Download Plant Hormones

Plant Responses and Adaptations
• In what may be its last
moments, an ant peers
down into a pitcher
plant's specialized leaf
• The leaf is lined with
slippery hairs and is filled
with digestive enzymes
that will extract nutrients
from any unsuspecting
prey
Plant Responses and Adaptations
Hormones and Plant Growth
• Unlike most animals, plants do not have a rigidly
set organization to their bodies
• Cows have four legs, ants have six, and spiders
have eight; but tomato plants do not have a
predetermined number of leaves or branches
• However, plants show distinct patterns of
growth
• As a result, you can easily tell the difference
between a tomato plant and a corn plant,
between an oak tree and a pine tree
Patterns of Plant Growth
• Although plant growth is not determined
precisely, it still follows general patterns that
differ among species
• What controls these patterns of
development?
• Biologists have discovered that plant cells
send signals to one another that indicate
when to divide and when not to divide, and
when to develop into a new kind of cell
Patterns of Plant Growth
• There is another difference between growth in plants
and animals
– Once most animals reach adulthood, they stop growing
– In contrast, even plants that are thousands of years old
continue to grow new needles, add new wood, and produce
cones or new flowers, almost as if parts of their bodies
remained “forever young”
• As you have learned, the secrets of plant growth are
found in meristems, regions of tissue that can
produce cells that later develop into specialized
tissues
• Meristems are found at places where plants grow
rapidly—the tips of growing stems and roots, and along
the outer edges of woody tissues that produce new
growth every year
Patterns of Plant Growth
• If meristems are the source of plant growth, how is that growth
controlled and regulated?
– Plants grow in response to environmental factors such as light,
moisture, temperature, and gravity
• But how do roots “know” to grow down, and how do stems
“know” to grow up toward light?
• How do the tissues of a plant determine the right time of year to
produce flowers?
• How do plants ensure that their growth is evenly balanced—
that the trunk of a tree grows large enough to support the
weight of its leaves and branches?
• The answers to these questions involve the actions of
chemicals that direct, control, and regulate plant growth
Plant Hormones
• In plants, the division, growth, maturation,
and development of cells are controlled by a
group of chemicals called hormones
• A hormone is a substance that is produced in
one part of an organism and affects another
part of the same individual
• Plant hormones are chemical substances
that control a plant's patterns of growth and
development, and the plant's responses to
environmental conditions
PLANT HORMONES
• Chemicals that control the internal factors
of plant growth
• Organic compounds that are effective in
small concentrations
• Synthesized in one part of the plant and
transported to target tissue elsewhere in
the plant triggering a physiological
response
– Many hormones work together
Plant Hormones
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The general mechanism of hormone action
in plants is shown in the diagram
As you can see, the hormone moves
through the plant from the place where it
is produced to the place where it triggers
its response
The portion of an organism affected by a
particular hormone is known as its target
cell or target tissue
To respond to a hormone, the target cell
must contain a hormone receptor—
usually a protein—to which the hormone
binds
If the appropriate receptor is present, the
hormone can exert an influence on the
target cell by changing its metabolism,
affecting its growth rate, or activating the
transcription of certain genes
– Cells that do not contain receptors are
generally unaffected by hormones
Hormone Action in Plants
• Plant hormones are
chemical substances that
control patterns of
development as well as plant
responses to the
environment
• Hormones are produced in
apical meristems, in young
leaves, in roots, and in growing
flowers and fruits
• From their place of origin,
hormones move to other
parts of the plant, where
target cells respond in a way
that is specific to the
hormone
Hormone Action in Plants
Hormone Action in Plants
• Different kinds of cells may have
different receptors for the same
hormone
