Although a growing plant needs access to the right proportions of sunlight, water, and nutrients to be healthy, plants are not simply passive recipients of nature's bounty. Despite their fixed positions in the earth, they move and grow in response to a variety of environmental cues.
Chief among any plant's requirements is light. Without this energy source, plants are unable to create the tissues that support them and the food that sustains them. Through the process of photosynthesis, plants use sunlight to convert water and carbon dioxide into carbohydrates. Mature plants can survive no more than a few days without light. Young plants that are not yet able to photosynthesize rely on energy stored in seeds to sustain them until their leaves develop.
As plants develop, they follow distinct growth and movement patterns, called tropisms. These directional movements occur in response to environmental stimuli, such as gravity, light, and touch. Because most seeds germinate underground and in darkness, gravity is one of the earliest and most important cues. As the first root emerges from the seed during germination, it grows downward in the direction of gravity's force—where there is apt to be a steady source of water and nutrients.
The plant's first shoot grows in the opposite direction, away from the force of gravity and up in the direction of sunlight. This upward growth proceeds rapidly as cells grow both in number and length. Only when a shoot encounters light does upward growth slow or deviate at all, as leaves open and photosynthesis begins. Patterns of growth that maximize the exposure to light continue for the life of most plants. Upward growth is usually sufficient to meet a plant's energy needs, but a plant may also bend, twist, or climb other plants and/or objects if a better source of light exists somewhere other than straight up.
Plants rely on specialized structures in their cells to sense environmental stimuli such as gravity and light. These structures, called amyloplasts, rest at the low end of the specialized cells in which they reside. A change in the orientation of a plant causes amyloplasts to shift, thus informing the plant's cells as to their direction in relation to gravity. Photoreceptors called phototropins enable plants to detect differences in the intensity of the light striking various cells.
A group of hormones, called auxins, enable plants to alter their direction of growth according to the stimuli they receive. These hormones promote cell division and elongation. They typically accumulate in growing plant stems on the side opposite the direction of a light source or other valuable resource. The increased growth on this side of a stem causes the stem's tip to bend and grow toward a stronger source of light or other resource.