Transcript root - McGraw Hill Higher Education

The Living World
Fifth Edition
George B. Johnson
Jonathan B. Losos
Chapter 23
Plant Form and Function
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
23.1 Organization of a Vascular
Plant
• A vascular plant is organized along a
vertical axis
 the part below ground is called the root
• the root penetrates the soil and absorbs water and
ions
• it also anchors the plant
 the part above ground is called the shoot
• the shoot consists of the stem and leaves
– the stem serves as a framework for positioning the
leaves
– the leaves are where most photosynthesis takes place
Figure 23.1 The body of a plant
23.1 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
23.1 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
23.1 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
23.2 Plant Tissue Types
• Most plants have three tissue types
 ground tissue
• in which the vascular tissue is embedded
 dermal tissue
• the outer protective covering of the plant
 vascular tissue
• conducts water and dissolved minerals up the
plant and conducts the products of photosynthesis
throughout
23.2 Plant Tissue Types
• There are three kinds of cells in plant ground tissue
 parenchyma cells
• they are alive at maturity
• they carry out the basic functions of living, including photosynthesis,
cellular respiration, and food and water storage
 collenchyma cells
• they are also living at maturity
• they provide much of the support for plant organs in which
secondary growth has not occurred
 sclerenchyma cells
• they usually do not contain living cytoplasm when mature
• they have tough cell walls called secondary cell walls
Ground Tissue Examples
Figure 23.2 Parenchyma cells
Figure 23.3 Collenchyma cells
23.2 Plant Tissue Types
• There are two types of sclerenchyma
 fibers which are long, slender cells that
usually form strands
 sclereids which are variable in shape but
often branched
• clusters of sclereids form the gritty texture one
feels in the flesh of pears
Figure 23.4 Sclerenchyma cells in
sclereids
23.2 Plant Tissue Types
• All parts of the outer layer of a primary
plant body are covered by flattened
epidermal cells, which are often covered
by a waxy layer called the cuticle
 they protect the plant and provide an effective
barrier against water loss
23.2 Plant Tissue Types
• There are several specialized epidermal cells that make
up dermal tissue
 guard cells are paired cells that flank an opening called a
stoma
• the guard cells regulate the passage of oxygen, carbon dioxide, and
water vapor across the epidermis
 trichomes are outgrowths of the epidermis that occur on the
shoot and give it a “fuzzy” appearance
• they play an insulating role and affect heat and water balance
 root hairs are extensions of the epidermis below ground and
keep the root in intimate contact with soil particles
• root hairs increase the surface area of the root
Figure 23.5 Guard cells and trichomes
23.2 Plant Tissue Types
• There are two types of vascular tissues
 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 food-conducting
tissue
23.2 Plant Tissue Types
• There are two principal conducting cells in the
xylem, both of which are dead at maturity
 tracheids are elongated cells that overlap at their
ends
• water flows from tracheid to tracheid through openings called
pits
 vessel elements are elongated cells that line up endto-end
• the end walls of vessel elements are almost completely open
or be perforated to allow for the flow of water
• vessels conduct water much more efficiently than tracheids
Figure 23.6 Comparison of vessel
elements and tracheids
23.2 Plant Tissue Types
• Food conduction in phloem is carried out
through two kinds of elongated cells
 sieve cells have smaller perforations
between cells
 sieve-tube members have some sieve areas
with larger pores than do sieve cells
• these areas are called sieve plates
• sieve-tube members occur end to end, forming
longitudinal series called sieve tubes
– specialized parenchyma cells, known as companion
cells, occur in association with the sieve tubes
Figure 23.7 Sieve tubes
23.3 Roots
• Roots have a central column of xylem with
radiating arms
 alternating within the radiating arms of xylem
are strands of primary phloem
 surrounding the central column, and forming
its boundary, is a cylinder of cells called the
pericycle
• branch, or lateral, roots are formed from cells of
the pericycle
23.3 Roots
• The outer layer of the root is the epidermis
 the mass of parenchyma in which the root’s
vascular tissue is located is called the cortex
• its innermost layer lies just outside the pericycle
and is called the endodermis
– the cells making up the endodermis are encircled by a
thickened, waxy band called the Casparian strip
– this strip blocks the movement of water between the
endodermal cells and instead forces the movement of
water through the plasma membrane of the endodermal
cells
Figure 23.9 A root cross section
23.3 Roots
• The apical meristem of a root is really three
primary meristems
 protoderm becomes the epidermis
 procambium produces primary vascular tissues
 ground meristem differentiates into ground tissues,
which is comprised of parenchyma tissue
• If apical meristem growth is outward, the cell
division forms a thimblelike mass of unorganized
cells called the root cap
