A tropism is a directional growth response in which the direction of the response is determined by the direction of external stimulus.
Tropisms include:
- Phototropism - shoots grow towards light, enables photosynthesis
- Geotropism - roots grow towards pull of gravity
- Chemotropism - on a flower, pollen tubes grow down the style, attracted by chemicals, towards where fertilisation takes place
- Thigmotropism - shoots of climbing plants wind around objects and gain support
Hormones coordinate plant responses to environmental stimuli. When hormones reach their target cells, they bind to specific complementary receptors on the plasma membrane. Allows hormones to acts upon only the correct tissues. Hormones moves around the plant by active transport, diffusion and mass flow in the xylem/phloem. Different hormones have different effects.
Auxins promote cell elongation; cytokinins promote cell division; gibberellins promote seed germination; abscisic acid inhibits seed germination/growth; ethene promotes fruit ripening.
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