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Growth Control in Plants Learning Objectives • Explain tropisms—phototropism, geotropism/gravitropism and thigmotropism. What is a Hormone? What do hormones do? Can you think of any examples? Chemical Messengers Oestrogen Glucagon ADH Coordinate systems What is a Hormone? What do hormones do? Insulin Can you think of any examples? Testosterone Travel in the blood, secreted by glands HGH Causes a change in how the body works Control Growth, blood sugar, sex development, mood, metabolism. Hormones in Plants What do hormones control in plants? Anything Relating to Growth! Roots and Shoots Seed Germination Leaf Fall Disease Resistance Fruit Formation and Ripening Flowering Time Bud formation Tropisms “Directional growth response” Plants grow in a certain direction in response to a stimulus Let’s Explore Tropisms! Use your observations from the following to complete the table Tropism 1. Phototropism 2. Geotropism/ Gravitropism 3. Chemotropism 4. Thigmotropism 5. Hydrotropism Stimulus Response • Look at the picture. • What is happening? • The seedlings are bending toward the light! • An example of phototropism • Since they bend TOWARD the light, it is POSITIVE tropism. • What is happening here? • The seedling has been knocked over and its shoot is shown to have changed its direction of growth. • This is an example of geotropism/ gravitropism. The stimulus is gravity. The shoot part of the plant grow AWAY from gravity, so it is NEGATIVE tropism. • What is happening here? • The top picture shows a Venus flytrap capturing a fly. The stimulus is touch. • This shows thigmotropism. • Since it closes TOWARD the touch, it is POSITIVE. • The bottom picture also shows POSITIVE tropism as the vine touches a surface and winds around it. Tropism Stimulus Response 1. Phototropism Light Plant grows toward light 2. Geotropism/ Gravitropism Gravity Shoots grow away from gravity; roots grow toward gravity 3. Thigmotropism Touch Plant part either moves toward or away from what touched it What Causes Tropisms? • Hormone Auxin • Auxin promotes growth Don’t need to know for unit test – just thought it was fascinating!! What Causes Tropisms? • Shoots of plants contain Auxin A) When light shines directly above tip, growth is equal towards light source What Causes Tropisms? B)When light shines from an angle: • Sunlight breaks down Auxin • So the side of the tip that has the most direct sunlight will have the least amount of Auxin • Area of the plant that is more shaded will have more Auxin • More growth on shaded side • Lop sided growth & plant bends towards light Tropisms Montage! Match the correct scenario to the tropism. Roots Growing Downwards into Dark Soil Positive Phototropism Shoots Growing Upwards towards light. Negative Geotropism Shoots Growing Upwards Negative Phototropism Roots Growing Downwards. Positive Geotropism Weed killers Selective weed killers kill some plants but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass, or getting rid of thistles in a field without killing the wheat plants. The selective weed killer contains growth hormone that causes the weeds to grow too quickly. The weed killer is absorbed in larger quantities by the weeds than the beneficial plants Agent Orange Rooting Powders • Powder contains auxins • Cuttings are dipped in powder • Promotes root growth Fruit Ripening • Ethene – promotes fruit ripening • Naturally given off by ripe fruit • Converts starch into sugar Plenary Using Plant Hormones Auxiins cause Match the sentences upwards Hormone rooting powder from cell to cell in solution Cuttings can produce clones of the parent plant Hormone weed killers cell growth Shoots grow downwards Flowering can be induced a source of light Plant hormones can be used Roots grow Auxins can diffuse Plants grow towards to ripen fruit causes roots to grow by the use of plant hormones are not usually harmful to animals Using Plant Hormones Hormone rooting powder causes roots to grow Auxiins cause cell growth Cuttings can produce clones of the parent plant Hormone weed killers are not usually harmful to animals Shoots grow upwards Flowering can be induced by the use of plant hormones Plant hormones can be used to ripen fruit Roots grow downwards Auxins can diffuse from cell to cell in solution Plants grow towards a source of light
