the plant body
Cells divide, expand and differentiate
155
Review: Key Concepts:
•
•
•
•
•
•
•
•
What is a kingdom?
Why study plants?
What makes a plant a plant?
The hierarchy of structure – plant cells,
tissues and organs
Growth
Primary growth – elongation
Secondary growth – diameter expansion
Morphogenesis occurs during growth
156
Hands On
• Go downstairs and find a living woody
plant
• Snap off a twig – be gentle!
• Locate bark – peel off and describe
157