Download Vascular tissue - Cloudfront.net

SEED VASCULAR PLANTS
• Division Cycadophyta: palms (naked seeds)
• Division Ginkgophyta: ginkgo (naked seeds)
• Division Gnetophyta: only few species (naked
seeds)
• Division Coniferophyta: cone plants: pine,
spruce, fir, larch, yew (naked seeds)
• Division Anthophyta: flowering plants (seeds in
fruit)
DIVISION ANTHOPHYTA
•
•
•
•
Flowering plants
Sex organs in flowers
Seeds in ovary that ripens into fruit
Two classes:
– Monocotyledoneae (Monocot)
– Dicotyledoneae (Dicot)
Roots, Stems, and Leaves
• Cacti leaves are
modified into thin,
sharp spines
• The reduced-leaf
surface area prevents
excess water loss
Roots, Stems, and Leaves
Specialized Tissues in Plants
• If you look deep inside a living plant, that first
impression of inactivity vanishes
• Instead, you will find a busy and complex
organism packed with specialized systems
and subsystems
• Materials move throughout the plant, and growth
and repair take place continuously
• Plants may act at a pace that seems slow to us,
but their cells work together in remarkably
effective ways to ensure the plant's survival
Seed Plant Structure
• The cells of a seed plant are
organized into different
tissues and organs, as
shown in the figure
• Three of the principal organs
of seed plants are roots,
stems, and leaves
• These organs are linked
together by systems and
subsystems that run the length
of the plant, performing
functions such as transport
and protection and
coordinating plant activities
Tissues in a Vascular Plant
• Vascular plants
consist of roots,
stems, and leaves
• Each of these organs
contains dermal
tissue, vascular
tissue, and ground
tissue, as shown by
the cross sections of
the leaf, stem, and
root
Tissues in a Vascular Plant
Roots
• The root system of a plant absorbs water and
dissolved nutrients
• Roots anchor plants in the ground, holding
soil in place and preventing erosion
• Root systems also protect the plant from
harmful soil bacteria and fungi, transport
water and nutrients to the rest of the plant,
and hold plants upright against forces such
as wind and rain
Stems
• A stem has a support system for the plant body, a
transport system that carries nutrients, and a
defense system that protects the plant against
predators and disease
• Stems can be as short as a few millimeters or as tall as
100 meters
• Whatever its size, the support system of a stem must
be strong enough to hold up its leaves and branches
• Similarly, the stem's transport system must contain
subsystems that can lift water from roots up to the
leaves and carry the products of photosynthesis
from the leaves back down to the roots
Leaves
• Leaves are the plant's main photosynthetic
systems
• The broad, flat surfaces of many leaves help
increase the amount of sunlight plants absorb
• Leaves also expose a great deal of tissue to the
dryness of the air and, therefore, must contain
subsystems to protect against water loss
• Adjustable pores in leaves help conserve
water while letting oxygen and carbon
dioxide enter and exit the leaf
Plant Tissue Systems
• Within the roots, stems, and leaves of plants are
specialized tissue systems
• Plants consist of three main tissue systems:
– Dermal tissue: is the skin of a plant in that it is the
outmost layer of cells
– Vascular tissue: is the plant's bloodstream,
transporting water and nutrients throughout the plant
– Ground tissue: and ground tissue is everything else
Dermal Tissue
• The outer covering of a plant consists of dermal tissue, which
typically consists of a single layer of epidermal cells
• The outer surfaces of these are often covered with a thick waxy
layer that protects against water loss and injury
– The thick waxy coating of the epidermal cells is known as the cuticle
• Some epidermal cells have tiny projections known as
trichomes, which help protect the leaf and also give it a fuzzy
appearance
• In roots, dermal tissue includes root hair cells that provide a
large amount of surface area and aid in water absorption
• On the underside of leaves, dermal tissue contains guard cells,
which regulate water loss and gas exchange
Vascular Tissue
•
•
•
•
Vascular tissue forms a transport
system that moves water and
nutrients throughout the plant
The principal subsystems in
vascular tissue are xylem, a waterconducting tissue, and phloem, a
food-conducting tissue
Vascular tissue contains several
types of specialized cells:
– Xylem consists of tracheids and
vessel elements
– Phloem consists of sieve tube
elements and companion cells
As you can see in the figure, both
xylem and phloem are made up of
networks of hollow connected cells
that carry fluids throughout the plant
Vascular Tissue in a Stem
• Vascular tissue is made up of
several different types of cells
• Xylem consists of tracheids
and vessel elements
• Phloem consists of sieve
tube elements and
companion cells
• Xylem tissue (left) conducts
water from the roots to the rest
of the plant
• Phloem tissue (right)
conducts a variety of materials,
mostly carbohydrates,
throughout a plant
Vascular Tissue in a Stem
Xylem
• All seed plants have a type of xylem
cell called a tracheid
• Recall that tracheids are long, narrow
cells with walls that are impermeable to
water
– These walls, however, are pierced by
openings that connect neighboring cells to
one another
– When tracheids mature, they die, and their
cytoplasm disintegrates
Xylem
• Angiosperms have another kind of xylem cell
that is called a vessel element
– Vessel elements are much wider than tracheids
– Like tracheids, they mature and die before they
conduct water
• Vessel elements are arranged end to end on top
of one another like a stack of tin cans
• The cell walls at both ends are lost when the
cells die, transforming the stack of vessel
elements into a continuous tube through
which water can move freely
Phloem
• The main phloem cells are sieve tube elements
– These cells are arranged end to end, like vessel elements, to
form sieve tubes
– The end walls of sieve tube elements have many small holes
in them
• Materials can move through these holes from one
adjacent cell to another
• As sieve tube elements mature, they lose their nuclei
and most of the other organelles in their cytoplasm
– The remaining organelles hug the inside of the cell wall
• The rest of the space is a pipeline through which
sugars and other foods are carried in a watery
stream
Phloem
• Companion cells are phloem cells that
surround sieve tube elements
– Companion cells keep their nuclei and
other organelles through their lifetime
• Companion cells support the phloem cells
and aid in the movement of substances in
and out of the phloem stream
Ground Tissue
• The cells that lie between dermal and vascular tissues make up
