Lecture #7
Angiosperms: Form & Function
Plant cells
• very similar to animal cells
• however, they are different in
a couple of ways
• 1. structural cell wall –
principle component is
cellulose
• 2. central vacuole – storage of
water, salts and wastes
• 3. plastids – unique organelles
with a variety of functions
– chloroplast – photosynthesis
– amyloplast - storage
chloroplast
The Cell Wall
• all cells of the plant have cell
walls (except the sperm)
• contains large amounts of
cellulose
• all cells have a thin primary
cell wall
– made of cellulose
– also contains the
polysaccharide called pectin
that allows for plant growth
• some plant cells can have a
secondary cell wall in
between the primary wall
and the plasma membrane
– made with lignin – for
strength
middle lamina – “cement”-like
layer of pectin found between
two plant cells
• cells walls present a problem if plant cells want to communicate
with each other
• evolved connections between cells = plasmodesmata
• several plasmodesmata cluster together to form a pit field
Plant Cells
parenchyma
• three types of plant cells:
•
classified based on the nature of their cell
walls
• 1. parenchyma – only have thin
primary walls
• 2. collenchyma – primary cell walls
thin in some areas, thick in others
• 3. sclerenchyma – primary and
secondary walls
collenchyma
– cells are dead when mature
sclerenchyma
Plant Tissues
• these three types of cells make up plant
tissues
• 3 types of plant tissues found throughout
the plant
• 1. dermal – outer covering
• 2. vascular & xylem and phloem
• 3. ground – “filler” tissue
• plant’s outer protective covering
• usually a single layer of parenchyma cells joined together very
tightly – epidermis
• usually covered with a waxy cuticle to minimize water loss and
protect against pathogens
• BUT cuticle can inhibit the entry of CO2 needed for
photosynthesis = PROBLEM!
• epidermis contains pairs of guard cells that surround pores in the
epidermis
• guard cells + stomatal pore = stoma (plural = stomata)
• guard cells control the size of the pore – control CO2 entry
Dermal
Tissue
Vascular tissues
• vascular tissue: xylem & phloem
– xylem: for the conduction of water &
minerals
• sclerenchyma cells (dead): 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
• living parenchyma cells: sieve tube members
• phloem picks up sugar from where it is
abundant in the plant and transports it to
where it is needed
SUGAR-CONDUCTING CELLS OF THE PHLOEM
Sieve-tube members:
longitudinal view
(LM)
Sieve-tube
member
Sieve
plate
Vascular Bundles
• xylem and phloem
occur together in roots
and stems as vascular
bundles
• 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
Ground Tissue
• tissues that are neither dermal or vascular
– includes cells specialized for storage, photosynthesis,
and support
– made up of all three types of plant 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 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
Stem
• 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
main axis
• stems or shoots consist of:
– a. an alternating system of nodes – where leaves are
attached
– b. internodes – between the nodes
3 yr old
stem
Apical or Terminal bud
Bud scale
Axillary buds
Leaf scar
This year’s growth
(one year old)
Stem
Node
Internode
Last year’s growth
(two years old)
Growth of two
years ago (three
years old)
Terminal bud
Leaf scar
One-year-old
Shoot formed
from axillary bud
near shoot apex
Scars left by terminal
bud scales of previous
winters
• a stem has two kinds of buds: apical/terminal and axillary
• the apical is at the top of the stem or shoot and is responsible for
growth of the stem in length
– can give rise to a flower when the plant wants to reproduce
• the axillary buds are the other buds – can give rise to new shoots
or leaves or flowers along the stem or shoot
• most stem growth is concentrated in the 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: Internal Anatomy
• epidermis – layer of parenchyma cells covered with a cuticle
• cortex – interior to the epidermis
– simple in most stems
– composed of photosynthetic parenchyma and sometimes collenchyma
• pith – most interior portion of the stem
– region of parenchyma found in dicots
– similar to the parenchyma of the cortex
• vascular bundles: xylem and phloem cells running up and
down the stem parallel to each other
– unique arrangement depending on whether the plant is a dicot
or a monocot
– monocots – bundles are distributed as a complex network
throughout the inner part of the stem
• frequently described as “scattered” in arrangement
– eudicots and gymnosperms – vascular bundles are arranged in
the periphery
• surrounding the pith
• “monkey face monocot”
• cap of sclerenchyma on top of phloem
• below this is a region of large xylem
cells
• smaller tracheids and vessel elements
below this is primary xylem
• chin of sclerenchyma
Monocot Stem
Vascular
Bundles
Phloem
Xylem
Air space
Eudicot Stem Vascular Bundles
•
•
•
•
•
•
“Russian priest”
cap of sclerenchyma
region of phloem
below this is a region of “stem cells” = vascular cambium
below this is a region of xylem
“chin” of sclerenchyma
Sclerenchyma
Phloem
Vascular cambium
Xylem
Sclerenchyma
Evolutionary Adaptations of Stems
• Rhizomes – a horizontal shoot that grows
just below the surface
• 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
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
Blade
Vein
• contains vascular tissue in the
form of veins
– monocots – parallel veins
– dicots – branched network
Petiole
Axillary bud
Leaves
Simple leaf
Petiole
• blade morphology:
– simple – single undivided blade
– compound – blade consists of
multiple leaflets
– double compound – each leaflet
divides into smaller leaflets
• like compound leaves – this
morphology may resist tearing by
strong winds
Axillary bud
Leaflet
Compound leaf
Petiole
Axillary bud
Doubly compound leaf
Leaflet
Petiole
Axillary bud
Phyllotaxy
• 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
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: very similar to stems
• cuticle: minimizes water loss
• single layer of epidermal cells (parenchyma): covered by the cuticle
• guard cells & stomata: upper and lower epidermis have stomata
• 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: two layers of ground tissue located between the
epidermal layers
• upper layer contains long, columnar parenchyma cells = palisade
parenchyma or palisade mesophyll
– main photosynthetic tissue of the plant- usually one layer thick
• layer below is made of spongy parenchyma or spongy mesophyll
– loosely packed cells with many intercellular air spaces to permit diffusion of
CO2 toward the palisade cells – for photosynthesis
vascular
bundles
• 3. vascular tissues or veins
– embedded in the mesophyll
– in dicots – branched venation
– in monocots – parallel venation
DICOT LEAF
MONOCOT LEAF
• dicot leaf
– 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
collenchyma
DICOT LEAF
• Monocot leaf
– parallel running veins with a distinct
pattern of phloem and xylem
• 3 large xylem vessel elements
• “monkey face” again
bundle sheath
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
• needles – capable of photosynthesis
• storage leaves – adapted to storing
water
• bracts – often mistaken for petals;
modified leaves that surround a group of
flowers
– e.g. pointsettia
Spines.
