Plant hormones (or Phytohormones) are chemicals and signal molecules that affect the growth, formation, development or division of plant cells. All plants cells are capable of producing growth regulator chemicals, as opposed to animals in which specific glands exist in order to produce vital hormones. Plant hormones control seed growth, time of flowering, senescence of leaves and fruits, the direction of tissue growth (whether roots grow down or stems grow up) and many other characteristics. There are five accepted major classes of plant growth regulators: gibberellins (such as Gibberellic Acid 3), auxins (such as IAA, NAA or IBA), abscisic acid (ABA), cytokinins (such as Zeatin or Trans Zeatin Riboside), and ethylene, as well as a number of other regulators that do not fit cleanly into these specific classes.