the ground tissues, shown in figure
• In most plants, ground tissue consists mainly of parenchyma
– Parenchyma cells have thin cell walls and large central vacuoles
surround by a thin layer of cytoplasm
• In leaves, these cells are packed with chloroplasts and are the site of
most of a plant's photosynthesis
• Ground tissue may also contain two types of cells with thicker cell
walls:
– Collenchyma cells have strong, flexible cell walls that help support
larger plants
• Collenchyma cells make up the familiar “strings” of a stalk of celery
– Sclerenchyma cells have extremely thick, rigid cell walls that make
ground tissue tough and strong
Ground Tissues
•
•
•
•
•
Ground tissue is made of cells
whose cell walls have different
thicknesses
Parenchyma cells have thin
walls and function mainly in
storage and photosynthesis
The root cells shown are filled with
purple-staining starch grains
Collenchyma and sclerenchyma
cells both function in support
Collenchyma cells have
irregularly shaped walls, but the
walls of sclerenchyma cells are
much thicker and harder
Ground Tissues
Plant Growth and Meristematic Tissue
• Most plants have a method of development that
involves an open, or indeterminate, type of growth
– Indeterminate growth means that they grow and produce
new cells at the tips of their roots and stems for as long as
they live
• These cells are produced in meristems, clusters of
tissue that are responsible for continuing growth
throughout a plant's lifetime
• The new cells produced in meristematic tissue are
undifferentiated—that is, they have not yet become
specialized for specific functions, such as transport
Plant Growth and Meristematic Tissue
• Near the end, or tip, of each
growing stem and root is an
apical meristem
• An apical meristem is a group
of undifferentiated cells that
divide to produce increased
length of stems and roots
• The figure at right shows
examples of root and shoot
apical meristems
• Meristematic tissue is the
only plant tissue that
produces new cells by
mitosis
Plant Growth and Meristematic Tissue
• Meristematic tissue
produces new cells by
mitosis
• Apical meristems, which
consist of many actively
dividing cells, are located
at the tips of shoots
(left) and roots (right)
• The apical meristem of a
root is surrounded by a
root cap that protects the
root as it grows through
the soil
Plant Growth and Meristematic Tissue
Plant Growth and Meristematic Tissue
• At first, the cells that originate in meristems look
very much alike: They divide rapidly and have
thin cell walls
• Gradually, these cells develop into mature
cells with specialized structures and
functions, a process called differentiation
• As these cells differentiate, they produce each of
the tissue systems of the plant, including dermal,
ground, and vascular tissue
Plant Growth and Meristematic Tissue
• The highly specialized cells found in flowers, which make
up the reproductive systems of flowering plants, are also
produced in meristems
– Flower development begins when certain genes are turned
on in a shoot apical meristem
• The actions of these genes transform the apical
meristem into a floral meristem, producing the
modified leaves that become the flower's colorful
petals, as well as the reproductive tissues of the
flower
• Many plants also grow in width as a result of
meristematic tissue that lines the stems and roots of
a plant
Roots
• As soon as a seed begins to grow, it puts out its first
root to draw water and nutrients from the soil
• Other roots soon branch out from this first root, adding
length and surface area to the root system
• The overall size of a plant's root system can be
astonishing: The total surface area of the root system of
a rye plant was measured at more than 600 square
meters—130 times greater than the combined surface
areas of both the stems and leaves
ROOT
• Absorbs water and dissolved mineral for
the plant
• Anchors the plant in the soil
• Types:
– Fibrous:
• Greatly branched
– Tap:
• Single large root
Types of Roots
•
•
The two main types of roots are:
– Taproots, which are found
mainly in dicots
– Fibrous roots, which are found
mainly in monocots
In some plants, the primary root
grows long and thick while the
secondary roots remain small
– This type of primary root is called
a taproot, shown in figure
– Taproots of oak and hickory trees
grow so long that they can reach
water far below Earth's surface
– Carrots, dandelions, beets, and
radishes have short, thick taproots
that store sugars or starches
Types of Roots
• Plants have
taproots, fibrous
roots, or both
• Taproots have a
central primary root
and generally grow
deep into the soil
• Fibrous roots are
usually shallow and
consist of many thin
roots.
Types of Roots
Types of Roots
• In other plants, such as grasses, fibrous
roots branch to such an extent that no
single root grows larger than the rest
• The extensive fibrous root systems
produced by many plants help prevent
topsoil from being washed away by
heavy rain
Root Structure and Growth
•
•
•
•
•
•
•
•
•
•
Roots contain cells from the three tissue
systems—dermal, vascular, and ground tissue
A mature root has an outside layer, the
epidermis, and a central cylinder of vascular
tissue
Between these two tissues lies a large area of
ground tissue
The root system plays a key role in water and
mineral transport
Its cells and tissues, as shown in the figure, contain
a number of subsystems that carry out these
functions
The root's epidermal subsystem performs the dual
functions of protection and absorption
Its surface is covered with tiny cellular projections
called root hairs
These hairs penetrate the spaces between soil
particles and produce a large surface area through
which water can enter the plant
Just inside the epidermis is a spongy layer of
ground tissue called the cortex
This layer extends to another layer of cells, the
endodermis
– The endodermis completely encloses the
root's vascular subsystem in a region called
the vascular cylinder
Root Structure and Growth
•
•
•
•
•
•
•
•
A root consists of a central vascular cylinder
surrounded by ground tissue and the
epidermis
Compare how cells in different regions of the
root are structurally specialized for different
functions
Root hairs along the surface of the root aid
in water absorption
Only the cells in the root tip divide
In the area just behind the root tip, the
newly divided cells increase in length,
pushing the root tip farther into the soil
The root cap, located just ahead of the
root tip, protects the dividing cells as
they are pushed forward
Dicot roots, such as the one shown in the
cross section, have a central column of
xylem cells arranged in a radiating
pattern
How are root hairs structurally specialized?