Storage
leaves.
Bracts.
NEXT CLASS
ROOTS and SECONDARY GROWTH IN
DICOTS
Roots
• organ that bears no leaves or node
• have multiple functions:
– 1. anchors the vascular plant in the soil – done by
the lateral roots
– 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
• numerous types of roots
• but two types of root systems: fibrous and
taproot
• found in dicots 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 so it
can’t form a taproot
– forms a fibrous system instead
• system of small roots and smaller 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
– tip of the root contains a meristem
(stem cells) for growth
• root cap:
– the meristem is protected by a root
cap as the root pushes through soil
– root cap cells secrete a mucilage or
slime to help the root push through
the soil
Meristem
Root cap
• Growth occurs just behind the
root tip, in three zones of
cells:
– Zone of cell division
• unspecialized cells undergoing mitosis to
increase the length of the root
– Zone of elongation – located
just behind the root apical
meristem
Root growth
Cortex
Epidermis
Root hair
• cells begin to specialize in this region and
form the tissues of the root
• area of water absorption
– Zone of maturation – also called
the root hair zone
• many of the epidermal cells extend out to
form root hairs
• impermeable to water
• vascular bundles for water and mineral
transport form in this zone
Vascular bundles
Zone of
maturation
Zone of
elongation
Root
meristem
Root cap
Zone of cell
division
Roots: Internal Anatomy
• from outermost to innermost:
– 1. epidermis
– 2. cortex – ground tissue
– 3. endodermis – single layer of cells that encircles the vascular
cylinder
– 4. vascular cylinder/bundle – contains xylem and phloem
endodermis
Monocot Roots:
Vascular Bundle
-Monocot roots: vascular
cylinder/bundle is a ring of
alternative xylem and phloem cells
surrounding a core of parenchyma
(similar to the pith of a stem)
Endodermis
Xylem
Phloem
Core of
parenchyma
cells
Epidermis
Cortex
Vascular
cylinder
Dicot Roots:
Vascular Bundle
-Eudicots and gymnosperms:
-vascular bundle is called a stele
-surrounded endodermis
-unique “X” shaped xylem cells
100 µm
Phloem
Xylem
Epidermis
Cortex
Vascular
cylinder
Endodermis
Epidermis
Cortex
Vascular
cylinder
Endodermis
Core of
parenchyma
cells
100 µm
100 µm
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.
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
Roots
• numerous types of roots
– adventitious – form from
unusual locations – e.g. leaves
or stems
– aerial – growth above ground
• e.g. orchids
– aerating – grow up above the
ground or water
– 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
– 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.
Types of Plants
• two designations: herbaceous (herb – nonwoody) and woody plants
• herbaceous plants – annuals that live one year
• woody plants – perennials that grow many years
– form wood
– wood: forms through the continued growth of the
plant in the second year and beyond  Secondary
growth
– wood contains secondary xylem
Vascular Cambium
• secondary growth happens because of the vascular cambium
• in herbaceous plants – there is no vascular cambium (because it
disappears)
• BUT in woody plants – the VC continues to divide in the
second year and beyond
• each year it forms new xylem and phloem – secondary X &
P
Secondary Xylem and Phloem
• comes from the vascular cambium
• form every year the plant grows
• secondary phloem – thin layer of phloem that is found
between the rings of the tree (i.e. the wood) and the
bark of the tree
• secondary xylem – known as the wood of the tree
– forms rings with each year of growth
Secondary growth: Wood
• secondary xylem that forms each year
of tree growth
• made up of tracheids and vessel
elements just like the xylem made in
the first year (primary xylem)
• forms annual rings – vascular
cambium goes “quiet” (quiescent)
during times of stress (extreme hot
and cold) and stops forming new cells
• when conditions improve 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 (larger xylem cells)
– later in the season – late wood or
summer wood
• late wood + early wood = annual ring
Secondary growth: Wood
• most commercial dicot woods are
strong and tough (hardwoods)
• wood from conifers have a softer
consistency – softwoods
• 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
heartwood
sapwood
BTW- what the heck is bark???
– non-technical term
– all tissues outside of the
vascular cambium
– includes the:
• secondary phloem
• outer layers of the bark called cork