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 Meristems are found at places where plants grow rapidly—therapidly—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 The answers to these questions involve the actions of chemicals that direct, control, and regulate plant growthchemicals that direct, control, and regulate plant growth

Plant Hormones

• In plants, the division, growth, maturation, and development of cells are controlled by a are controlled by a group of chemicals called hormonesgroup 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• 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 To respond to a hormone, the target cell must contain a hormone receptor—must contain a hormone receptor—usually a protein—to which the hormone usually a protein—to which the hormone bindsbinds

• 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

• PhototropismPhototropism 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  • 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 lateral buds is inhibited by auxinsinhibited 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 dominanceapical 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 If you snip off the tip of a plant, the side branches

begin to grow more quickly, resulting in a begin to grow more quickly, resulting in a rounder, fuller plantrounder, 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 mimicmimic 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 (2,4-

dichlorophenoxyacetic acid), which is used to kill weeds

• A mixture containing 2,4-D was used as Agent A mixture containing 2,4-D was used as Agent Orange, a chemical defoliant sprayed during the Orange, a chemical defoliant sprayed during the Vietnam WarVietnam 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 Cytokinins often produce effects opposite to those of auxinsof 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 show that the rate of cell growth in most plants is determined by the ratio of thein most plants is determined by the ratio of the concentration of auxins to cytokininsconcentration 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 produced a growth-promoting substance that was named gibberellinwas 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 Gibberellins produce dramatic increases in size, particularly in stems and fruitsize, 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 ethyleneethylene, 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 ripenEthylene 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 Just before they are delivered to market, the fruits

are treated with synthetic ethylene to produce a are treated with synthetic ethylene to produce a ripe color quicklyripe color quickly

• This trick does not always produce a ripe This trick does not always produce a ripe flavor, which is one reason why naturally flavor, which is one reason why naturally ripened fruits often taste much betterripened 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 stimuliexternal stimuli are called tropismstropisms, from a Greek word that means “turning”– Plant tropisms include gravitropismgravitropism,

phototropismphototropism, and thigmotropismthigmotropism

• 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 plantsshort-day plants

• Plants such as spinach and irises flower when days are long and are therefore known as long-long-day plantsday 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 darknessperiods 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   • 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 plantsLong-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 is responsible for photoperiodismphotoperiodism

• 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 The change in the relative amounts of these two hormones starts a series of events that gradually hormones starts a series of events that gradually shut down the leaf 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 Finally, an abscission layer of cells at the petiole seals the leaf off from the plant's vascular systemseals 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 Before long, the leaf falls to the ground, a sign that the tree is fully prepared for winterthe 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 Enclosed in its coat of scales, a terminal bud can survive

the coldest winter daysthe 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 These molecules act like antifreeze in a car, preventing the

tree's sap from freezing, thus making it possible to survive tree's sap from freezing, thus making it possible to survive the bitter coldthe 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?How is this possible?

– Through natural selectionnatural 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 air-filled 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 The leaves of these plants have specialized cells that pump salt out of the plant tissues and onto the leaf surfaces, salt out of the plant tissues and onto the leaf surfaces, where it is washed off by rainwhere 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   • 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 where there is very little or no nitrogen presentvery 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 parasiteEpiphytes 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 plant-eating 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 nicotine is a natural insecticide that disrupts the nervous system insecticide that disrupts the nervous system of many insects, protecting tobacco plants of many insects, protecting tobacco plants from potential predatorsfrom potential predators