20. tropism: Tropism is when an organism turns or grows in a certain direction due to an external stimulus. Unlike taxis, tropism suggests that the turning/growth is an involuntary response. Tropism is important in plants because it allows the plant to grow in the direction of sunlight in order to maximize photosynthesis potential and it allows the roots of the plants to grow downwards to maximize the water availability. The cedar tree in the photo has branches that have grown towards the direction of light and roots that have grown downwards underground.
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