MANY DIFFERENT TYPES of ecosystems throughout the world are described as grasslands because they are dominated by species of grass with a range of other plant types as subordinates within the community. Grasslands occur at most latitudes and altitudes reflecting the wide range of environmental tolerances that characterize this huge taxonomic group of plants known as the Gramineae. There are approximately 9,000 species within this group. They are described as monocotyledons because there is only one leaf protecting the seed, in contrast to most flowering plants, which have two seed leaves and are known as dicotyledons. Grasses reproduce by generating and dispersing seed; many species spread through the production of underground stems called tillers. These two characteristics have been widely exploited in agricultural systems. First, many species producing particularly carbohydrate-rich seeds have been domesticated, a process begun more than 10,000 years ago, and today they are major cereal crops of arable agriculture, notably rice, maize (corn), wheat, barley, oats, rye, and sorghum. Second, the ability of grasses to produce a turf ground cover through tillering and the placement of the growing points close to the ground facilitates grazing by herbivorous animals and thus underpins pastoral agriculture worldwide. Broadly, grasslands may be classified as natural, seminatural, and artificial. Each of these groups can be further subdivided. Natural grasslands are widespread and tend to occur where climatic or soil conditions prohibit the growth of shrubs and trees; annual precipitation regimes are especially influential, as a pronounced dry season or seasons is characteristic of grasslands, along with a low annual precipitation. The occurrence of natural fire may have also played a role in the formation of continental grasslands. Other factors may give rise to grasslands locally, such as persistently cold, high-speed or salt-loaded winds in coastal areas.