Root Structure and Growth
Root Structure and Growth
• Roots grow in length as their apical meristem produces new
cells near the root tip
– These fragile new cells are covered by a tough root cap that protects
the root as it forces its way through the soil
– As the root grows, the root cap secretes a slippery substance that
lubricates the progress of the root through the soil
• Cells at the very tip of the root cap are constantly being
scraped away, and new root cap cells are continually added by the
meristem
• Most of the increase in root length occurs immediately behind
the meristem, where cells are growing longer
• At a later stage, these cells mature and take on specialized
functions
• The process by which unspecialized cells change to become
specialized in structure and function is known as cell differentiation
ROOT GROWTH
•
•
•
•
Meristem:
– Located just behind the root cap
– Zone of rapidly dividing cells (become cells in the zone of elongation or root cap)
Root cap:
– Tip of root
– Protects meristem
– As root grows through the soil, cells are rubbed off and replaced by meristematic
cells
Zone of elongation:
– Cells cease to divide but enlarge
– Enlargement of cells pushes the root deeper into the soil
Zone of maturation:
– Cells differentiate
Root Functions
• Roots anchor a plant in the ground and
absorb water and dissolved nutrients from
the soil
• How does a root go about the job of
absorbing water and minerals from the soil?
– Although it might seem to, water does not just
“soak” into the root from soil
• It takes energy on the part of the plant to absorb
water
• Our explanation of this process begins with a
description of soil and plant nutrients
ROOT FUNCTION
•
•
•
•
Anchors the plant in the soil
Absorption of water and dissolved mineral
– Root hair
• Fingerlike extension of a single epidermal cell
• Greatly increase surface area of the root enabling the root to absorb
water and dissolved mineral from the soil
• Contains many dissolved substances such as minerals, sugars, and
amino acids
– Soil contains fewer dissolved substances
– Concentration gradient develops between the soil and root hair
– Water moves into the root hair by osmosis
Absorbs macronutrients (Nitrogen/Potassium) in large amount and
micronutrients (Manganese) in small amounts by active transport
Food storage
Uptake of Plant Nutrients
• An understanding of soil helps explain how plants
function
• Soil is a complex mixture of sand, silt, clay, air, and
bits of decaying animal and plant tissue
• Soil in different places and at different depths contains
varying amounts of these ingredients
• Sandy soil, for example, is made of large particles
that retain few nutrients, whereas the finely textured
silt and clay soils of the Midwest and southeastern
United States are high in nutrients
• The ingredients define the soil and determine, to a large
extent, the kinds of plants that can grow in it
Uptake of Plant Nutrients
•
•
•
•
•
To grow, flower, and produce seeds,
plants require a variety of inorganic
nutrients in addition to carbon dioxide
and water
– The most important of these
nutrients are nitrogen, phosphorus,
potassium, magnesium, and calcium
The functions of these essential nutrients
within a plant are described in the table
These nutrients are located in varying
amounts in the soil and are drawn up by the
roots of a plant In addition to these essential
nutrients, trace elements are required in
small quantities to maintain proper plant
growth
Trace elements include sulfur, iron, zinc,
molybdenum, boron, copper, manganese,
and chlorine
Large amounts of trace elements in the soil
can be poisonous
Essential Plant Nutrients
• Soil contains several
nutrients that are
essential for plant growth
• Each nutrient plays a
different role in plant
functioning and
development, and
produces distinct effects
when deficient in the soil
• If you notice that a plant
is becoming paler and
more yellow, what
nutrient might need to be
added?
Essential Plant Nutrients
Active Transport of Minerals
• The cell membranes of root hairs and
other cells in the root epidermis
contain active transport proteins
– These proteins use ATP (an energy source) to
pump mineral ions from the soil into the plant
• The high concentration of mineral ions in
the plant cells causes water molecules to
move into the plant by osmosis, as shown
in the figure at right
Active Transport and Osmosis in a Root
•
•
•
•
•
•
Roots absorb water and dissolved
nutrients from the soil
Most water and minerals enter a plant
through the tiny root hairs
Water moves into the cortex, through
the cells of the endodermis, and into
the vascular cylinder
Finally, water reaches the xylem,
where it is transported throughout
the plant
Cells in the endodermis are made
waterproof by the Casparian strip
The Casparian strip is another
example of how cells are specialized
to perform a particular function—in
this case, preventing the backflow
of water out of the vascular cylinder
into the root cortex
Active Transport and Osmosis in a Root
Active Transport of Minerals
•
•
You may recall that osmosis is the
movement of water across a
membrane toward an area where
the concentration of dissolved
material is higher
– By using active transport to
accumulate ions from the soil,
cells of the root epidermis
create conditions under which
osmosis causes water to
“follow” those ions and flow
into the root
Note that the root does not actually
pump water
– But by pumping dissolved
minerals into its own cells, the
end result is almost the same—
the water moves from the
epidermis through the cortex
into the vascular cylinder
Movement Into the Vascular Cylinder
• Both osmosis and active
transport cause water and
minerals to move from the
root epidermis into the
cortex
– From there, the water and
dissolved minerals pass
the inner boundary of the
cortex and enter the
endodermis
• This process is shown in the
figure Active Transport and
Osmosis in a Root
Movement Into the Vascular Cylinder
•
•
•
•
The endodermis encloses the vascular
cylinder and stretches up and down the
entire length of the root, like a cylinder
– It is composed of many individual cells,
each shaped a bit like a brick
– Each of these cells is surrounded on
four sides by a waterproof strip
called a Casparian strip
To imagine what the Casparian strip looks
like, think of a brick with a thick rubber band
stretched around it
The rubber bands stick together like mortar
between the bricks
Imagine many of these bricks placed edge
to edge to build a cylinder
– When a root is viewed in cross
section, the endodermis forms a
circle
Movement Into the Vascular Cylinder
• Recall that water moves into
the vascular cylinder by
osmosis
• Because water and minerals
cannot pass through the
waxy Casparian strip, once
they pass through the
endodermis, they are
trapped in the vascular
cylinder
• As a result, there is a oneway passage of materials
into the vascular cylinder in
plant roots
Root Pressure
• Why do plants “need” a system that ensures
the one-way movement of water and
minerals?
