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The stimulus of geotropism is gravity. Plants respond to gravity by growing in a specific direction - roots grow towards gravity (positive geotropism) and shoots grow away from gravity (negative geotropism).
The all-or-none principle in physiology states that once a nerve impulse reaches a certain threshold, a neuron will fire at its maximum potential. If the stimulus is strong enough to trigger an action potential, the response will occur at full strength. If the stimulus is below the threshold, no response will be triggered.
Roots mainly grow towards regions with higher concentrations of nutrients, water, and oxygen. They also grow away from areas with high levels of toxins or unfavorable soil conditions. Additionally, roots grow towards gravity, a phenomenon known as positive geotropism, with the root tip sensing gravity and growing downwards.