Presentation on theme: "Plant Responses 6.L.2.2 Explain how plants respond to external stimuli (including dormancy and forms of tropism) to enhance."— Presentation transcript:
1Plant Responses6.L.2.2 Explain how plants respond to external stimuli (including dormancy and forms of tropism) to enhance survival in an environment.
2What are plant responses? A stimulus is anything in the environment that causes a response in an organism.A stimulus may come from outside (external) or inside (internal) the organism.All living organisms, including plants, respond to stimuli.A Venus’s-flytrap hasthree small trigger hairs on thesurface of its toothed leaves. Whentwo hairs are touched at the sametime, the plant responds by closingits trap in less than 1 second.
3TropismsPlants can change their growth in response to their environment. These changes are called tropisms. Positive tropism - a plant growing toward a stimulus. Negative tropism – a plant growing away from a stimulus.
4Tropisms (Cont.) Plants can exhibit the following kinds of tropisms: Phototropism - the way a plant grows or bends in response to light.Geotropism - the way a plant grows or bends in response to gravity.Hydrotropism - the way a plant grows or bends in response to water.Thigmotropism - the way a plant grows or bends in response to touch.
5PhototropismPositive phototropism - a plant growing or bending toward the light.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 toward the light.
6GeotropismThe downward growth of plant roots is a positive response to gravity or positive geotropism.A stem growing upward is a negative response to gravity or negative geotropism.
7HydrotropismHydrotropism Turning or bending towards moisture, as roots.The most common example is that of plant roots growing in humid air bending toward a higher relative humidity level.
8ThigmotropismThigmotropism - is the directional response of a plant organ to touch or physical contact with a solid object. The plant’s stem bends and twists around any object it touches.
9Plant HormonesHormones control the changes in growth that result from tropisms and affect other plant growth. Ethylene:Many plants produce the hormone ethylene gas and release it into the air around them. Ethylene is produced in cells of ripening fruit, which stimulates the ripening process.Fruits such as oranges and bananas are picked when they are unripe and the green fruits are exposed to ethylene during shipping so they will ripen.
10Plant Hormones (Cont.) Auxin: A plant growth-regulating substance found in plants that stimulates cell elongation in plant tissues. It promotes root formation and bud growth. It also causes plant leaves and stems to exhibit positive phototropisms.
11Plant Hormones (Cont.) Gibberellins: Are plant growth substances involved in promotion of stem elongation, mobilization of food reserves in seeds and other processes. Its absence results in the dwarfism of some plant varieties.Cytokinins:Are a class of plant growth substances (plant hormones) active in promoting cell division, and are also involved in cell growth.
12Plant Hormones (Cont.) Abscisic Acid: The substance that keeps seeds from sprouting and buds from developing during the winter. It also causes the stomata to close and helps plants respond to water loss on hot summer days.
13PhotoperiodsSome plant species produce flowers at specific times during the year. Ex.) Sunflowers bloom in the summer, and cherry trees flower in the spring.Photoperiodism is the effect of day and night length on plant flowering. Some plants are long day, requiring hours of sunlight per day to flower. Others are short day requiring only hours. Others are day neutral and unaffected by day length.
14Darkness and FlowersLong-day plant is a plant that generally requires short nights—less than 10 to 12 hours of darkness—to begin the flowering process. Ex.) Spinach, lettuce, and beetsShort-day plant is a plant that generally requires long nights—12 or more hours of darkness—to begin the flowering process. Ex.) Poinsettias, strawberries, and ragweed
15Day-Neutral PlantsDay-Neutral Plant is a plant that doesn’t require a specific photoperiod and can begin the flowering process over a range of night lengths.
16QuestionsA plant is placed inside near a window. Over time, it grows toward the window. Why does the plant grow toward the window? A. to receive more sunlight B. to receive more fresh air C. to get away from the air conditioning D. to get away from the light in the room
17A student places a small plant on a shelf for three weeks. What has most likely happened to the plant?A. The plant was growing toward the source of light.B. The plant was too heavy to be supported by its stem.C. The plant was blown by air from a vent in the house.D. The plant was moved toward the window by the student.
18Which is the primary stimulus that makes a stem from a seed grow up and out of the ground, instead of down into it?A. airB. gravityC. lightD. pressure