growth inhibitor A substance that slows the growth of a plant, for example the plant hormones abscisic acid and eth-ylene (ethene), which act at very low concentrations. Compare growth retardant.
growth retardant A synthetic compound used to reduce stem elongation in plants, for example to produce more compact plants or to prevent the stems of cereal crops becoming too long and thin. Compare growth inhibitor.
growth ring See annual rings.
guano The accumulated droppings of birds, bats, or seals, which are collected and used as fertilizer. Guano is rich in nutrients, especially calcium phosphate, and in places is mined on an industrial scale. It tends to accumulate at the sites of dense breeding colonies, such as seabird colonies on offshore islands or bat colonies in caves.
guild A group of species that live in the same environment and exploit the habitat and its resources in a similar way.
Gulf Stream A warm ocean current that flows north from Florida to Newfound land, with an eastern extension, the North Atlantic Drift, which curves east to northwest Europe. Its temperature and salinity are relatively constant. The warming effect of the North Atlantic Drift on northwest Europe is considerable, ameliorating the winters. There is concern that if global warming continues, increased meltwater from the Arctic may disrupt the Gulf Stream, and especially the North Atlantic Drift, causing climate change.
gully erosion Severe erosion of land in which water flowing over slight depression erodes them into trenches more than 30 cm deep. It is typical of sloping land in areas of high rainfall or sporadic fierce storms where the natural vegetation has been removed for agriculture or firewood or reduced by overgrazing, e.g. many parts of the uplands of Haiti.
guttation Loss of water as liquid from the surface of a plant. Water is normally lost as vapor during transpiration but, if the atmosphere is very humid, water may also be forced from the leaves through hy-dathodes (special secretory structures on leaves) as a result of root pressure (a form of hydrostatic pressure).
gymnosperms A general term for any seed plants except the angiosperms (see Angiospermophyta), characterized by bearing naked seeds, i.e. seeds not enclosed in an ovary (fruit). Gymnosperms lack endosperm and many also have advanced tracheids with a structure reminiscent of angiosperm vessels, whereas in other gym-nosperms the xylem is composed solely of more primitive tracheids.
gyre A circular or spiral ocean current, like a giant eddy. There is a major gyre in each of the main ocean basins about 30° on either side of the Equator, caused mainly by surface winds. Gyres flow clockwise in the northern hemisphere and counterclockwise in the southern hemisphere as a result of the coriolis force.
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