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PLANT FORM AND
FUNCTION
CHAPTER 33
ORGANIZATION OF A VASCULAR
PLANT
Apical meristem
Terminal bud
• A vascular plant is organized Primary
growth zone
along a vertical axis.
• The root penetrates the soil and meristems
absorbs water and ions and it Internode
anchors the plant.
• The shoot consists of the stem
and leaves.
• The stem serves as a
Vascular
system
framework for positioning
the leaves.
• The leaves are where most
photosynthesis takes place.
Blade
Leaf
Vein
petiole
Axilary
bud
Lateral
Apical meristem
Node
Shoot
Pith
Lateral root
Root
Primary
root
Primary growth
zone
ORGANIZATION OF A VASCULAR
PLANT
• Plants contain growth zones of
unspecialized cells called meristems.
• Meristems are not only areas of actively dividing
cells that result in plant growth, but also
continuously replenish themselves.
• In this way, meristem cells function much like stem
cells in animals.
ORGANIZATION OF A VASCULAR
PLANT
• Primary growth is initiated at the tips (of roots
and shoots) by the apical meristems.
• The growth of these meristems results primarily in
the extension of the plant body.
• Secondary growth involves the activity of
the lateral meristems.
• The continued divisions of their cells results
primarily in the thickening of the plant body.
ORGANIZATION OF A VASCULAR
PLANT
• There are two kinds of lateral meristems:
• Vascular cambium - gives rise to thick
accumulations of secondary xylem and phloem.
• Cork cambium - forms the outer layers of bark on
both roots and shoots.
PLANT TISSUE TYPES
• Most plants have three tissue types:
• Ground tissue - in which the vascular tissue is
embedded.
PLANT TISSUE TYPES
• Dermal tissue - the outer protective
covering of the plant. Flattened epidermal
cells are the most abundant cells in the
plant’s outer layer, or epidermis.
• The epidermis is often covered by a waxy layer
called the cuticle.
• The epidermis and cuticle protect the plant and
provide an effective barrier against water loss.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(a)
Trichome
Trichomes
Root
hairs
186 µm
Epidermal cells Stomatal opening Guard cells
Stomata
137 µm
(c)
Root hairs
(b)
(top): © Andrew Syred/Science Photo Library/Photo Reseachers;(middle): © Dr. Jeremy Burgess/Science Photo Library/Photo
Researchers; (bottom): © Dennis Drenner/Visuals Unlimited, Inc.
PLANT TISSUE TYPES
• Vascular tissue - conducts water
and dissolved minerals up the
plant and conducts the products
of photosynthesis throughout.
• There are two types of vascular
tissue:
• Xylem is the plant’s principal waterconducting tissue.
• It forms a continuous system that
runs throughout the plant body.
• Water (and dissolved minerals)
pass from the roots to the shoots.
• When water reaches the
leaves, most exits through the
stomata.
• Phloem is the principal foodconducting tissue.
Xylem
Tracheids
Pits
Pores
Vessel
Tracheid
Vessel
element
Vessel
element
(a)
(b)
(c)
Courtesy of Wilfred Cote, SUNY College of Environmental Forestry
ROOTS
Primary Primary
Endodermis
Pericycle
phloem xylem
• Roots have a
central column
of xylem with
radiating arms.
• Alternating
within the
radiating arms
of xylem are
strands of
primary phloem.
Cortex
Epidermis
Casparian
strip
Sectioned
endodermal
cells
H2O
H2O
ROOTS
• The root elongates rapidly just behind its tip
in the area known as the zone of elongation.
• Abundant root hairs, extensions of single
epidermal cells, form above the elongation
zone.
• This area is called the zone of differentiation.
Epidermis
Phloem
Epidermis
Xylem
Cortex
Protoderm
Ground
meristem
Procambium
Roothair
Pericycle
Endodermis
Primary phloem
Primary xylem
Zone of
differentiation
Protoderm
Zone of
elongation
Apical meristem
Root cap
(a) Dicot
Ground
meristem
Procambium
Apical
meristem
Root cap
(b) Monocot
ROOTS
• Roots branching is
initiated as a result
of cell divisions in
the pericycle.
• The developing
lateral roots grow
out of the cortex
toward the surface
of the root.
Developing
lateral root
Pericycle
Cortex
Endodermis
Epidermis
STEMS
• Stems often experience both primary and
secondary growth.