 the root cap protects the root’s apical meristem as it
grows through the soil
23.3 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
23.3 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
Figure 23.10 Lateral roots
23.4 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
23.4 Stems
• The places of the stem where leaves form
are called nodes
 the portions of the stem between these leaf
attachment points are called internodes
• As the leaves expand to maturity, a bud
develops in the angle between the leaf
and the stem from which it arises
 this area is called the axil
23.4 Stems
• Buds have their own immature leaves
called stipules
 buds may either elongate or remain dormant
• their activity is controlled by a hormone that moves
downward from the terminal bud of the shoot
• the hormone suppresses expansion of buds in the
upper portions of the stem
• buds begin forming in the lower down portions of
the stem where the amount of hormone is reduced
Figure 23.11 A woody twig
23.4 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
Figure 23.12 A comparison of dicot and
monocot stems
23.4 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
23.4 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 cells
23.4 Stems
• The cork, the cork cambium, and the
parenchyma cells collectively make up a layer
called the periderm
 the periderm is the plant’s outside protective covering
• 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
Figure 23.13 Vascular cambium
and secondary growth
23.4 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
Figure 23.14 Annual rings in a
section of pine
23.5 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
• once a leaf is fully expanded, its marginal
meristems cease to grow
23.5 Leaves
• Additional leaf structures include
 a slender stalk called a petiole
 two leaflike organs, called stipules, may flank
the base of the petiole where it joins the stem
 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
Figure 23.16 Dicot and monocot
leaves
23.5 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 vein
• palmately compound describes leaflets that are
radiate out from a common point at the blade end
of the petiole
Figure 23.17 A leaf in cross section
23.5 Leaves
• Leaves can be arranged in different
patterns
 alternate leaves spiral around a shoot
 opposite occur on opposite sides of a shoot
 whorl circle the stem as a group
23.5 Leaves
• A typical leaf contains masses of parenchyma,
called mesophyll, through which the vascular
bundles, or veins, run
 a closely packed, columnlike layer or layers of
parenchyma cells are found underneath the upper
epidermis of a leaf
• this is called the palisade mesophyll
• it is packed with chloroplasts
 the rest of the leaf interior, except for the veins,
consists of spongy mesophyll
• it has lots of interior spaces for gas exchange
Figure 23.17 A leaf in cross section
23.6 Water Movement
• Several factors are at work to move water
up the height of a plant
 the initial movement of water into the roots of
a plant involves 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
23.6 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
Figure 23.18 Capillary action
23.6 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
23.6 Water Movement
• The combination of gravity, tensile
strength, and cohesion affects water
movement
 the whole process is explained by the
cohesion-adhesion-tension theory
23.6 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
Figure 23.19 How transpiration
works
23.6 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
23.6 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
Figure 23.20 How guard cells regulate
the opening and closing of stomata
23.6 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
 minerals also enter the root hairs because they
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
Figure 23.21 Root hairs
Figure 23.22 The flow of materials
into, out of, and within a plant
23.7 Carbohydrate Transport
• Translocation is the process by which most of
the carbohydrates manufactured in plants are
moved through the phloem
 the movement is a passive process
• the mass flow of materials transported occurs because of
water pressure generated by osmosis
– 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
Figure 23.23 How translocation works
23.8 Essential Plant Nutrients
• Minerals are involved in plant metabolism in
many ways
 nitrogen (N) is an essential part of proteins and
nucleic acids
 potassium (K) ions are used to regulate turgor
pressure in guard cells
 calcium (Ca) is an essential part of cell walls
 magnesium (Mg) is a part of the chlorophyll molecule
 phosphorous (P) is a part of ATP and nucleic acids
 sulfur (S) is a key component of the amino acid,
cysteine
23.8 Essential Plant Nutrients
• Other essential minerals for plant health
include chlorine (Cl), iron (Fe), boron (B),
manganese (Mn), zinc (Zn), copper (Cu),
and molybdenum (Mb)
• Most plants acquire minerals from the soil,
although some carnivorous plants are
able to use other organisms directly as
sources of nitrogen, just as animals do
Figure 23.24 A carnivorous plant
Inquiry & Analysis
• In the 23-cm section,
is more 42K found in
xylem or phloem?
• Above and below the
23-cm section, is
more 42K found in
xylem or phloem?
Graph of Movement of 42K Through
a Stem