Stems grow<span> upward, or </span>away<span> from the center of Earth, and </span>towards<span> light. </span>Roots grow <span>downward, or </span>towards<span> the center of Earth, and </span>away<span> from light. These responses to external stimuli are called tropisms. Plants' growth response to </span>gravity<span> is known as gravitropism; the growth response to light is phototropism.</span>