– As a result, a single hormone may affect
two different tissues in different ways
• For example:
– a particular hormone may stimulate growth
in stem tissues but inhibit growth in root
tissues
Auxins
• The experiment that led to the discovery of the
first plant hormone was carried out by Charles
Darwin
• In 1880, Darwin and his son Francis published a
book called The Power of Movement in Plants
• In this book, they described an experiment in
which oat seedlings demonstrated a
response known as phototropism
• Phototropism is the tendency of a plant to grow
toward a source of light
Auxins
• The activity Effect of Light on a Growing Plant shows an
experiment similar to the one carried out by the Darwins
• Notice that the tip of one of the oat seedlings was
covered with an opaque cap
– This plant did not bend toward the light, even though the rest of
the plant was uncovered
• However, if an opaque shield was placed a few
centimeters below the tip, the plant would bend
toward the light as if the shield were not there
– Clearly, something was taking place at the tip of the
seedling
AUXINS
• Hormones that regulate the growth of plant cells
– Stimulates/inhibits cell elongation depending on concentration
• Tropisms:
– Phototropism: response to light
– Geotropism (gavitropism): response to gravity
– Thigmotropism: response to touch
• Synthetic:
– Weed killers: 2,4-D
– Fruit harvest: naphthaleneacetic acid (NAA)
• Harvest fruit at sametime
• Stimulates root development
Auxins and Phototropism
• The Darwins suspected that the tip of each seedling produced
substances that regulated cell growth
– Forty years later, these substances were identified and named
auxins
• Auxins are produced in the apical meristem and are
transported downward into the rest of the plant
– They stimulate cell elongation
• When light hits one side of the stem, a higher concentration of
auxins develops in the shaded part of the stem
– This change in concentration stimulates cells on the dark side to
elongate
– As a result, the stem bends away from the shaded side and toward the
light
• Recent experiments have shown that auxins migrate toward the
shaded side of the stem, possibly due to changes in membrane
permeability in response to light
Auxins and Gravitropism
• Auxins are also responsible for gravitropism,
which is the response of a plant to the force
of gravity
– By mechanisms that are still not understood, auxins
build up on the lower sides of roots and stems
• In stems, auxins stimulate cell elongation,
helping turn the trunk upright, as shown in
photo
• In roots, however, the effects of auxins are
exactly the opposite
– There, auxins inhibit cell growth and elongation,
causing the roots to grow downward
Gravitropism in a Stem
• Auxins are
responsible for the
plant response called
gravitropism
• Auxins caused the tip
of this tree stem to
grow upright
Gravitropism in a Stem
Gravitropism in a Stem
• Auxins are also involved in the way roots
grow around objects in the soil
• If a growing root is forced sideways by an
obstacle such as a rock, auxins accumulate on
the lower side of the root
• Once again, high concentrations of auxins inhibit
the elongation of root cells
• The uninhibited cells on the top elongate more
than the auxin-inhibited cells on the bottom of
the root
• As a result, the root grows downward
Auxins and Branching
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Auxins also regulate cell
division in meristems
As a stem grows in length, it
produces lateral buds
– A lateral bud is a meristematic
area on the side of a stem that
gives rise to side branches
•
Most lateral buds do not start
growing right away
– The reason for this delay is that
growth at the lateral buds is
inhibited by auxins
– Because auxins move out from
the apical meristem, the closer a
bud is to the stem's tip, the more it
is inhibited
•
This phenomenon is called
apical dominance
Apical Dominance
• Apical dominance, shown
here, is controlled by
the relative amounts of
auxins and cytokinins
• During normal growth
(A), lateral buds are
kept dormant because
of the production of
auxins in the apical
meristem
• If the apical meristem is
removed (B), the
concentration of auxins
drops
Apical Dominance
Apical Dominance
• Although not all gardeners have heard of auxins,
most of them know how to overcome apical
dominance
– If you snip off the tip of a plant, the side branches
begin to grow more quickly, resulting in a rounder,
fuller plant
• Why does this happen?