– That system is how the plant generates enough
pressure to move water out of the soil and up into
the body of the plant
• As minerals are pumped into the vascular
cylinder, more and more water follows by
osmosis, producing a strong pressure
– If the pressure were not contained, roots would
expand as they filled with water
Root Pressure
• Instead, contained within the Casparian strip, the water has just
one place to go—up
• Root pressure, produced within the cylinder by active transport,
forces water through the vascular cylinder and into the xylem
• As more water moves from the cortex into the vascular
cylinder, more water in the xylem is forced upward through the
root into the stem
• In the figure, you can see a demonstration of root pressure in a
carrot root
• Root pressure is the starting point for the movement of water
through the vascular system of the entire plant
• But it is just the beginning
• Once you have learned about stems and leaves, you will see
how water and other materials are transported within an entire
plant
Demonstration of Root Pressure
• As a carrot root
absorbs water, root
pressure forces water
upward into the glass
tube, which takes the
place of the stem and
leaves of the carrot in
this demonstration
Demonstration of Root Pressure
Stems
• What do a barrel cactus, a tree trunk, a
dandelion stem, and a potato have in
common?
– They are all types of stems
• Stems vary in size, shape, and method of
development
– Some grow entirely underground; others reach high
into the air
• Stems also vary in structure and internal
arrangement of cells
Stem Structure and Function
• In general, stems have three important functions:
– They produce leaves, branches, and flowers
– They hold leaves up to the sunlight
– They transport substances between roots and leaves
• Stems make up an essential part of the water and mineral
transport systems of the plant
• The vascular tissue in stems conducts water, nutrients, and other
compounds throughout the plant
• Xylem and phloem, the major subsystems of the transport
system, form continuous tubes from the roots through the
stems to the leaves
– These vascular tissues link all parts of the plant, allowing water and
nutrients to be carried throughout the plant
• In many plants, stems also function as storage systems and in the
process of photosynthesis
Stem Structure and Function
• Like the rest of the plant, the stem is
composed of three tissue systems:
dermal, vascular, and ground tissue
• Stems are surrounded by a layer of
epidermal cells that have thick cell walls
and a waxy protective coating
STEM FUNCTION
• Three functions:
– Support
– Transport
– Storage
Stem Structure and Function
•
•
•
In most plants, stems contain
distinct nodes, where leaves are
attached, and internode regions
between the nodes, as shown in
figure
– Small buds are found where
leaves attach to the nodes
Buds contain undeveloped tissue
that can produce new stems and
leaves
In larger plants, stems develop
woody tissue that helps support
leaves and flowers
Buds, Nodes, and Internodes
• Stems produce leaves
and branches and hold
leaves up to the
sunlight, where they
carry out
photosynthesis
• Leaves are attached to a
stem at structures called
nodes
• These nodes are
separated by regions of
the stem called
internodes
Buds, Nodes, and Internodes
Stems Adapted for Storage and Dormancy
• For example, many kinds
of plants have modified
stems that store food
• Tubers, rhizomes, bulbs,
and corms can remain
dormant during cold or
dry periods until favorable
conditions for growth
return
• Examples of these are
shown in the figure at
right
Stems Adapted for Storage and Dormancy
• Many kinds of plants
have modified stems
that store food
• Tubers, rhizomes,
bulbs, and corms can
remain dormant
during cold or dry
periods until
favorable conditions
for growth return
Stems Adapted for Storage and Dormancy
STEM TYPES
• Different types:
–
–
–
–
–
Upright: herbaceous (green) / woody
Stolon: grow along ground surface (strawberry)
Tuber: food storage (potato)
Fleshy: water storage and photosynthesis (cactus)
Corms: food storage (gladiolus/crocus/some
begonias)
– Bulb: food storage (tulip/onion/daffodil)
– Rhizome: grow horizontally underground (iris/water
hyacinth/some types of blueberries)
Monocot and Dicot Stems
•
•
•
•
•
The arrangement of tissues in a
stem differs among seed plants
In monocots, vascular bundles
are scattered throughout the
stem
In dicots and most
gymnosperms, vascular
bundles are arranged in a
cylinder
Recall that monocots and dicots
are two types of flowering plants,
or angiosperms
For a comparison of monocot and
dicot stems, look at the figure
Monocot and Dicot Stems
• The arrangement of
vascular bundles in the
stem of a monocot differs
from that in the stem of a
dicot
• In a monocot, vascular
bundles are scattered
throughout the stem
• In a dicot, vascular
bundles are arranged in
a ring
Monocot and Dicot Stems
Monocot Stems
• The cross section of a young monocot stem
shows all three tissue systems clearly
• The stem has a distinct epidermis, which
encloses a series of vascular bundles, each of
which contains xylem and phloem tissue
– Phloem faces the outside of the stem, and xylem
faces the center
• In monocots, these bundles are scattered
throughout the ground tissue
• The ground tissue is fairly uniform, consisting
mainly of parenchyma cells
Dicot Stems
• Young dicot stems have vascular bundles,
but they are generally arranged in an
organized, ringlike pattern
• The parenchyma cells inside the ring of
vascular tissue are known as pith, while
those outside form the cortex of the stem
• In dicots, these relatively simple tissue
patterns become more complex as the plant
grows larger and the stem increases in
diameter
STEM STRUCTURE
• Terminal bud:
– Tip of a twig (develops into leaves or flowers)
– Contains meristem tissue which adds to the length of the stem
