Plants need to absorb energy from the sun in the form of solar radiation in order to grow (using a biochemical process called photosynthesis). Most leaf surfaces are created to soak up the maximum amount of radiation, and plants have a specific shape or structure in order to ensure that they are as efficient as possible in collecting sunlight. Over time, crop plants have been selected and bred in order to ensure that this shape is such that the plant can intercept as much radiation as possible. For example, corn hybrids have been selected over time to have leaves that are not as upright, but rather grow outward so that more surfaces of the leaf can intercept radiation. Also, some plants have the biological ability to change direction based upon the direction of the sun, called heliotropism. One flower capable of this is the common sunflower.