– When the tip is removed, the apical meristem—the
source of the growth-inhibiting auxins—goes with
it
– Without the influence of auxins, meristems in the
side branches grow more rapidly, changing the
overall shape of the plant
Auxinlike Weed Killers
• Chemists have produced many compounds that
mimic the effects of auxins
• Because high concentrations of auxins inhibit growth,
many of these compounds are used as herbicides,
which are compounds that are toxic to plants
– Herbicides include a chemical known as 2,4-D (2,4dichlorophenoxyacetic acid), which is used to kill weeds
• A mixture containing 2,4-D was used as Agent
Orange, a chemical defoliant sprayed during the
Vietnam War
CYTOKININS
• Promote cell division
• Influence the development of root, stems,
and differentiation of xylem and phloem
Cytokinins
• Cytokinins are plant hormones that are
produced in growing roots and in
developing fruits and seeds
• In plants, cytokinins stimulate cell
division and the growth of lateral buds,
and cause dormant seeds to sprout
• Cytokinins also delay the aging of
leaves and play important roles in the
early stages of plant growth
Cytokinins
• Cytokinins often produce effects opposite to those
of auxins
– For example, auxins stimulate cell elongation, whereas
cytokinins inhibit elongation and cause cells to grow thicker
• Auxins inhibit the growth of lateral buds, whereas
cytokinins stimulate lateral bud growth
• Recent experiments show that the rate of cell growth
in most plants is determined by the ratio of the
concentration of auxins to cytokinins
• In growing plants, therefore, the relative concentrations
of auxins, cytokinins, and other hormones determine
how the plant grows
GIBBERELLINS
• Promote cell enlargement
– Increasing length between nodes in stem
• Elongation of stem
• Taller plant
• 65 different types
• Stimulates seed germination
• Promotes formation of seedless fruits
Gibberellins
• For years, farmers in Japan knew of a
disease that weakened rice plants by causing
them to grow unusually tall
• They called the disease the “foolish seedling”
disease
• In 1926, Japanese biologist Eiichi Kurosawa
discovered that this extraordinary growth
was caused by a fungus: Gibberella fujikuroi
• His experiments showed that the fungus
produced a growth-promoting substance that
was named gibberellin
Gibberellins
• Before long, other researchers had learned that
plants themselves produce more than 60
similar compounds, all of which are now
known as gibberellins
• Gibberellins produce dramatic increases in
size, particularly in stems and fruit
• Gibberellins are also produced by seed tissue
and are responsible for the rapid early growth of
many plants
Ethylene
• When natural gas was used in city street
lamps in the nineteenth century, people
noticed that trees along the street
suffered leaf loss and stunted growth
• This effect was eventually traced to
ethylene, one of the minor components
of natural gas
Ethylene
• Today, scientists know that plants produce
their own ethylene, and that it affects
plants in a number of ways
• In response to auxins, fruit tissues
release small amounts of the hormone
ethylene
• Ethylene then stimulates fruits to ripen
Ethylene
• Commercial producers of fruit sometimes
use this hormone to control the ripening
process
• Many crops, including lemons and tomatoes,
are picked before they ripen so that they can
be handled without damage to the fruit
– Just before they are delivered to market, the fruits
are treated with synthetic ethylene to produce a
ripe color quickly
• This trick does not always produce a ripe
flavor, which is one reason why naturally
ripened fruits often taste much better
Plant Responses
• Like all living things, plants respond to
changes in their environments
• Some biologists call these responses “plant
behavior,” which is a useful way of thinking
about them
• Plants generally do not respond as quickly as
animals do, but that does not make their
responses any less effective
• Some plant responses are so fast that even
animals cannot keep up with them!
Tropisms
• Plants change their patterns and directions of
growth in response to a multitude of cues
• The responses of plants to external stimuli
are called tropisms, from a Greek word that
means “turning”
– Plant tropisms include gravitropism,
phototropism, and thigmotropism
• Each of these responses demonstrates the
ability of plants to respond effectively to
external stimuli, such as gravity, light, and
touch
Gravitropism and Phototropism
• You have already read about gravitropism, the
response of a plant to gravity, and phototropism,
the response of a plant to light
• Both of these responses are controlled by
the hormone auxin
• Gravitropism causes the shoot of a germinating
seed to grow out of the soil—against the force of
gravity
• It also causes the roots of a plant to grow with
the force of gravity and into the soil
Gravitropism and Phototropism
• Phototropism causes a plant to grow
toward a light source
• This response can be so quick that young
seedlings reorient themselves in a matter
of hours
Thigmotropism in a Grapevine
• Plant tropisms include
gravitropism,
phototropism, and
thigmotropism
• One effect of
thigmotropism—growth
in response to touch—
is that plants curl and
twist around objects, as
shown by the stems of
this grapevine
Thigmotropism in a Grapevine
Rapid Responses
• Some plant responses do not involve growth
– In fact, they are so rapid that it would be a mistake to call them tropisms
• If you touch a leaf of Mimosa pudica, appropriately called the
“sensitive plant,” within only two or three seconds, its two
leaflets fold together completely
• The secret to this movement is changes in osmotic pressure
• Recall that osmotic pressure is caused by the diffusion of water into
cells
• The leaves are held apart due to osmotic pressure where the two
leaflets join
• When the leaf is touched, cells near the center of the leaflet
pump out ions and lose water due to osmosis
• Pressure from cells on the underside of the leaf, which do not
lose water, force the leaflets together
Rapid Responses
• The carnivorous Venus' flytrap also
demonstrates rapid responses
• When a fly triggers sensory cells on the
inside of the flytrap's leaf, electrical signals
are sent from cell to cell
• A combination of changes in osmotic
pressure and cell wall expansion causes
the leaf to snap shut, trapping the insect
inside
Photoperiodism
• To every thing there is a season
• Nowhere is this more evident than in the
regular cycles of plant growth
– Year after year, some plants flower in the spring,
others in summer, and still others in the fall
• Plants such as chrysanthemums and poinsettias
flower when days are short and are therefore
called short-day plants
• Plants such as spinach and irises flower when
days are long and are therefore known as longday plants
Photoperiodism
• How do all these plants manage to time their flowering
so precisely?