• Lateral bud:
– Above leaf scars (scars left by vascular bundle of dead leaves)
– Adds lateral branches
• Lenticels: raised areas (blisters) that allow the exchange of gases
between the atmosphere and the inner tissues of the stem
• Vascular bundles:
– Monocot: scattered
– Dicot: arranged in a ring
Primary Growth of Stems
•
•
•
•
•
Plants grow in ways that are
distinctly different from other
organisms
– For their entire life, new
cells are produced at the
tips of roots and shoots
This method of growth,
occurring only at the ends of a
plant, is called primary growth
The increase in length produced
by primary growth from year to
year is shown in the figure
Primary growth of stems is
produced by cell divisions in
the apical meristem
It takes place in all seed plants
Primary Growth in a Stem
• All seed plants undergo
primary growth, which
is an increase in length
• Every year, apical
meristems, shown in red,
divide to produce new
growth
• The primary growth for
one season consists of a
stem and several leaves
Primary Growth in a Stem
STEM GROWTH
• Vertical growth (height)only at the tip
• Vertical/horizontal growth at buds
• Horizontal growth (circumference) at the lateral
meristem
– Vascular Cambium:
• Between the xylem and phloem
• Zone of cell division adding cells to the phloem (outside) and
xylem (inside)
– Spring: cells wide and thin
– Summer: cells small and thick
» Density difference creates annual rings
Secondary Growth of Stems
• If a plant is to grow larger year after year, its stems must
increase in thickness as well as in length
– They have more mass to support and more fluid to move through
their vascular tissues
• Yet, only meristematic tissue can produce new cells for growth
• Some monocots, such as palm trees, produce thick stems from a
meristem that becomes wider as the plant grows
• However, most monocots, such as grasses, produce only
fleshy growth and do not grow very tall
• Many dicots grow extremely tall and also grow in width to
support this extra weight
– This growth occurs as a result of meristems other than the apical
meristem
Secondary Growth of Stems
•
•
•
•
•
•
•
The method of growth in which stems
increase in width is called secondary
growth
In the figure at right, you can see the pattern
of secondary growth in a dicot stem
In conifers and dicots, secondary growth
takes place in lateral meristematic
tissues called the vascular cambium and
cork cambium
The type of lateral meristematic tissue called
vascular cambium produces vascular
tissues and increases the thickness of
stems over time
Cork cambium produces the outer covering
of stems
Another kind of cambium enables roots to
grow thicker and branch
The addition of new tissue in these cambium
layers increases the thickness of the stem
Secondary Growth in a Dicot Stem
• Dicots produce secondary
growth from meristematic
tissue called vascular
cambium
• This tissue forms between the
xylem and phloem of the
individual vascular bundles, as
shown in A
• Once the tissue forms, as
shown in B, it divides to
produce xylem cells toward
the center of the stem and
phloem cells toward the
outside
• These different tissues form
the bark and wood of a mature
stem, shown in C
Secondary Growth in a Dicot Stem
Formation of the Vascular Cambium
• In a young dicot stem produced by primary growth,
bundles of xylem and phloem are arranged in a ring
• Once secondary growth begins, the vascular cambium
appears as a thin layer situated between clusters of
vascular tissue
• This new meristematic tissue forms between the xylem
and phloem of each vascular bundle
• Divisions in the vascular cambium give rise to new
layers of xylem and phloem
– As a result, the stem becomes wider
– The cambium continues to produce new layers of vascular
tissue, causing the stem to become thicker and thicker
Formation of Wood
•
•
•
•
Most of what we call “wood” is
actually layers of xylem
– These cells build up year after
year, layer on layer
As woody stems grow thicker, the
older xylem near the center of the
stem no longer conducts water and
instead becomes what is known as
heartwood
– Heartwood usually darkens with
age because it accumulates
impurities that cannot be
removed
Heartwood is surrounded by
sapwood, which is active in fluid
transport and therefore usually
lighter in color
Both heartwood and sapwood are
shown in the figure at left
Layers in a Mature Tree
• In a mature tree that has
undergone several years
of secondary growth, the
vascular cambium lies
between layers of xylem
to the inside, and layers
of phloem to the
outside
• The youngest xylem,
called sapwood,
transports water and
minerals
• Which layer contains
meristematic cells?
Layers in a Mature Tree
Formation of Wood
•
•
•
In most of the temperate zone, tree
growth is seasonal
When growth begins in the spring,
the vascular cambium begins to
grow rapidly, producing large, lightcolored xylem cells with thin cell
walls
– The result is a light-colored
layer of wood called early wood
As the growing season continues,
the cells become smaller and have
thicker cell walls, forming a layer of
dark wood
– This darker wood is called late
wood
Formation of Wood
• This alternation of dark and light wood produces
what we commonly call tree rings
– Each ring is composed of a band of light wood
and a band of dark wood
– Thus, a ring corresponds to a year of growth
– By counting the rings in a cross section of a tree, you
can estimate its age
• The size of the rings may even provide information
about weather conditions, such as wet or dry years
– Thick rings indicate that weather conditions were
favorable for tree growth, whereas thin rings indicate
less favorable conditions
Formation of Bark
•
•
•
•
•
•
•
•
•
On most trees, bark includes all of the
tissues outside the vascular cambium, as
shown in the figure at right
– These tissues include phloem, the
cork cambium, and cork
How does bark form?
