What Is Life? A Guide To BiologyFirst Edition

Jay Phelan

© 2011 W. H. Freeman and Company

CHAPTER 19Plants Respond to Their Environments

PLANTS RESPOND TO THEIR ENVIRONMENT

The leaves of the arrowleaf plant take on dramatically different forms depending on the environment in which the plant grows.

PLANTS RESPOND TO THEIR ENVIRONMENT

Three cuttings are taken from the same plant and grown in different environments.

Deep water Shallow water Land

The plant grows long, ribbon-like

leaves.

The plant grows large, round

leaves.

The plant grows arrow-shaped

leaves.

PLANT HORMONES

Gibberellins

HORMONE FUNCTION LOCATION

Increase the speed of seed germination; promote stem elongation; induce early blooming of flowers; increase fruit size

Stimulate stem elongation; control seedling orientation; stimulate root branching; promote fruit development

Increases the speed at which fruit ripens; stimulates leaf dropping and the death of flowers

Inhibits growth and reproduction; inhibits seed germination; stimulates closure of stomata

Cause rapid cell division, in conjunction with auxin; induce seed germination; initiate new branches from lateral buds

Shoot and rootapical meristems;seeds

Apical meristems; immature plant tissue

All parts of the plant including the fruits

Leaves; fruits; root tips; seeds

Roots and fruits,primarily

Auxins

Ethylene

Abscisic acid

Cytokinins

THE EFFECTS OF GIBBERELLINS

THE EFFECTS OF GIBBERELLINS

SPEEDING SEED GERMINATIONGibberellins initiate the production of enzymes that help break down nutrients stored within the seed’s endosperm, allowing quicker and more efficient use of the seed’s energy reserves.

STEM ELONGATIONGibberellins affect stem elongation by increasing the distance between nodes, thus spacing the branch points farther apart.

INDUCING EARLY BLOOMING OF FLOWERSGibberellins can cause flower production in the absence of a triggering event from the external environment. ENLARGEMENT OF FRUITSSeedless grapes sprayed with large amounts of gibberellins grow larger and, due to the stem-elongation effects, have more space between the grapes on the bunch.

Gibberellins are powerful growth stimulators and, when applied in unnaturally large concentrations, can produce giant plants!

THE EFFECTS OF AUXINS

THE EFFECTS OF AUXINS

STIMULATE SHOOT ELONGATIONAuxins enhance the effect of gibberellins in shoot elongation.

CONTROL SEEDLING ORIENTATIONAuxins direct the growth of shoots and roots, making sure the correct ends are up and down.

STIMULATE ROOT BRANCHINGAuxins induce the formation of roots.

PROMOTE FRUIT DEVELOPMENTAuxins produced within an embryo promote the maturation of the ovary wall and development of the fruit.

AUXINS’ INFLUENCE ON PLANT ORIENTATION

Sunlight Auxin molecules

Auxins are produced near the growing tips of shoot, roots, and branches.

The auxin molecules are directed downward by gravity and move away from light.

In regions of higher auxin concentration, cells elongate more rapidly than in regions of lower auxin concentration, causing the shoot to bend toward the light.

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ETHYLENE

Bananas picked before they are ripe are exposed to ethylene gas just prior to their delivery to market, initiating the ripening of all the fruit simultaneously.

Bananas picked before they are ripe are exposed to ethylene gas just prior to their delivery to market, initiating the ripening of all the fruit simultaneously.

Some flower merchants briefly soak cut-flower stems in a chemical solution of silver salts, which inhibits the deteriorating effects of ethylene on a flower’s petals.

Some flower merchants briefly soak cut-flower stems in a chemical solution of silver salts, which inhibits the deteriorating effects of ethylene on a flower’s petals.

ABSCISIC ACID

THE PRIMARY EFFECTS OF ABSCISIC ACID•Inhibits growth and reproductive activities when environmental conditions are stressful•Signals the stomata on a plant’s leaves to close, increasing water conservation

CYTOKININS

THE PRIMARY EFFECTS OF CYTOKININS•Cause rapid cell division in conjunction with auxins•Induce seed germination•Initiate new branches from lateral buds•Retard leaf death

PHOTOTROPISM

PHOTOTROPISM

Auxins produced in the plant move away from the light source to the shaded side of a stem, stimulating a greater rate of growth than on the side with less auxin. The uneven growth causes the plant to bend toward the light.

Auxins produced in the plant move away from the light source to the shaded side of a stem, stimulating a greater rate of growth than on the side with less auxin. The uneven growth causes the plant to bend toward the light.

