accumulation, such as those affecting the PGM1 (which encodes the enzyme phosphoglucomutase) gene in Arabidopsis, causing plastids – the presumptive statoliths – to be less dense and, in support of the starch-statolith hypothesis, less sensitive to gravity. Other examples of gravitropic mutants include those affecting the transport or response to the hormone auxin. In addition to the information about gravitropism which such auxin-transport or auxin-response mutants provide, they have been instrumental in identifying the mechanisms governing the transport and cellular action of auxin as well as its effects on growth. There are also several cultivated plants that display altered gravitropism compared to other species or to other varieties within their own species. Some are trees that have a weeping or pendulate growth habit; the branches still respond to gravity, but with a positive response, rather than the normal negative response. Others are the lazy (i.e. ageotropic or agravitropic) varieties of corn (Zea mays) and varieties of rice, barley and tomatoes, whose shoots grow along the ground. See also Amyloplast – starch organelle involved in sensing gravitropism Astrobotany – the field of science concerned with plants in a spaceflight environment Clinostat – a device used to the effects of gravitational pull Random positioning machine – a device used to negate the effects of gravitational pull Free fall machine – a device used to negate the effects of gravitational pull Large diameter centrifuge – a device used to create a hyper-gravity pull Prolonged sine – reaction of plants to turning from their usual vertical orientation References Tropism Auxin action Articles containing video clips