• Stems are the source of an economically
important product—wood.
• In the primary growth of a shoot, leaves first
appear as leaf primordia.
• These are rudimentary leaves that cluster around
the apical meristem.
• They unfold and grow as the stem elongates.
STEMS
• The places on the stem
where leaves form are
called nodes.
• The portions of the stem
between these leaf
attachment points are
called internodes.
Terminal
bud
Axillarybud
arising from
theaxil
Node
Blad
• As the leaves expand to
Internode
maturity, a bud
develops in the angle
between the leaf and
the stem from which it
arises.
Terminal
• This area is called the axil. budscale
scars
Petiole
STEMS
• Within soft, young stems,
the vascular tissue
strands are arranged
differently in dicots
versus monocots.
• In dicots, vascular bundles
(containing primary xylem
and primary phloem) are
arranged around the
outside of the stem.
• In monocots, vascular
bundles are scattered
throughout the stem.
Epidermis
(outerlayer)
Collenchyma
(layers below
epidermis)
Pith
Vascular
bundle
Xylem
Phloem
Cortex
(a)
Xylem
Phloem
Ground tissue
Vascular bundles
(b)
STEMS
• In stems, secondary growth is initiated by the
differentiation of the vascular cambium.
• This is a thin layer of actively dividing cells located
between the bark and the main stem in woody
plants, running between the xylem and the
phloem.
• Cells that divide from the vascular cambium
outwardly become secondary phloem.
• Cells that divide from the vascular cambium
inwardly become secondary xylem.
VASCULAR CAMBIUM AND
SECONDARY GROWTH
Primary Secondary Primary Secondary
xylem
xylem
phloem phloem
Cork
Cork
cambium
Vascular
Secondary cambium
phloem
Annual
growth
layers
Periderm
STEMS
• While the vascular cambium is being
established, a second kind of lateral
cambium develops in the stem’s outer layer.
• The cork cambium consists of plates of dividing
cells that move deeper and deeper into the stem
as they divide.
• Outwardly, this cambium divides to form
densely packed cork cells.
• Inwardly, this cambium divides to produce a
layer of parenchyma (ground) cells.
STEMS
• The term bark refers to all of the tissues of a
mature stem or root outside of the vascular
cambium.
• Wood is accumulated secondary xylem.
STEMS
• Because of the way it is
accumulated, wood
often displays rings.
• The vascular cambium
divides more actively in
the spring and the
summer than in the fall
and winter.
• The growth rate
differences are reflected
in alternating rings of
growth of different
thickness.
LEAVES
• Leaves are usually the
most prominent shoot
organ and are
structurally diverse.
• Growth occurs by means
of marginal meristems.
• The marginal meristems
grow outward and
ultimately form the
blade (the flattened
portion) of the leaf.
LEAVES
• Leaf blades come in a variety of forms:
• Simple leaves have a single, undivided blade.
• Compound leaves have a blade divided into
leaflets.
• Pinnately compound describes leaflets that are
arranged in pairs along a central axis.
• Palmately compound describes leaflets that
radiate out from a common point at the blade
end of the petiole.
LEAVES
• Veins, comprised of xylem and phloem, run
through the leaf.
• In most dicots, the veins have a net or reticulate
venation.
• In most monocots, the veins are parallel.
LEAVES
• Leaves can be arranged in different patterns:
• Alternate leaves spiral around a shoot.
• Opposite leaves occur on opposite sides of a shoot.
• Whorled leaves circle the stem as a group.
Alternate (spiral):
Ivy
Opposite:
Periwinkle
Whorled:
Sweet woodruff
LEAVES
• A typical leaf contains masses of
parenchyma, called mesophyll, through
which the vascular bundles, or veins, run.
Vein
Upper
epidermis
Cuticle
Palisade
mesophyll
Spongy
mesophyll
Lower
epidermis
Air spaces
Stoma
Guard cell
Guard cell
Stoma
Vein
Air spaces
WATER MOVEMENT
• Vascular plants have conducting systems for
transporting fluids and nutrients throughout the
plant.
• Water and minerals enter a plant through the roots
and are transported in the xylem.
• Carbohydrates synthesized by photosynthesis are
transported throughout the plant in the phloem.