Picture a tree as new xylem is being laid
down
It is expanding in width, or girth
Recall that the phloem tissue lies to the
outside of this xylem
Phloem must grow to accommodate the
larger size of the tree
As the vascular cambium increases in
diameter, it forces the phloem tissue
outward
This expansion causes the oldest tissues to
split and fragment as they are stretched by
the expanding stem
Were this expansion left unchecked, the
outer covering of the stem might eventually
split and break
Formation of Bark
• Another layer of growing tissue, the cork
cambium, solves this potential problem
• The cork cambium surrounds the cortex and
produces a thick protective layer of cork
• Cork consists of cells that have thick walls
and usually contain fats, oils, or waxes
• These waterproof substances help prevent
the loss of water from the stem
• The outermost cork cells are usually dead
• As the stem increases in size, this dead bark
often cracks and flakes off in strips or patches
Leaves
• The leaves of a plant are its main
organs of photosynthesis
– In a sense, plant leaves are the world's
most important manufacturers of food
• Sugars, starches, and oils manufactured
by plants in their leaves are sources of
food for virtually all land animals
Leaves
• Recall from Chapter 8 that photosynthesis uses
carbon dioxide and water to produce sugars and
oxygen
• Leaves, therefore, must have a way of obtaining
the materials needed for photosynthesis as well
as distributing its end products
• Much of the internal structure of leaves can be
understood in terms of their functions in carrying
out photosynthesis
LEAF
• Main organ of photosynthesis
– Uses carbon dioxide and water in the
presence of sunlight, enzymes, and
chlorophyll to produce sugar and oxygen
Leaf Structure
•
•
•
•
•
•
•
•
The structure of a leaf is optimized for
absorbing light and carrying out
photosynthesis
As you can see in the figure below, leaves
may differ greatly in shape, yet share certain
structural features
To collect sunlight, most leaves have thin,
flattened sections called blades
The blade is attached to the stem by a thin
stalk called a petiole
Like roots and stems, leaves have an outer
covering of dermal tissue and inner regions
of ground and vascular tissues
As shown in the figure at right, leaves are
covered on the top and bottom by epidermis
made of a layer of tough, irregularly shaped
cells
The epidermis of many leaves is also
covered by the cuticle
Together, the cuticle and epidermal cells
form a waterproof barrier that protects
tissues and limits the loss of water through
evaporation
Tissues in a Leaf
• Leaves absorb light and
carry out most of the
photosynthesis in a plant
• Some of the most important
manufacturing sites on Earth
are found in the leaves of
plants
• The cells in plant leaves are
able to use light energy to
make carbohydrates
• Compare the structure of the
different kinds of cells in a
leaf
Tissues in a Leaf
LEAF PARTS
• Node: region of the stem where the leaf is
attached
• Leaf:
– Two parts:
• Blade: flattened portion
• Petiole: stemlike portion that connects the leaf to the stem
• Bud: at the base of each petiole
• Veins: vascular bundles (xylem/phloem)
– Monocot: parallel
– Dicot: net
Leaf Shapes
• Most of a leaf
consists of a blade
attached to the stem
by a petiole. The
blade of a simple leaf
(left) can be different
shapes
• In a compound leaf
(right), the blade is
divided into many
separate leaflets
Leaf Shapes
Leaf Shapes
• The vascular tissues of leaves are connected
directly to the vascular tissues of stems,
making them part of the plant's transport
system
• In leaves, xylem and phloem tissues are
gathered together into bundles that run from
the stem into the petiole
• Once they are in the leaf blade, the vascular
bundles are surrounded by parenchyma and
sclerenchyma cells
LEAF SHAPE
• Toothed
• Smooth
• Lobed
LEAF ARRANGEMENT
• Opposite
• Whorled
• Alternate
SIMPLE/COMPOUND
LEAF
•
•
•
•
Simple
Pinnately compound
Palmately compound
Bipinnately compound
Leaf Functions
• A leaf can be considered a system
specialized for photosynthesis
• Subsystems of the leaf include tissues that
bring gases, water, and nutrients to the
cells that carry out photosynthesis
LEAF CROSS SECTION
• Epidermis:
– Protects the leaf from injury and desiccation
– Most cells do not contain contain chloroplast
– Waxy covering (cuticle) prevents water evaporation
• Amount varies from species to species
– Some contain epidermal hairs (extension of the cell)
which slow the rate of water evaporation
– Upper: thicker cuticle layer
– Lower: contain guard cells and stoma
Photosynthesis
• The bulk of most leaves
consists of a specialized
ground tissue known as
mesophyll, shown in the
figure at right
• Photosynthesis in most
plants occurs in the
mesophyll
• The carbohydrates produced
move into phloem vessels of
the transport system, which
carry them to the rest of the
plant
Tissues in a Leaf
Photosynthesis
•
•
•
•
•
•
A leaf has specialized cells that enable it to
carry out photosynthesis
Just under the epidermis is a layer of
mesophyll cells called the palisade
mesophyll
These closely packed cells absorb light that
enters the leaf
Beneath the palisade layer is the spongy
mesophyll, a loose tissue with many air
spaces between its cells
These air spaces connect with the exterior
through stomata (singular: stoma), porelike
openings in the underside of the leaf that
allow carbon dioxide and oxygen to diffuse
into and out of the leaf
Each stoma consists of two guard cells, the
specialized cells in the epidermis that control
the opening and closing of stomata by
responding to changes in water pressure
LEAF CROSS SECTION
•
Lower epidermis:
– Pair of guard cells ( containing chloroplast) surrounds a pore (stoma) that
opens and closes
• Size of opening depends on the shape of the guard cells (when open gases
move in and out)
• Guard Cell:
– Cell wall next to stoma is thick and inflexible
– Cell wall next to epidermis is thin and elastic
• During photosynthesis (sunny day) the guard cells become swollen with
water (turgid) pushing the thinner wall into the neighboring epidermal cell
and pulling the thicker walls away from each other thus opening the stoma
– Carbon dioxide enters
– Oxygen and water vapor exit
• When it is dark (no photosynthesis), the guard cells are less turgid (loss
water) and the stoma closes
– No oxygen nor carbon dioxide exchange
– Water is conserved
LEAF CROSS SECTION
• Palisade Mesophyll:
– Cells upright and parallel
• Maximum exposure to sunlight
– Main region of photosynthesis
• Spongy Mesophyll:
– Loosely packed
• Air pockets
– Gases exchange
Transpiration
• The surfaces of spongy mesophyll cells are kept
moist so that gases can enter and leave the cells
easily
• This also means that water evaporates from
these surfaces and is lost to the atmosphere
• Transpiration is the loss of water through its
leaves
• This lost water is replaced by water drawn into
the leaf through xylem vessels in the vascular
tissue
Gas Exchange
• Leaves take in carbon dioxide and give off
oxygen during photosynthesis
• When plant cells use the food they make, the
cells respire, taking in oxygen and giving off
carbon dioxide (just as animals do)
• Plant leaves allow gas exchange between air
spaces in the spongy mesophyll and the
exterior by opening their stomata
Gas Exchange
•
•
•
•
It might seem that stomata should
be open all the time, allowing gas
exchange to take place and
photosynthesis to occur at top
speed
This is not what happens!
