<P> Plant hormones, known as plant growth regulators (PGRs) or phytohormones, are chemicals that regulate a plant's growth . According to a standard animal definition, hormones are signal molecules produced at specific locations, that occur in very low concentrations, and cause altered processes in target cells at other locations . Unlike animals, plants lack specific hormone - producing tissues or organs . Plant hormones are often not transported to other parts of the plant and production is not limited to specific locations . </P> <P> Plant hormones are chemicals that in small amounts promote and influence the growth, development and differentiation of cells and tissues . Hormones are vital to plant growth; affecting processes in plants from flowering to seed development, dormancy, and germination . They regulate which tissues grow upwards and which grow downwards, leaf formation and stem growth, fruit development and ripening, as well as leaf abscission and even plant death . </P> <P> The most important plant hormones are abscissic acid (ABA), auxins, ethylene, gibberellins, and cytokinins, though there are many other substances that serve to regulate plant physiology . </P> <P> While most people know that light is important for photosynthesis in plants, few realize that plant sensitivity to light plays a role in the control of plant structural development (morphogenesis). The use of light to control structural development is called photomorphogenesis, and is dependent upon the presence of specialized photoreceptors, which are chemical pigments capable of absorbing specific wavelengths of light . </P>

Name and describe five ways in which plants can respond to their environment