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Section 25-1: Hormones and Plant Growth
Plant hormones are chemical substances that control a plant's patterns of growth and development and the plant's responses to environmental conditions.
Auxins are produced in the apical meristem and are transported downward into the rest of the plant. They stimulate cell elongation.
Cytokinins are produced in growing roots and in developing fruits and seeds. They stimulate cell division and the growth of lateral buds, and cause dormant seeds to sprout.
Gibberellins produce dramatic increases in size, particularly in stems and fruit.
In response to auxins, fruit tissues release small amounts of the hormone ethylene. Ethylene then stimulates fruits to ripen.
Section 25-2: Plant Responses
Plant tropisms include gravitropism, phototropism, and thigmotropism. Each of these responses demonstrates the ability of plants to respond effectively to conditions in which they live.
Photoperiodism in plants is responsible for the timing of seasonal activities such as flowering and growth.
As cold weather approaches, deciduous plants turn off photosynthetic pathways, transport materials from leaves to roots, and seal leaves off from the rest of the plant.
Section 25-3: Plant Adaptations
To take in sufficient oxygen, many aquatic plants have tissues with large air-filled spaces through which oxygen can diffuse.
Plant adaptations to a desert climate include extensive roots, reduced leaves, and thick stems that can store water.
Plants that have specialized features for obtaining nutrients include carnivorous plants and parasites.
Many plants defend themselves against insect attack by manufacturing compounds that have powerful effects on animals.