– If stomata were kept open all the
time, water loss due to
transpiration would be so great
that few plants would be able to
take in enough water to survive
So, plants maintain a kind of balance
Plants keep their stomata open just
enough to allow photosynthesis to
take place but not so much that
they lose an excessive amount of
water
Opening and Closing of Stomata
• Plants regulate the opening
and closing of their stomata
to balance water loss with
rates of photosynthesis
• A stoma opens or closes in
response to the changes in
pressure within the guard
cells that surround the
opening
• When the guard cells are
swollen with water (left), the
stoma is open
• When the guard cells lose
water (right), the opening
closes, limiting further water
loss from the leaf
Opening and Closing of Stomata
Gas Exchange
•
•
•
•
•
Guard cells are epidermal cells found on
the undersides of leaves
They are structurally specialized to
control stomata and thus regulate the
movement of gases, especially water
vapor, into and out of leaf tissues
The stomata open and close in response
to changes in water pressure within the
guard cells, as shown in the figure at
right
– When water pressure within the
guard cells is high, the thin outer
walls of the cells are forced into a
curved shape
• This pulls the thick inner walls
of the guard cells away from
one another, opening the stoma
– When water pressure within the
guard cells decreases, the inner walls
pull together and the stoma closes
Guard cells respond to conditions in the
environment, such as wind and temperature,
helping to maintain homeostasis within a
leaf
Notice how the structure of guard cells,
which is quite different from the structure of
other epidermal cells, helps them to carry
out this task
Gas Exchange
• In general, stomata are open during the
daytime, when photosynthesis is active,
and closed at night, when open stomata
would only lead to water loss
• However, stomata may be closed even in
bright sunlight under hot, dry conditions in
which water conservation is a matter of life
and death
CHEMICAL ACTIVITY
• PHOTOSYNTHESIS
– Only during sunlight
• RESPIRATION
– 24 hours a day
Transport in Plants
• The pressure created by water entering the tissues
of a root can push water upward in a plant stem
• This creates more than enough pressure to force water
into the vascular system and out of the root
• However, root pressure does not exert enough
pressure to lift water up into trees, such as the
topmost needles of a redwood tree 90 meters above
the ground
• To draw water to such great heights, plants take
advantage of some of water's most interesting physical
properties
Water Transport
• Recall that xylem tissue forms a continuous set of tubes that stretch
from roots through stems and out into the spongy mesophyll of
leaves
• This set of tubes forms a complex transport system within a plant
• The transport is carried out by a subsystem of cells and tissues
• Active transport and root pressure cause water to move from soil
into plant roots
• Root pressure alone, however, cannot account for the
movement of water and dissolved materials throughout an
entire plant
– Obviously, other forces are at work
– These include capillary action and transpiration
• The combination of root pressure, capillary action, and
transpiration provides enough force to move water through the
xylem tissue of even the tallest plant
• As you will learn, transpiration is the most powerful of these forces
STEM TRANSPORT
• Transpiration (xylem: dead cells at
maturity)
• Transpiration-cohesion theory (xylem:
dead cells at maturity)
• Translocation: Pressure-flow hypothesis
(phloem: living cells)
Capillary Action
•
•
•
•
•
•
•
Water molecules are attracted to one
another by a force called cohesion
Recall from Chapter 2 that cohesion is the
attraction of molecules of the same
substance to each other
– Because of cohesion, water
molecules have a tendency to form
hydrogen bonds with each other
Water molecules can also form hydrogen
bonds with other substances
– This results from a force called
adhesion, which is attraction
between unlike molecules
Place empty glass tubes of various widths
into a dish of water, as shown below, and
you will see both forces at work
The tendency of water to rise in a thin
tube is called capillary action
Water is attracted to the walls of the tube,
and water molecules are attracted to one
another
The thinner the tube, the higher the water
will rise inside it
Capillary Action
• Capillary action—the
result of water
molecules' ability to
stick to one another
and to the walls of a
tube—contributes to
the movement of water
up the cells of xylem
tissue
• As shown here, capillary
action causes water to
move much higher in a
narrow tube than in a
wide tube
Capillary Action
Capillary Action
• What does capillary action
have to do with water
movement through xylem?
