Warm-Up #31 Complete the Analyzing Data #1-4 on text

page 637.

Hormones and Plant Growth and Plant Responses

Objectives Describe patterns of plant growth. Explain what plant hormones are. Describe how auxins, cytokinins, gibberellins

and ethylene affect plant growth. Explain what plant tropisms are.

Patterns of Plant Growth 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.

Unlike animals, plants grow throughout their lifetime.

Plant growth occurs in the meristems. Meristems are found at places where plants

grow rapidly—the tips of growing stems and roots, and along the outer edges of woody tissues that produce new growth every year.

Plant Hormones How do plants know when and where to grow? Plant hormones are chemical substances that

control a plant’s patterns of growth and development, and the plant’s responses to environmental conditions.

Just like in animals, target cells for particular hormones have specific receptors for that hormone.

The Hormones Auxins—produced in the apical meristem and are

transported downward into the rest of the plant—stimulate cell elongation.

Cytokinins—produced in growing roots and in developing fruits and seeds--stimulate cell division and the growth of lateral buds—cause dormant seeds to sprout.

Gibberellins—produce dramatic increases in size, particularly in stems and fruit.

Ethylene—produced in fruits, aging leaves—in response to auxins, fruits release a small amount that stimulates the fruits to ripen. Controls aging of leaves.

Auxins and Phototropism Darwin and his son Francis

—phototropism Conclusion--Auxins are

produced in the apical meristem and are transported downward into the rest of the plant.

They stimulate cell elongation—cells on the shaded side develop more auxin than the other side. Cells on that side get longer causing the plant to bend toward the light.

Auxins and Gravitropism Auxins build up on lower sides of roots and

stems. In stems, auxins stimulate cell elongation

helping turn the trunk upright. In roots, the opposite happens. Auxins in the

roots inhibit cell growth and elongation causing the roots to grow downward.

Auxins and Branching As a stem grows in length, it produces lateral

buds (meristematic tissue on the side of stem that gives rise to side branches).

Lateral buds that are close to the apical meristem are inhibited from growing. (apical dominance)

If you snip off the tip of a plant, this inhibition is removed and the side branches will begin to grow creating a rounder fuller plant.

Cytokinins Stimulate cell division and the growth of

lateral buds Cause dormant seeds to sprout Delay aging of leaves Opposite of auxin

Inhibit cell elongation/cause cells to grow thicker Stimulate lateral buds Ratio of auxin to cytokinin determines how the

plant grows

Gibberellins Produce dramatic increases in size,

particularly in stems and fruit. Also produced by seed tissue and are

responsible for rapid early growth of plants

Ethylene In response to auxins, fruit tissues release a

small amount of ethylene that causes fruit to ripen.

Leaf Abscission—in the fall auxin production drops off, but production of ethylene increases. Shuts down the leaf for winter.

Plant Responses Tropisms—responses of plants to external

environment Gravitropism—response of a plant to gravity

(controlled by auxin) Phototropism—response of a plant to light

(controlled by auxin) Thigmotropism—response of plants to touch

Some plants can be stunted in growth if touched too much

When the tip a vine encounters an object it wraps around it

Summary What are plant hormones? What are auxins? What are cytokinins? What are gibberelins? What is ethylene? What is phototropism? What is gravitropism? What is thigmotropism?

Seed and Fruit Lab

Homework Homework Packets

Warm-Ups #27-32—12 points Study Guide—8 points Leaf Lab—5 points Flower/Fruit/Seed Lab—8 points Extra Credit—on back of warm-up #28—2 points

to test Study for Test Study Guide 99-100