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Section 25-1
25–1 Hormones and Plant Growth
A. Patterns of Plant Growth - meristems are the source of
plant growth. Plants never stop growing.
B. Plant Hormones - are chemical substances that controls a
plant’s patterns of growth and development, and the plant’s
responses to environmental conditions
The four main hormones are
1. Auxins
2. Cytokinins
3. Gibberellins
4. Ethylene
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Section:
Figure 25-2 Hormone Action on Plants
Section 25-1
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 one
part of the plant to control another
part of the plant.
The cells affected by
a hormone must
have a hormone
receptor
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Section Outline
Section 25-1
25–1 Hormones and Plant Growth - Continued
C.Auxins - are produced in the apical meristem and are
transported downward into the rest of the plant. They stimulate
cell elongation.
D.Cytokinins - stimulate cell division and the growth of lateral
buds, and cause dormant seed to sprout
E.Gibberellins - produce dramatic increase in size, particularly in
stems and fruit
F. Ethylene - stimulates fruit to ripen
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Figure 25–3 Auxins and Phototropism
Section 25-1
High
concentration
of auxin
Low
concentration
of auxin
Control
Tip
removed
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Section:
Opaque
cap
Clear
cap
Opaque shied
over base
Figure 25–5 Apical Dominance
Section 25-1
Apical meristem
Lateral buds
Auxins produced in the apical meristem
inhibit the growth of lateral buds.
Apical meristem removed
Without the inhibiting effect of auxins
from the apicial meristem, lateral buds
produce many branches.
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Section Outline
Section 25-2
25–2 Plant Responses
A. Tropisms - A plant’s ability to respond to external stimuli.
1. Gravitropism - a plants response to the force of gravity.
Controlled by auxins
2. Phototropism - the tendency of a plant to grow toward a source of
light. Auxins stimulate the elongation of stem cells on the dark side
to elongate and the plant bends toward the light
3. Thigmotropism - a plants response to touch. Vines and climbing
plants like grapes have tendrils that wrap tightly around objects
they encounter
B. Rapid Responses - movement occurs from rapid changes in
osmotic pressure. An example of a rapid change is the closing of a
Venus fly trap leaf to capture a insect.
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Tropisms
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Section Outline
Section 25-2
25–2 Plant Responses - continued
C.Photoperiodism - is responsible for the timing of seasonal activities
such as flowering and growth.
D.Winter Dormancy - 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.
1. Leaf Abscission - photosynthesis stops and the green pigments
are destroyed revealing the other colors that were there all along.
Most water and available nutrients are removed from the leaf, and
an abscission layer forms at the base of the leaf.
2. Overwintering of Meristems - produce thick waxy scales over the
new leaf buds for the next spring. Xylem and phloem tissues pump
themselves full of ions and organic compounds to act as an
antifreeze during the cold winter.
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Photoperiodism and Flowering
Section 25-2
Short-Day Plant Long-Day Plant
Midnight
Noon
Long Day
Midnight
Noon
Short Day
Midnight
Noon
Interrupted Night
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Section:
Section Outline
Section 25-3
25–3 Plant Adaptations
A. Aquatic Plants - to take in sufficient oxygen, many aquatic plants
have tissues with large air filled spaces through which oxygen can
diffuse. Other adaptations include seeds that float and quick growth
following germination to allow the shoot to reach the surface of the
water.
B. Salt-Tolerant Plants - take in more salt than the plant can use to
allow the roots to still absorb water by osmosis. The roots have
adapted to tolerate the salt conditions that would destroy most plants.
Excess salt is pumped out of the plant onto the leaves where it is
washed away by the rain.
C. Desert Plants - adaptations to a desert climate include extensive
roots, reduces leaves, and thick stems that can store water. The
spines on a cactus are actually modified leaves. Many seeds of
desert plants can remain dormant for years.
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Plant Adaptations
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Section Outline
Section 25-3
25–3 Plant Adaptations - continued
D. Nutritional Specialists - have specialized features for obtaining
nutrients.
1. Carnivorous Plants - many live in bogs where there is little or no
nitrogen present in the soil. Plants that live in these type of
ecosystems have specialized leaves to capture and digest insects
for a source of nitrogen. Ex) Venus fly trap and picture plant
2. Parasites - plants that extract water and nutrients directly from a
host plant.
E. Epiphytes - plants that are not rooted in the soil but instead grow
directly on the bodies of other plants. Epiphytes are not parasites they
gather their own moisture and produce their own food.
F. Chemical Defenses - many plants defend themselves against insect
attack by manufacturing compounds that have powerful affects on
animals
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Compare/Contrast Table
Section 25-3
Comparing Carnivorous Plants, Epiphytes, and Parasites
Characteristics
Carnivorous
Plants
Epiphytes
Parasites
Environment
bog
host plant
host plant
Method of
obtaining
nutrients
leaves that trap
gather moisture
and digest insects from rainfall and
produce their own
food
extract moisture
and nutrients from
host plant
Examples
pitcher plant,
sundew, Venus’
flytrap
dodder, mistletoe
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Spanish moss,
orchid
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