• In the early 1920s, scientists discovered that tobacco
plants flower according to the number of hours of light
and darkness they receive
• Additional research showed that many other plants
also respond to periods of light and darkness, a
response called photoperiodism
• This type of response is summarized in the figure
• Photoperiodism in plants is responsible for the
timing of seasonal activities such as flowering and
growth.
PHOTOPERIODISM
• Plant response to changes in day length
– Long-day plants: flower when exposed to
longer days (Spring/Summer)
– Short-day plants: flower when exposed to
shorter days (Fall)
– Day neutral plants: flowering not affected by
length of day (tomato, dandelion)
Effect of Photoperiod on Flowering
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Photoperiodism controls the
timing of flowering and
seasonal growth
The response of flowering, shown
here, is controlled by the
amount of darkness plants
receive
Short-day plants, such as
chrysanthemums, flower only
when exposed to an extended
period of darkness every night—
and thus a short period of light
during the day
Long-day plants, such as irises,
flower when exposed to a short
period of darkness or to a long
period of darkness interrupted by
a brief period of light
Effect of Photoperiod on Flowering
Effect of Photoperiod on Flowering
• It was later discovered that a plant pigment
called phytochrome is responsible for
photoperiodism
• Phytochrome absorbs red light and activates
a number of signaling pathways within plant
cells
• By mechanisms that are still not understood
completely, plants respond to regular changes in
these pathways
• These changes determine the patterns of a
variety of plant responses
Winter Dormancy
• Phytochrome also regulates the changes
in activity that prepare many plants for
dormancy as winter approaches
• Dormancy is the period during which
an organism's growth and activity
decrease or stop
Winter Dormancy
• The changes that prepare a plant for dormancy are important
adaptations that protect plants over the cold winter months
• As cold weather approaches, deciduous plants turn off
photosynthetic pathways, transport materials from leaves to
roots, and seal leaves off from the rest of the plant
• In early autumn, the shorter days and lower temperatures gradually
reduce the efficiency of photosynthesis
• With these changing conditions, the plant gains very little by keeping
its leaves alive
• In fact, the thin, delicate leaves produced by most flowering plants
would have little chance of surviving a tough winter, and their
continued presence would be costly in terms of water loss
Leaf Abscission
• In temperate regions, most flowering plants lose their
leaves during the colder months
• During the warm growing season, auxins are produced
in leaves
• At summer's end, the phytochrome in leaves
absorbs less light as days shorten and nights
become longer
• Auxin production drops, but the production of
ethylene increases
• The change in the relative amounts of these two
hormones starts a series of events that gradually
shut down the leaf
Leaf Abscission
• The chemical pathways for chlorophyll synthesis
stop first
• When light destroys the remaining green
pigment, other pigments that have been
present all along—including yellow and
orange carotenoids—become visible for the
first time
• Production of new plant pigments—the reddish
anthocyanins—begins in the autumn
• The brilliant colors of autumn leaves are a direct
result of these processes
Leaf Abscission
• Behind the scenes, enzymes extract nutrients from the
broken-down chlorophyll
• These nutrients are then transported to other parts of the
plant, where they are stored until spring
• Every available carbohydrate is transported out of the
leaf, and much of the leaf's water is extracted
• Finally, an abscission layer of cells at the petiole
seals the leaf off from the plant's vascular system
• The location of the abscission layer is shown in the
diagram
• Before long, the leaf falls to the ground, a sign that
the tree is fully prepared for winter
Leaf Abscission
• Deciduous plants
undergo changes in
preparation for winter
dormancy
• Photosynthetic
pathways in leaves shut
down
• An abscission layer of
cells forms at the petiole
to seal the leaf off from
the rest of the plant
• Eventually, the leaf falls
off.
