What is Plant hormone?
[[File:Auxin.jpg|thumb|Lack of the plant hormone auxin can cause abnormal growth (right)|261x261px|right]] '''Plant hormones''' (or '''Phytohormones''') are [[chemical]]s that regulate [[plant]] growth. In the [[UK]] they are called 'plant growth substances'. The best known plant hormone is [[auxin]]. Plant [[hormones]] are [[signal transduction|signal molecule]]s produced within the [[plant]]. They occur in extremely low [[concentration]]s. Hormones regulate [[cell]]ular processes in targeted [[Cell (biology)|cells]]. [[Hormones]] also govern the formation of [[flowers]], [[Plant stem|stems]], [[leaves]], the [[Moulting|shedding]] of [[leaves]], and the development and ripening of [[fruit]].<ref>[https://web.archive.org/web/20030908034154/http://www.sidwell.edu/sidwell.resources/bio/virtuallb/plant/hormone.html Simple plant hormone table] with location of synthesis and effects of application — this is the format used in the descriptions at the ends of the Wikipedia articles on individual plant hormones. </ref><ref>Srivastava L.M. 2002. ''Plant growth and development: hormones and environment''. Academic Press. {{ISBN|0-12-660570-X}}</ref> Plants, unlike [[animal]]s, lack [[gland]]s that produce and [[secretion|secrete]] hormones. Individual cells can produce hormones. They affect which tissues grow upward and which grow downward, leaf formation and stem growth, fruit development and ripening, plant [[longevity]], and even plant [[death]]. Hormones are vital to plant growth, and, lacking them, plants would be mostly a mass of undifferentiated cells. So they are also known as growth factors or growth hormones. Phytohormones are found not only in higher plants, but in [[algae]] too, with similar functions.<ref>Tarakhovskaya E.R. ''et al'' 2007. Phytohormones in algae. ''Russian Journal of Plant Physiology'' '''54'''(2): 163-170.</ref> They also occur in [[fungi]] and [[bacteria]], where they may be used to induce beneficial reactions in host plants.<ref>{{cite journal|url=https://doi.org/10.1007/BF00029903|tit