Question

Why does stem always grow against the gravity and roots towards the gravity

Answer 1

Stems of a plant show negative Geotropism and the roots show positive Geotropism. Roots grow deep into the soil in search of water while the stem has leaves and other structures which need sunlight for making food by the process of photosynthesis. This is why to perform their respective functions they grow this way.

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Answer 2

Plants' growth response to gravity is known as gravitropism; the growth response to light is phototropism. ... As a result, root cells on the upper side of the root grow longer, turning the roots downward into soil and away from the light. Roots also will change direction when they encounter a dense object, such as a rock.