Tropism 6 th Grade Science

Have you ever wondered how seedlings “know” which way to grow? The plant must send its roots down into the soil for minerals and water. The plant must send its stems up into the light and air so that it can manufacture food. Seeds grow the correct way no matter which way they are planted into the ground.

Tropism? Tropism is the growing or bending of a plant in response to an external source of stimulation. Several different types of tropism: –Phototropism (photo = light) –Geotropism (geo = earth) –Hydrotropism (hydro = water)

Geotropism In growing straight up and down, the plant is responding to a very powerful force – gravity. Geotropism is the way a plant grows or bends in response to the pull of gravity. The roots respond positively to gravity by growing down into the ground. The stems and leaves respond negatively to gravity by growing up into the light and air. Click here to see a video showing geotropism. (The video is courtesy of Roger Hangarter's lab)Click hereRoger Hangarter's lab

Phototropism Triggered by light. Phototropism is the way a plant grows or bends in response to light. The roots respond negatively to light by growing away from it. The stems and leaves respond positively to light by growing towards it. Click here to view a video showing phototropism. T he video is courtesy of Roger Hangarter's labClick hereRoger Hangarter's lab

Hydrotropism Triggered by water. The way a plant grows or bends in response to water. More difficult to observe; plant’s roots will grow towards or away from water.

Three important stimuli for plant growth Touch- Some plants show a response to touch called thigmotropism. The stems of vines, such as grapes coil around objects when they grow. Light- All plants exhibit a response to light called phototropism. The plants will grow towards the light, showing a positive phototropism. Gravity-Plants also respond to gravity. This is called gravitropism. Roots show positive gravitropism—they grow downward. Stems, show negative gravitropism— they grow upward.

Hormones and Tropisms Plants are able to respond to touch, light, and gravity because they have hormones. A hormone produced by a plant is a chemical that affects they way that they grow and develop. One important hormone is called auxin. Auxin speeds up the rate at which a plant’s cells grow.

Photoperiodism The amount of darkness a plant receives determines the time of flowering in many plants. A plant’s response to seasonal changes in length of night and day is called photoperiodism.

Winter Dormancy As winter draws near, many plants go into a state of dormancy. Dormancy is the period when an organism’s growth or activity stops. Dormancy helps plants survive during the winter months. It helps them survive freezing temperatures and lack of water.

Life Spans of Angiosperms Angiosperms are classified as annuals, biennials, and perennials. Flowering plants that complete a life cycle in one growing season are called annuals. Biennials will complete their life cycle in two years. In the first year, they germinate and grow roots, with very short stems and leaves. In the second year, they lengthen their stems and produce flowers and seeds. Perennials will live more than two years. Most will flower every year. They will die during the winter months, and new ones will grow during the spring. Brainpop