What causes unequal growth in plant stems, allowing them to bend toward light

Due to Auxin

Explanation:

Auxins are known as the growth chemicals in a plant body. The part of the plant which are kept in shade has  higher growth as compared to the part of the plant which is in the shade. This is so because,  Auxins on the sunnier side break down into smaller molecule compounds and hence the plant body on the sunnier side has lower growth rate while in case of side which is on the darker side, a plant receives lesser amount of sunlight, thus auxin remains intact.

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