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Jay Phelan
What Is Life? A Guide To Biology
First Edition
CHAPTER 19
Plants Respond to Their Environments
© 2011 W. H. Freeman and Company
PLANTS RESPOND TO THEIR ENVIRONMENT
PLANTS RESPOND TO THEIR ENVIRONMENT
Three cuttings are taken from the same plant
and grown in different environments.
Deep water
The plant grows
long, ribbon-like
leaves.
Shallow water
The plant grows
large, round
leaves.
Land
The plant grows
arrow-shaped
leaves.
 The leaves of the arrowleaf plant take on
dramatically different forms depending on
the environment in which the plant grows.
PLANT HORMONES
HORMONE
FUNCTION
LOCATION
Increase the speed of seed germination;
promote stem elongation; induce early
blooming of flowers; increase fruit size
Shoot and root
apical meristems;
seeds
Stimulate stem elongation; control
seedling orientation; stimulate root
branching; promote fruit development
Apical meristems;
immature plant
tissue
Increases the speed at which fruit
ripens; stimulates leaf dropping and the
death of flowers
All parts of the
plant including the
fruits
Inhibits growth and reproduction;
inhibits seed germination; stimulates
closure of stomata
Leaves; fruits; root
tips; seeds
Gibberellins
Auxins
Ethylene
Abscisic acid
Cause rapid cell division, in conjunction
with auxin; induce seed germination;
initiate new branches from lateral buds
Cytokinins
Roots and fruits,
primarily
THE EFFECTS OF GIBBERELLINS
THE EFFECTS OF GIBBERELLINS
SPEEDING SEED GERMINATION
Gibberellins 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 ELONGATION
Gibberellins affect stem elongation by
increasing the distance between nodes,
thus spacing the branch points farther
apart.
INDUCING EARLY BLOOMING OF
FLOWERS
Gibberellins can cause flower
production in the absence of a triggering
event from the external environment.
ENLARGEMENT OF FRUITS
Seedless 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
ELONGATION
Auxins enhance the effect of
gibberellins in shoot elongation.
CONTROL SEEDLING
ORIENTATION
Auxins direct the growth of
shoots and roots, making sure
the correct ends are up and
down.
STIMULATE ROOT
BRANCHING
Auxins induce the formation
of roots.
PROMOTE FRUIT
DEVELOPMENT
Auxins produced within an
embryo promote the maturation
of the ovary wall and
development of the fruit.
AUXINS’ INFLUENCE ON PLANT ORIENTATION
1
Sunlight
Auxin
molecules
2
3
1
Auxins are produced near the growing tips of shoot,
roots, and branches.
2
The auxin molecules are directed downward by gravity
and move away from light.
3
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.
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.
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
Auxin
molecules
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.
GRAVITROPISM
GRAVITROPISM
Auxin
molecules
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.
THIGMOTROPISM
Climbing plants produce tendrils,
which are specialized thread-like
leaves or stems that wrap
around whatever they touch.
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
Amount of daylight
All flowering plants fall into one of three categories when it
comes to regulating their flower production.
J
F
M
A
M
J
J
A
S
O
N
Month
LONG-DAY PLANTS
Flower production is triggered by shorter periods of
darkness (generally in spring).
D
PHOTOPERIODISM
Amount of daylight
All flowering plants fall into one of three categories when it
comes to regulating their flower production.
J
F
M
A
M
J
J
A
S
O
N
Month
SHORT-DAY PLANTS
Flower production is triggered by longer periods of
darkness (generally in late summer or fall).
D
PHOTOPERIODISM
Amount of daylight
All flowering plants fall into one of three categories
when it comes to regulating their flower production.
J
F
M
A
M
J
J
Month
A
S
O
N
D
DAY-NEUTRAL PLANTS
Flower production is triggered by a sufficient state of
maturity and not by periods of darkness.
 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.
MECHANICAL DEFENSES
IN PLANTS
THORNS, SPINES, AND HAIRS
Structures such as sharp spines or
fine hairs can significantly reduce
herbivory.
MECHANICAL DEFENSES
IN PLANTS
WAXES AND SAPS
Leaf secretions such as slippery
waxy compounds or sticky saps
significantly reduce herbivory.
MECHANICAL DEFENSES
IN PLANTS
DEFENSIVE MOVEMENTS
Rapid 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 cyanidecontaining 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.
METHODS OF SURVIVING
DRY HABITATS
SUCCULENT LEAVES AND STEMS
Cacti 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 SURVIVING
DRY HABITATS
DEEP TAPROOTS
Plants such as mesquite send down
unusually hardy and deep taproots
that can utilize water far beneath the
surface.
METHODS OF SURVIVING
DRY HABITATS
LONG-DORMANT SEEDS
Many 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.
 Plants living in cold and windy habitats tend to
grow close to the ground and have smaller-thanaverage leaves and shallow root systems.
PLANT HORMONES
HORMONE
Gibberellins
Auxins
Ethylene
Abscisic acid
Cytokinins
FUNCTION
LOCATION