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25–1 Hormones and Plant GrowthA. 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

Section 25-1

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Section 25-1

Figure 25-2 Hormone Action on Plants

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

25–1 Hormones and Plant Growth - ContinuedC.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

Section 25-1

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Section 25-1

Control Tipremoved

Opaquecap

Clearcap

Opaque shiedover base

Highconcentration

of auxin

Lowconcentration

of auxin

Figure 25–3  Auxins and Phototropism

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Section 25-1

Apical meristem

Lateral buds

Apical meristem removed

Auxins produced in the apical meristeminhibit the growth of lateral buds.

Without the inhibiting effect of auxinsfrom the apicial meristem, lateral budsproduce many branches.

Figure 25–5 Apical Dominance

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25–2 Plant ResponsesA. 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.

Section 25-2

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Tropisms

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Section Outline

25–2 Plant Responses - continuedC.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.

Section 25-2

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Section 25-2

Midnight

Noon

Long Day

Midnight

Noon

Short Day

Midnight

Noon

Interrupted Night

Short-Day Plant Long-Day Plant

Photoperiodism and Flowering

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25–3 Plant AdaptationsA. 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.

Section 25-3

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Plant Adaptations

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25–3 Plant Adaptations - continuedD.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

Section 25-3

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Section 25-3

Comparing Carnivorous Plants, Epiphytes, and ParasitesComparing Carnivorous Plants, Epiphytes, and Parasites

Characteristics

Environment

Method of obtaining nutrients

Examples

Characteristics

Environment

Method of obtaining nutrients

Examples

Carnivorous Plants

bog

leaves that trap and digest insects

pitcher plant, sundew, Venus’ flytrap

Carnivorous Plants

bog

leaves that trap and digest insects

pitcher plant, sundew, Venus’ flytrap

Epiphytes

host plant

gather moisture from rainfall and produce their own food

Spanish moss, orchid

Epiphytes

host plant

gather moisture from rainfall and produce their own food

Spanish moss, orchid

Parasites

host plant

extract moisture and nutrients from host plant

dodder, mistletoe

Parasites

host plant

extract moisture and nutrients from host plant

dodder, mistletoe

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