Auxin molecules

GRAVITROPISM

Starches within the cells of the stem sink downward in response to gravity, triggering the movement of auxin toward them. Auxin then stimulates faster growth in the regions where it occurs in higher concentration, causing the stem to bend upward.

Starches within the cells of the stem sink downward in response to gravity, triggering the movement of auxin toward them. Auxin then stimulates faster growth in the regions where it occurs in higher concentration, causing the stem to bend upward.

GRAVITROPISM

Auxin molecules

Climbing plants produce tendrils, which are specialized thread-like leaves or stems that wrap around whatever they touch.

Climbing plants produce tendrils, which are specialized thread-like leaves or stems that wrap around whatever they touch.

THIGMOTROPISM

THE BIOLOGICAL CLOCK IN PLANTS

Sunrise Noon Sunset Midnight

Plants have internal methods of keeping time—influenced by the external environment—that enable them to initiate various actions at the appropriate time.

When it comes to producing flowers—an energetically expensive task—a plant’s life can depend on choosing the right moment.

PHOTOPERIODISM

LONG-DAY PLANTSFlower production is triggered by shorter periods of darkness (generally in spring).

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All flowering plants fall into one of three categories when it comes to regulating their flower production.

PHOTOPERIODISM

SHORT-DAY PLANTSFlower production is triggered by longer periods of darkness (generally in late summer or fall).

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All flowering plants fall into one of three categories when it comes to regulating their flower production.

Some plants are triggered to produce flowers when the length of the nights is long (and the amount of daylight is relatively small). Others are triggered when nights are shorter, and the daylight lasts longer.

PHOTOPERIODISM

DAY-NEUTRAL PLANTSFlower production is triggered by a sufficient state of maturity and not by periods of darkness.

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All flowering plants fall into one of three categories when it comes to regulating their flower production.

MECHANICAL DEFENSES IN PLANTS

THORNS, SPINES, AND HAIRSStructures such as sharp spines or fine hairs can significantly reduce herbivory.

MECHANICAL DEFENSES IN PLANTS

WAXES AND SAPSLeaf secretions such as slippery waxy compounds or sticky saps significantly reduce herbivory.

MECHANICAL DEFENSES IN PLANTS

DEFENSIVE MOVEMENTSRapid movements, such as flattening leaves in response to touch, can decrease available surface area and significantly reduce herbivory.

Monarch butterfly caterpillars feed on milkweed and are able to tolerate the cyanide-containing molecules produced by the plant that are toxic to most other insects. The caterpillars are then able to store the toxic chemical and, in turn, become poisonous to the animals that try to eat them.

Monarch butterfly caterpillars feed on milkweed and are able to tolerate the cyanide-containing molecules produced by the plant that are toxic to most other insects. The caterpillars are then able to store the toxic chemical and, in turn, become poisonous to the animals that try to eat them.

The chemical compounds produced by plants to reduce herbivory can also have medicinal effects in humans.

Auxins produced in the plant move away from the light source to the shaded side of a stem, stimulating a greater rate of growth than on the side with less auxin. The uneven growth causes the plant to bend toward the light.

Auxins produced in the plant move away from the light source to the shaded side of a stem, stimulating a greater rate of growth than on the side with less auxin. The uneven growth causes the plant to bend toward the light.

METHODS OF SURVIVINGDRY HABITATS

SUCCULENT LEAVES AND STEMSCacti and other succulent plants have thick, fleshy, water-storing tissue within their leaves and stems that helps minimize water loss due to evaporation.

METHODS OF SURVIVINGDRY HABITATS

DEEP TAPROOTSPlants such as mesquite send down unusually hardy and deep taproots that can utilize water far beneath the surface.

METHODS OF SURVIVINGDRY HABITATS

LONG-DORMANT SEEDSMany plants have seeds that can remain dormant for long periods of time, then quickly germinate and grow in response to brief periods of moisture.

Mangroves are able to transport much of the salt absorbed through their roots and excrete it through their leaves. The salt sits on the leaf surfaces until it dries and blows away.

Mangroves are able to transport much of the salt absorbed through their roots and excrete it through their leaves. The salt sits on the leaf surfaces until it dries and blows away.

Plants living in cold and windy habitats tend to grow close to the ground and have smaller-than- average leaves and shallow root systems.

PLANT HORMONESHORMONE FUNCTION LOCATION

Gibberellins

Auxins

Ethylene

Abscisic acid

Cytokinins