A tropism is the manner in which a plant responds to an environmental stimulus, including light (phototropism), gravity (gravitropism) or touch (thigmotropism). Plants respond positively to some stimuli by moving toward them, and negatively to other stimuli by moving away from them.
There are several different examples of tropisms in plants. Gravitropism, or the response to gravity, causes roots to move towards gravity, or down into the soil, to reach water and minerals. It also causes stems to move away from gravity, so that leaves and stems will be in a position to intercept light.
Phototropism is the response in plants to unidirectional light coming from one source, such as a lamp or beam of sunlight. Cells on the shaded side of the stem elongate more than the cells on the lit side, causing the stem to turn toward the light.
Thigmotropism is plant response to touch. The most common example is the curling response of vines when in contact with an object. The response is controlled by specialized epidermal cells, which mediate the differential growth on each side of the stem. Elongation of cells on one side of the stem cause bending of the stem in the opposite direction.
Heliotropism is the response of an entire plant organ, such as a flower or leaf, to the sun's position in the sky. Plants orient leaves at different angles in relation to the sun to regulate temperature, evapotranspiration, and rate of photosynthesis in leaves.