Tropism is the term given to how a plant responds to light, touch and gravity. A plant's parts experience the phenomenon of geotropism, a process that, while researched by Charles Darwin, is still not fully understood. Reacting to both internal and external forces, a tropism forces a plant's major parts to grow in different directions at the same time. In most plants, this is evident by how the roots know how to grow down into the soil while the top of the plant, or shoot, knows how to grow upward. This is often thought of as the plant growing upward toward the light, but that's only half the story.
A plant's root is the first to emerge during a seed's germination. A series of events takes place in the cells at the very tip of the plant's main root that force it to move in a downward motion. Positive geotropism, also called gravitropism in the case of root growth, is caused by the Earth's gravitational pull. Dense, heavy particles called amyloplasts gather in highly developed root cap cells. These particles are pulled down by gravity toward the very tip of the root, causing it to elongate straight down into the soil. As the roots grow longer, the layers of new cells forming along one side of each root determine the direction it will grow in, which is usually away from the main root.
Conversely, the top part of a plant, represented by the shoot that grows above ground, experiences negative geotropism as it resists the gravitational pull and grows in the other direction. Shoot plant cells grow longer with the help of a hormone called auxin, which develops in greater concentrations along certain parts of the plant. Auxin plays a greater role in shoot development as it stimulates cell growth in such a way as to bend it toward the light. As the lowest cells in a plant shoot grow larger, they push the cells above it upward, a chain reaction causing the plant to grow upward. When any part of a plant starts to grow in a different direction, it is due to how quickly some cells elongate depending on how much auxin they have been exposed to.
In plants that depend on photosynthesis for their own food production, a new shoot is encouraged by negative gravitropism to grow toward the sun, which essentially completes a major component of the developmental process, as the shoot can now make use of the sun's energy to continue growing upward. If a plant is exposed to uneven sunlight and starts to bend toward the light, auxins once again take over to elongate cells on the plant's dark side, keeping it positioned in such a way as to make maximum use of what light is available.