MOLECULES THAT INFLUENCE PLANTS
Plant patterns of growth and development are controlled by a variety of molecules produced by the plants themselves. Some of the first molecules to be identified as controllers in plants were hormones. Hormones are organic molecules that act as highly specific chemical signals to cells, influencing the cells' growth and development. Charles Darwin performed some of the earliest experiments on plant hormone production. Five main classes of hormones have been traditionally studied in plants.

More recently, additional signaling molecules have been identified. Although little is understood about their actual mechanisms, many signaling molecules appear to function primarily in defensive responses of plants to disease organisms and insect pests.