Title: Plants and tropisms 13 th March 2014

1. Title: Plants and tropisms13th March 2014 Learning question: How do plants use hormones to respond to stimuli? Homework: Study for homeostasis test on Thursday Starter: • Key words… • Tropism • Positive phototropism • Negative phototropism • Geotropism • Auxins • gibberellins

2. Tropisms • Plants do not move actively from place to place • They do not possess muscle or a nervous system, but they do respond to: • Light (phototropism) • Gravity (geotropism) • Changes in temperature • Contact • These tend to initiate very small changes in growth or direction of growth. This is called tropisms.

3. Responses of different parts of the plant

7. Tropisms • A 'tropism' is a growth in response to a stimulus. • Plants grow towards sources of waterand light, which they need to survive and grow. • Auxinis a plant hormone produced in the stem tips and roots, which controls the direction of growth.

8. Auxins and phototropism Auxins are produced at the shoot tips . They make the cells here elongate. Causes plants to grow towards a light source

9. Auxins and gravitropism • Auxins in root tips have the opposite effect to those in shoot tips – they stop elongation of cells. • Encourages positive geotropism • Helps roots to anchor the plant in place

10. Seed structure Provides a source of energy for the growing plant Part of the seed that grows into a new plant Protects the seed before germination takes place

11. Seed Germination • Germinationis the development of the embryo root and shoots into a young plant. • During germination, the food reserves in the seed are used up to provide the energy for the growth of the root and the shoot.

12. Gibberellins • Another plant hormone called gibberellins causes starch stored in seeds to ne broken down into sugars. • Sugar is then used for respiration so that the root and shoots can grow • Gibberellins also stimulate flower and fruit production in some plants

13. Task • Complete questions 1 – 6 on pages 62-63 in your class book

14. Using plant hormones – Higher tier Weedkillers • Selective weedkillers kill some plants but not others. • This can be useful for getting rid of dandelions in a lawn without killing the grass • The selective weedkiller contains growth hormone that causes the weeds to grow too quickly. • The weedkiller is absorbed in larger quantities by the weeds than the beneficial plants.

15. Rooting powder • Rooting powder makes stem cuttings quickly develop roots. • Rooting powder contains growth hormones.

16. Controlling fruit ripening • Some hormones slow the ripening of fruits and others speed it up. • These hormones and their inhibitors are useful for delaying ripening during transport or when fruit is displayed in shops.

17. Seedless fruit • Some seedless fruit are produced using plant hormones. • The flowers are sprayed with plant hormones that cause fruits to develop but not their seeds • Other plants, like some varieties of grape, have naturally seedless but small fruits. The fruits are sprayed with gibberellins to increase their size

18. Dormancy • Dormancy stops seeds germinating until conditions are ideal for growth. • Hormones can be used to remove the dormancy of a seed so it can germinate at all times of year. • Buds and flowers can also be naturally dormant. • Hormones can also be used to make plants grow bushier, make them flower or control the growth of hedge plants.

19. Task • Complete questions 1 – 6 on pages 64-65 in your class book