Stems grow away from gravity, and roots grow toward gravity. this describes

stems grow up toward,or away from the center of earth, and towards light.

Explanation:

roots grow dowanwards ,or towards the center of the earth , and away from light. these responses to external stimuli are called tropisms . plants grow responses to gravity is known as gravitropism  the growth response to light is phototropism.

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its the cell wall and the they are unique to plant