Homework: Study for homeostasis test on Thursday Title: Plants and tropisms 13th 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

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.

Responses of different parts of the plant

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

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

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

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

Seed Germination Germination is 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.

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

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

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.

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

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.

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

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.

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