1. Warm-Up #31  Complete the Analyzing Data #1-4 on text page 637.

2. Hormones and Plant Growth and Plant Responses

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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

11. 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

12. 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.

14. 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

15. 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?

16. Seed and Fruit Lab

17. 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