H2O
Xylem
Carbohydrates
H2O
Spongy
mesophyll
layer
H2O
Phloem
Xylem
Water and minerals
Stoma
pass up through
Water exits the
H2O
xylem.
vapor plant through stomata
in leaves.
Carbohydrates
H2O and minerals
Water and carbohydrates
travel to all parts of the plant.
H2O
and
minerals
H2O
and
minerals
H2O
and
minerals
Water enters the
plant through the roots.
WATER MOVEMENT
• Several factors are at work to move water
up the height of a plant.
• Osmosis: Water moves into the cells of the root
because the fluid in the xylem contains more
solutes than the surroundings.
• This osmotic force is called root pressure but, by
itself, is not sufficient to “push” water up a
plant’s stem.
WATER MOVEMENT
• In addition to root pressure,
capillary action adds “pull” to
the movement of water up a
plant stem.
• Capillary action results from the
tiny electrical attractions of polar
water molecules to surfaces that
carry electrical charge.
• This attraction is called
adhesion.
• But capillary action, by itself, is not
strong enough to “pull” water up
the plant stem.
WATER MOVEMENT
• A final “pull” to the process of moving water
up a plant shoot is provided by transpiration.
• Water evaporating from the top (leaf) of the tube
pulls the column of water from the bottom (root).
• The column of water does not collapse because
water molecules are attracted to each other.
• This process is called cohesion.
• The narrower the diameter of the tube, the
more tensile strength, or resistance to
separation, of the water column.
WATER MOVEMENT
• The combination of gravity, tensile strength,
and cohesion affects water movement.
• The whole process is explained by the cohesionadhesion-tension theory.
WATER MOVEMENT
• Transpiration is the process by which water leaves a
plant.
• More than 90% of the water taken in by a plant is lost to the
atmosphere, mostly through the leaves.
• Water first passes into the pockets of air in the spongy
mesophyll and then evaporates through the stomata.
• High humidity and low temperatures increase transpiration
rates.
1
2
H2O
3
H2O
Dry air
H2O
H2O
H2O
H2O
Dry air passes across the leaves and
causes water vapor to evaporate out
of the stomata.
The loss of water from the leaves
creates a type of “suction” that draws
water up the stem through the xylem.
H2O
New water enters the plant through
the roots to replace the water
moving up the stem.
WATER MOVEMENT
• The only way that plants can control water
loss on a short-term basis is to close their
stomata.
• But plants need to balance closing their stomata
with keeping them open for providing access to
carbon dioxide.
• The stomata open and close because of
changes in the water pressure of their guard cells.
WATER MOVEMENT
• When the guard cells
are plump and swollen
with water, they are
said to be turgid and
the stoma is open.
• When the guard cells
lose water, the stoma
closes.
Guard cell
Chloroplasts
H2O
H2O
H2O
Epidermal cell
H2O
H2 O
H2O
Nucleus
H2O
H2O
Thickened
inner wall
H2O
H2O
H2O
Stoma open
(a)
H2O
Stoma closed
(b)
WATER MOVEMENT
• Root hairs greatly
increase the surface
area of roots.
• Root hairs are turgid
because they contain a
higher concentration of
dissolved solutes than the
soil.
• Root hairs also contain a variety of ion transport
channels that transport specific ions.
• This may involve active transport.
• The minerals are transported by the xylem while dissolved
in water.
CARBOHYDRATE TRANSPORT
• Translocation is the process by which most of
the carbohydrates manufactured in plants
are moved through the phloem.
• Carbohydrates are transported by mass flow, a
passive process.
• Mass flow occurs because of water pressure when carbohydrates are loaded into sieve
tubes, water also enters due to osmosis; the
water pressure forces the carbohydrates down
the plant.
CARBOHYDRATE TRANSPORT
• An area where sucrose is made is
called a source and an area where
sucrose is delivered from the sieve
tubes is called a sink.
• Sucrose moves from a source to a
sink by a process described by the
pressure-flow hypothesis.
1
Leaf cells
Sugar
Phloem
Xylem
Root cells
Sugar created in the leaves by photosynthesis (“source”)
enters the phloem by active transport.
2
H2O
When the sugar concentration in the phloem increases,
water is drawn into phloem cells from the xylem by osmosis.
3
Sugar
The addition of water from the xylem causes pressure to
build up inside the phloem and pushes the sugar down.
4
Sugar
Sugar from the phloem enters the root cells (“sink”) by
active transport.