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41.1 Plant growth is often guided by environmental cues.
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• Tropisms in plants are directional growth responses to external stimuli, such as light, gravity, or contact.
• Dormancy is a plant adaptation that carries a plant through unfavorable seasons or periods of drought.
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1. In general, which part of a plant is positively phototropic? What is the adaptive significance of this reaction?
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41.2 The hormones that guide growth are keyed to the environment.
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• Auxin in the shoot tip migrates away from light and promotes the elongation of plant cells on the dark side, causing stems to bend in the direction of light.
• Cytokinins are necessary for mitosis and cell division in plants. They promote growth of lateral buds and inhibit formation of lateral roots.
• Gibberellins, along with auxin, play a major role
in stem elongation in most plants. They also tend to hasten the germination
of seeds and to break dormancy in buds.
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2. How does auxin affect the plasticity of the plant cell walls?
3. Where are most cytokinins produced? From what biomolecule do cytokinins appear to be derived?
4. What plant hormones could be lacking in genetically dwarfed plants?
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41.3 The environment influences flowering.
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• The transition of a shoot meristem from vegetative to adult development is called phase change. During phase change, plants gain competence to produce a floral signal(s) and or perceive a signal.
• The light-dependent pathway uses information from light receptor molecules integrated with a biological clock to determine if the length of night is sufficient for flowering.
• The autonomous path functions independently of environmental
cues. Internal floral inhibitor(s) from roots and leaves and floral promoter(s)
from leaves move through the plant.
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5. A plant has undergone phase change. Although it is an adult, it does not flower. How might you get this plant to flower?
6. You have recently moved from Canada to Mexico and brought some seeds from your favorite plants. They germinate and produce beautiful leaves, but never flower. What went wrong?
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41.4 Many short-term responses to the environment do not require growth.
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• Changes in turgor pressure reflect responses to environmental signals that can protect plants from predation, and regulate stomatal opening, among other things.
• Other reversible movements in plants are caused by changes in turgor pressure that are regulated by internal circadian rhythms.
• Plants have the ability to recognize and respond to
invaders at the cellular, tissue/organ, and whole plant levels.
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7. What happens in the cells of the sensitive plant (Mimosa pudica) when its leaves are touched?
8. In what ways can a plant protect itself from pathogenic microbes? From animals?
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