• Recall that there are two main
types of xylem tissue in
flowering plants: tracheids and
vessel element
• Both tracheids and vessel
elements form hollow
connected tubes similar to a
thin, glass capillary tube
• Capillary action in the tubelike
structures formed by both
types of cells causes water to
rise well above the level of the
ground
Transpiration
•
•
•
•
For trees and other tall plants, the
combination of root pressure and
capillary action does not provide enough
force to lift water to the topmost
branches and leaves
The major force in water transport is
provided by the evaporation of water
from leaves during transpiration
– When water is lost through
transpiration, osmotic pressure
moves water out of the vascular
tissue of the leaf, as shown in the
figure
Then, like a locomotive pulling a train with
hundreds of cars, the movement of water
out of the leaf “pulls” water upward through
the vascular system all the way from the
roots
This process is known as transpirational
pull
Movement of Water Through
Plant
• Root pressure, capillary
action, and
transpiration contribute
to the movement of
water within a plant
• Transpiration is the
movement of water
molecules out of leaves
• The faster water
evaporates from a
plant, shown in A, the
stronger the pull of
water upward from the
roots, shown in B
Movement of Water Through
Plant
Transpiration
• How important is transpirational pull?
• On a hot day, even a small tree may lose
as much as 100 liters of water to
transpiration
• The hotter and drier the air, and the
windier the day, the greater the amount of
water lost
• As a result of this water loss, the plant
draws up even more water from the roots
TRANSPIRATION
•
•
Water eventually enters the leaf from the stem, where it is used in
photosynthesis and cell metabolism
Water evaporates through the stomata openings of the lower epidermis of
the leaf:
– As water evaporates from the stoma, cells close to the stoma absorb
water from neighboring cells by osmosis. Those cells, in turn, absorb
water from more distant cells, until eventually water is absorbed from
the xylem in the veins (vascular bundles) of the leaf
– Water is pulled up the xylem
– Evaporated water molecules pull other water molecules after them
– Molecules of water move upward through the xylem
– Water column extends from the root to the stoma
• Column is held together by adhesion and cohesion
– Water molecules in the soil replace those that leave the plant through its
leaves (xylem vessels are never empty)
TRANSPIRATION-COHESION
THEORY
•
•
•
•
Water is pulled up the xylem in a continuous column that stretches from the
roots to the leaves
The forces of adhesion and cohesion make the thin column of water in the
xylem behave like a dense, tightly-pulled wire
Cohesion: attraction between molecules of the same kind
– Water molecules are polar
– Positive (H) of one water molecule attracts the negative (O) of the
neighboring water molecule and so on
– A long chain of water molecules is formed
– Water molecules throughout the entire xylem system are held together
by these cohesion forces
Adhesion: attraction of unlike molecule
– Water molecules adhere to the surface of the xylem vessels (like water
adhering to the glass/plastic of a graduated cylinder)
Controlling Transpiration
• The leaf's gas exchange subsystem helps to
maintain homeostasis by keeping the water content
of the leaf relatively constant
– For example:
• When water is abundant, it flows into the leaf, raising water
pressure in the guard cells, which then open the stomata
– Excess water is then lost through the open stomata by
transpiration
• When water is scarce, the opposite occurs
• Water pressure in the leaf falls, and the guard cells
respond by closing the stomata
• This reduces further water loss by limiting transpiration
Transpiration and Wilting
• Osmotic pressure keeps a plant's leaves and stems
rigid, or stiff
• High transpiration rates can lead to wilting
• Wilting results from the loss of water—and therefore
of the pressure in a plant's cells
• Without this internal pressure to support them, the
plant's cell walls bend inward, and the plant's leaves
and stems wilt
• When a leaf wilts, its stomata close
– As a result, transpiration slows down significantly
– Thus, wilting helps a plant to conserve water
Nutrient Transport
Functions of Phloem
• Many plants pump sugars into their fruits
• This action often requires moving sugars out of
leaves or roots into stems, and then through stems
to the fruits
– All of this movement takes place in the phloem
• In cold climates, many plants pump food down into their
roots for winter storage
– This stored food must be moved back into the trunk and
branches of the plant before growth begins again in the
spring
• Phloem carries out this seasonal movement of
sugars within a plant
Nutrient Transport
Movement From Source to Sink
• A process of phloem transport moves
sugars through a plant from a source to a
sink
• The source can be any cell in which
sugars are produced by photosynthesis
• The sink is a cell where the sugars are
used or stored
• How does phloem transport take place?
Nutrient Transport
Movement From Source to Sink
•
•
•
•
•
One idea put forward by many plant
scientists is called the pressure-flow
hypothesis
As you can see in the figure at right,
sugars are pumped into the phloem
at one point, called the source
For example, sugars produced by
photosynthesis may move from a leaf
As concentrations of sugar increase
in the phloem, water from the xylem
moves in by osmosis
This movement causes an increase
in pressure at that point, forcing
nutrient-rich fluid to move through
the phloem away from nutrientproducing regions and toward a
region that uses these nutrients,
called the sink
The Pressure-Flow Hypothesis
• The diagram shows the
movement of sugars and
water throughout the phloem
and xylem as explained by
the pressure-flow
hypothesis
• Materials move from a source
cell, where photosynthesis
produces a high
concentration of sugars, to a
sink cell, where sugars are
lower in concentration
• What is the source of the water
that forces nutrients through
phloem tissue?
The Pressure-Flow Hypothesis
TRANSLOCATION
• Movement of food (dissolved sugar) from one part of a plant to
another
• Pressure-flow hypothesis:
– Food is transported through the phloem (sieve tubes/sieve tube
elements/companion cells) as a result of differences in pressure
– When sugar molecules produced in a leaf enter a particular
sieve element, the concentration of water in that element is
lowered
• Water enters from neighboring cells by osmosis increasing
the pressure in the cell pushing the contents of the element
into the next element
• Energy is required
Nutrient Transport
Movement From Source to Sink
• Conversely, if part of a plant actively absorbs nutrients
from the phloem, osmosis causes water to follow
• This movement of water decreases pressure and causes
a movement of fluid in the phloem toward the sink
• When nutrients are pumped into or removed from
the phloem system, the change in concentration
causes a movement of fluid in that same direction
• As a result, phloem is able to move nutrients in
either direction to meet the nutritional needs of the
plant