Plants can move when their cells grow as a response to either light or gravity, which they are able to detect even as seedlings. This type of movement is slow and permanent. Other kinds of plants are capable of quick, momentary bursts of movement, which is due to hydraulics.
Phototropism occurs when the pigment phototropin absorbs light. This pigment is present in the tips of plant shoots, which are the sources of directional growth in the plant. Once phototropin absorbs light, the growth hormone auxin is released, causing the cells of the plant to divide. Auxin directly responds to the direction of light, thus allowing the plant to grow where light is strongest.
Gravitropism, which also utilizes auxin, allows a plant to grow either toward or away from gravitational direction. This process is regulated by statoliths, or small packages of starch that are present in the shoot tips and roots of the plant. When the plant is tilted, the statoliths settle on whichever side gravity dictates. Auxin is then released in that direction.
Some predatory plants such as the Venus flytrap move as a response to physical stimulus. Tiny hairs cover the Venus flytrap's surface and allow the plants' cells to sense pressure. Once enough pressure is reached, chloride ions are released that allow an electrical signal to travel throughout the plant. This causes potassium ions to enter and exit cells. Water then follows, as parts of the plant function as levers.