In order to survive, plants must respond to changes in both their external and internal environments. For example plants must respond to: Light – stems grow towards the light
- gravity – plants need to be firmly anchored in the soil, roots are sensitive to gravity and grow in the direction of its pull.
- - water – almost all plant roots grow towards water, in order to absorb it for use in photosynthesis and other metabolic processes, as well as for support.
Plants respond to external stimuli by means of plant hormones or,more correctly, plant growth factors. The latter term is more descriptive because:
- - They exert their influence by affecting growth
- - Unlike animal hormones, they are made by cells located throughout the plant rather than in particular organs.
- - Unlike real animal hormones, some plant growth factors affect the tissuesthat release them rather than acting on a distant target organ.
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