Leaf Abscission
Overwintering of Meristems
• Hormones also produce important changes in apical
meristems
• Instead of continuing to produce leaves, meristems
produce thick, waxy scales that form a protective
layer around new leaf buds
– Enclosed in its coat of scales, a terminal bud can survive
the coldest winter days
• At the onset of winter, xylem and phloem tissues
pump themselves full of ions and organic
compounds
– These molecules act like antifreeze in a car, preventing the
tree's sap from freezing, thus making it possible to survive
the bitter cold
Plant Adaptations
• Flowering plants grow in a variety of biomes—in
deserts, savannas, and tundras—to name a few
• They also grow in various aquatic ecosystems, such
as ponds and streams
• Angiosperms can survive in many different locations
• How is this possible?
– Through natural selection they have evolved tolerances and
structural and physiological adaptations to meet the
conditions of each biome
• In this section, we explore how plants have become
adapted to various environments through evolutionary
change
Aquatic Plants
• Aquatic plants are able to tolerate mud that
is saturated with water and nearly devoid of
oxygen
• To take in sufficient oxygen, many aquatic
plants have tissues with large air-filled
spaces through which oxygen can diffuse
• In waterlilies there are large open spaces in
the long petioles that reach from the leaves
down to the roots at the bottom
– Oxygen diffuses from these open spaces into the
roots
Waterlilies
• Aquatic plants have airfilled spaces in their
tissues that allow for
the uptake and
diffusion of oxygen
• These waterlilies
transport oxygen from the
air to their roots through
large spaces in their
petioles
Waterlilies
Aquatic Plants
• Many other plants show similar adaptations
• Several species of mangrove trees grow in shallow water
along tropical seacoasts
• Mangroves tolerate this environment by means of
specialized air roots with air spaces in them, just like
waterlily stems
– These spaces conduct air down to the buried roots, allowing the
root tissues to respire normally
• Stately bald cypress trees thrive in freshwater swamps
in the southern United States
– These trees grow structures called knees, which protrude above
the water
– The knees bring oxygen-rich air down to the roots
Aquatic Plants
• The reproductive adaptations of aquatic
plants include seeds that float in water and
delay germination for long periods
• Many aquatic plants grow quickly after
germination, extending the growing shoot
above the water's surface
Salt-Tolerant Plants
• When plant roots take in dissolved minerals, a
difference in the concentration of water molecules is
created between the root cells and the surrounding
soil
– This concentration difference causes water to enter the root
cells by osmosis
• For plants that grow in salt water, such as
mangroves, this means taking in much more salt
than the plant can use
– The roots of salt-tolerant plants are adapted to salt
concentrations that would quickly destroy the root hairs on most
plants
– The leaves of these plants have specialized cells that pump
salt out of the plant tissues and onto the leaf surfaces,
where it is washed off by rain
Desert Plants
• Plants that live in the desert biome are called
xerophytes
• Xerophytes must tolerate a variety of extreme
conditions, including strong winds, daytime heat,
sandy soil, and infrequent rain
• Rainwater sinks rapidly through desert soils instead of
staying near the surface
• The hot, dry air quickly removes moisture from any wet
surface, making life difficult for plants
• Plant adaptations to a desert climate include
extensive roots, reduced leaves, and thick stems
that can store water
Desert Plants
• One familiar group of desert plants is the cactus
(family Cactaceae)
• Cactuses have root systems that either
spread out for long distances just beneath
the soil surface or that reach deep down into
the soil
• In addition, the roots have many hairs that
quickly absorb water after a rainstorm,
before the water sinks too deeply into the
soil
Cactuses
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Desert plants have evolved
different adaptations to survive
desert conditions
For example, the shallow root
systems of cactuses allow them to
pick up surface water
The deep taproots of the
mesquite tree and the sagebrush
collect underground water
Spines, which are found on many
desert plants, are actually reduced
leaves that carry out little or no
photosynthesis and, as a result,
lose little water
– Most of a plant's
photosynthesis is carried out in
its fleshy stem
Cactuses
