Plants show directional growth responses which are known as tropisms. Plants are limited by the fact they cannot move but still need to respond to changes in the environment so possess chemical communication systems called hormones. Plants have evolved a range of hormones such as auxins, gibberellins, abscisic acid and ethene which have a wide range of functions within the plant.Plants also produce other chemicals e.g. for defence against pests or to communicate with other plants.Plant hormones work at very low concentrations and have multiple interactions making them difficult for scientists to study.