Tropism is the involuntary growth or
orientation of an organism towards or away from a
stimulus. In comparison, voluntary movement towards or away from a stimulus is called a
taxis. The term tropism is usually applied to and observed in plants.
Plant hormones that exist throughout the organism direct its growth. The hormones that control growth in plants are known as auxins and are responsible for tropism. The stems of plants exhibit positive phototropism and negative geotropism, meaning that they grow towards sources of light and away from gravity. Light activates the auxins that exist in the stem, and growth is accelerated there. In portions of the stem where there is an absence of light, those hormones are inhibited and growth is stunted. If a plant is placed in such a way that one side of it is lit and the other is not for an extended period of time, it will actually grow towards the source of light.
Different parts of a plant may exhibit different kinds of tropism. For instance, the stem of a plant may be negatively geotropic, but the roots will be positively geotropic. The stem grows up and out while the roots grow down to absorb more water and minerals. This allows the plant to grow up above ground while expanding roots deeper below ground.