Cactuses
• To reduce water loss due to transpiration, cactus
leaves have been reduced to thin, sharp spines
• Cactuses also have thick green stems that
carry out photosynthesis and are adapted to
store water
• The stems of cactuses swell during rainy periods
and shrivel during dry spells, when the plants
are forced to use up their water reserves
Desert Plants
• Seeds of many desert plants can remain
dormant for years, germinating only when
sufficient moisture guarantees them a
chance for survival
• Other desert plants have bulbs, tubers, or
other specialized stems that can remain
dormant for years
• When rain does come, the plants mature,
flower, and set seed in a matter of weeks or
even days, before the water disappears
Nutritional Specialists
• Some plants grow in environments that
have low concentrations of nutrients in the
soil
• Plants that have specialized features
for obtaining nutrients include
carnivorous plants and parasites
Carnivorous Plants
• Some plants live in bogs, wet and
acidic environments where there is
very little or no nitrogen present
• Because conditions are too wet and too
acidic, bacteria that cause decay
cannot survive
– Without these bacteria, neither plant nor
animal material is broken down into the
nutrients plants can use
Carnivorous Plants
• A number of plants that live in these habitats obtain
nutrients using specialized leaves that trap and
digest insects
• Pitcher plants drown their prey in pitcher-shaped leaves
that hold rainwater and digestive enzymes
• Sundews trap insects on leaf hairs tipped with sticky
secretions
• The best known of the carnivorous plants is the Venus'
flytrap
– This plant has leaf blades that are hinged at the middle
– If an insect touches the trigger hairs on the leaf, the leaf folds up
suddenly, trapping the animal inside
– Over a period of several days, the leaf secretes enzymes that
digest the insect and release nitrogen for the plant to use
Venus' Flytrap
• Plants that have specialized
features for obtaining
nutrients include
carnivorous plants and
parasites
• Carnivorous plants, such as
the Venus' flytrap, digest
insects—and occasionally
frogs—as a source of nutrients
• Parasites grow into the tissues
of their host plant and extract
water and nutrients, causing
harm to the host
Venus' Flytrap
Parasites
• Some plants extract water and nutrients directly
from a host plant
• Like all parasites, these plants harm their host
organisms and sometimes even pose a serious
threat to other species
• The dodder plant Cuscuta is a parasitic plant that has
no chlorophyll and thus does not produce its own food
– The plant grows directly into the vascular tissue of its host
– There, it extracts nutrients and water
• Mistletoe grows as a parasite on many plants, including
conifers in the western United States
Epiphytes
• Epiphytes are plants that are not rooted in soil but
instead grow directly on the bodies of other plants
– Most epiphytes are found in the tropical rain forest biome, but
they grow in other moist biomes as well
• Epiphytes are not parasite
– They gather their own moisture, generally from rainfall, and
produce their own food
• One of the most common epiphytes is Spanish moss
• This plant is actually not a moss at all but a member of
the bromeliad family
• Over half the species of orchids are epiphytes
Chemical Defenses
• Seed plants and insects have had such a
long relationship that each has had plenty of
time to adapt to the other
– The beginnings of the relationship are obvious—
plants represent an important source of food for
insects
• Plants, therefore, fall prey to a host of planteating insects
• Because plants cannot run away, you might
think that they are defenseless against insects
that are armed with biting and sucking structures
• But plants have their own defenses
Chemical Defenses
• Many plants defend themselves against
insect attack by manufacturing compounds
that have powerful effects on animals
– Some of these chemicals are poisons that can be
lethal when eaten
– Other chemicals act as insect hormones, disrupting
normal growth and development and preventing
insects from reproducing
• These chemicals include those used in aspirin,
codeine, and scores of other drugs that humans
use as medicines
Chemical Defenses
• As you may know, nicotine is a chemical that
is found in tobacco plants
• When a person smokes tobacco in the form of
cigarettes, the nicotine in the tobacco affects
the human nervous system
• Biologists hypothesize that nicotine is a natural
insecticide that disrupts the nervous system
of many insects, protecting tobacco plants
from potential predators