Biology: Plant Behaviour
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Plants respond to various external stimuli through tropisms and rhythmic behaviors. Tropisms are plant growth responses to stimuli like light, gravity, touch, and water. For example, phototropism is the growth of a plant towards or away from light due to the plant hormone auxin. Auxin levels determine which side of the plant grows more, causing it to bend. Rhythmic behaviors follow circadian rhythms or responses to day length, influencing behaviors like flowering times in short-day or long-day plants. Plants use stimuli responses and rhythms to optimize processes like photosynthesis and reproduction.
Biology: Plant Behaviour
- 1. Plant Behaviour Chapter 11 (Behavioural Adaptions) page. 362.
- 3. What do plants need to survive?
- 4. Tropisms External factors that influence plant growth and development: Light, gravity, touch, water. Tropism: The growth of a plant in response to the direction of a stimulus, such as light, gravity, or touch. Positive tropism: Plant grows towards a stimulus Negative tropism: Plant grows away from a stimulus
- 5. Phototropism: direction of light as a stimulus When a plant moves or grows in response to light. Movement towards light = Positive or negative phototropism?
- 7. Phototropism: direction of light as a stimulus Charles Darwin (1809-1882) found that he could prevent phototropism by covering the growing tips of seedlings (coleoptiles). Conclusion: The tip of the coleoptile influences the bending that occurs in phototropism.
- 8. Phototropism: direction of light as a stimulus We now know: The plant hormone auxin is produced in the tip of a coleoptile that causes growth of cells. The tip is the site of reception of the light stimulus. Auxin moves away from the light source to the darker side of the tip. Increased concentration of auxin in cells of that region increase the growth of the cells (greater elongation). The uneven growth of cells results in bending of the coleoptile. Draw diagram.
- 10. Geotropism: direction of gravity as a stimulus Gravity causes most of the auxin produced to accumulate in the lower sides of the horizontal shoot and root. Auxin Stimulates growth of cells in shoots. Inhibits the rate of growth of cells in roots. How do you think the shoots and roots would grow? Draw diagram.
- 13. Thigmotropism: direction of touch as a stimulus E.g. Grapevines- Tendrils and twining parts of the climbing plant wrap around support structures or other plants. The twisting is caused by uneven growth of cells along the tendril as it comes into contact with an object. What do you think happens to the distribution of auxin when a growing plant comes into contact with another object? Draw diagram.
- 15. Heliotropism/Solar Tracking direction of the sun as a stimulus (also time) Also a rhythmic behaviour because direction of the sun depends on the time. Leaves and flowers of many plants are able to move during the day so that they are oriented either perpendicular (at right angle to) or parallel to the sun’s direct rays. How do you think this would assist in survival?
- 17. Rhythmic Behaviour Plants also have a biological clock!
- 18. Rhythmic Behaviour (Time as a stimulus) Circadian rhythm (or circadian cycle) = An activity that follows a 24-hour cycle. Include the opening and closing of flowers, and nectar and perfume production. This kind of movement is called nastic movement: Does not depend on the direction of the stimulus.
- 19. Plants that use a circadian rhythm
- 21. Photoperiodism: The response of plants to particular periods of light and dark. Photoperiod: The relative of day and night length Exposure to “darkness/night-time” is important for the flowering of some plants (can affect the timing of flowering). Short-day plants and long-day plants. Rhythmic Behaviour (time as a stimulus) Nastic Movement: Timing of flowering
- 22. Short-day plants Flower only after being exposed to day lengths that are shorter than a certain critical length. E.g. Chrysanthemums, poinsettia, datura stramonisum.
- 23. Long-day plants Flower only after being exposed to day lengths that are longer than a certain critical length. E.g. Hibiscus, carnations.
- 24. Day-neutral plants Flowering is not influenced by length of exposure to light/darkness. Example: tomatoes, maize, dandelions.
- 25. Key ideas Factors external to a plant influence growth and development of the plant. The direction of response of a plant is sometimes influenced by the direction of the external stimulus. Day-night lengths are critical for some plant activities.