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Pg. 139-145 What are plant responses? • A stimulus is anything in the environment that causes a response in an organism. The response often involves movement either toward the stimulus or away from the stimulus. • A stimulus may come from outside (external) or inside (internal) the organism. What are plant responses? • Internal stimuli are usually chemicals produced by organisms. • Many of these chemicals are hormones. Hormones are substances made in one part of an organism for use somewhere else in the organism. What are plant responses? • Many different chemicals are known to act as hormones in plants. • Plants respond to external stimuli such as touch, light, and gravity. • Some responses are rapid. Other plant responses are slower because they involve changes in growth. Tropisms • Some responses of a plant to an external stimuli are called tropisms. • A tropism is a response of a plant to an external stimulus. • For example, plants might grow toward a stimulus a positive tropism or away from a stimulus a negative tropism. Touch • One stimulus that can result in a change in a plant’s growth is touch. • When some plants touch a solid object, they respond by growing faster on one side of its stem than on the other side. • As a result the stem bends and twists around any object it touches. Light • Light is an important stimulus to plants. • When a plant responds to light, the cells on the side of the plant opposite the light get longer than the cells facing the light. • Because of this uneven growth, the plant bends towards the light. Light • When a plant grows toward light it is called a positive response to light, or positive phototropism. Gravity • The downward growth of plant roots is a positive response to gravity. • A stem growing upward is a negative response to gravity. Plant Hormones • Hormones control the changes in growth that result from tropisms and affect other plant growth. Ethylene • Ethylene is produced in cells of ripening fruit, which stimulates the ripening process. • Helps fruit ripen Auxin • Auxin is a type of plant hormone that causes plant stems and leaves to grow toward the light. • When light shines on a plant from one side, the auxin moves to the shaded side of the stem where it causes a change in growth. Auxin • Development of many parts of the plant, including flowers, roots, and fruit, is stimulated by auxins. • Because auxins are so important in plant development, synthetic auxins have been developed for use in agriculture. Gibberellins and Cytokinins • Gibberellins are chemical substances that were isolated first from a fungus. • The fungus infects the stems of plants and causes them to grow too tall. Gibberellins and Cytokinins • Gibberellins can be mixed with water and sprayed on plants and seeds to stimulate plant stems to grow and seeds to germinate. Gibberellins and Cytokinins • Cytokinins also cause rapid growth. • Cytokinins promote growth by causing faster cell divisions. Abscisic Acid • Abscisic acid is the substance that keeps seeds from sprouting and buds from developing during the winter. • This plant hormone also causes stomata to close and helps plants respond to water loss on hot summer days. Photoperiods • A plant’s response to the number of hours of daylight and darkness it receives daily is photoperiodism. • The hours of daylight and darkness vary with the seasons. • These changes in lengths of daylight and darkness affect plant growth. Darkness and Flowers • Generally, plants that require less than 10 to 12 hours of darkness to flower are called longday plants. Darkness and Flowers • Plants that need 12 or more hours of darkness to flower are called shortday plants. Day-Neutral Plants • Day-neutral plants have no specific photoperiod, and the flowering process can begin within a range of hours of darkness. • In nature, photoperiodism affects where flowering plants can grow and produce flowers and fruit.