Students explore how plants respond to their environment without a nervous system, discovering the chemical signalling systems that coordinate plant growth and development.
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Teach: auxin, phototropism, gravitropism, hormone, stimulus, response, elongation, germination. The key asymmetry — auxin promotes growth in shoots but inhibits it in roots — is the conceptual challenge that explains both phototropism and gravitropism.
Focus on auxin and phototropism before introducing gravitropism and other hormones.
Can students explain phototropism in terms of auxin distribution and differential cell elongation? Can they explain why the same hormone has opposite effects in shoots and roots?
Growing seedlings in a box with a small hole demonstrates phototropism freely using sunlight and any available seeds. No specialist equipment needed.
Students often think plants grow toward light because they need it — this is correct but not the mechanism. The mechanism is asymmetric auxin distribution causing differential growth. Understanding the mechanism distinguishes biological explanation from functional description.
Plant hormones have major agricultural applications — from rooting powder to weed killers to fruit ripening agents. Understanding them connects plant biology to food production and horticulture.
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