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Poaceae (Poe-ay-see-ay) or Gramineae is a large and nearly ubiquitous family of monocotyledonous flowering plants known as grasses . Poaceae includes the cereal grasses , bamboos and the grasses of natural grassland and cultivated lawns and pasture . Grasses have stems that are hollow except at the nodes and narrow alternate leaves borne in two ranks. The lower part of each leaf encloses the stem, forming a leaf -sheath. With around 780 genera and around 12,000 species, Poaceae are the fifth-largest plant family , following the Asteraceae, Orchidaceae, Fabaceae and Rubiaceae. Grasslands such as savannah and prairie where grasses are dominant are estimated to constitute 40.5% of the land area of the Earth , excluding Greenland and Antarctica . Grasses are also an important part of the vegetation in many other habitats, including wetlands , forests and tundra . The Poaceae are the most economically important plant family , providing staple foods from domesticated cereal crops such as maize , wheat , rice, barley , and millet as well as forage, building materials ( bamboo , thatch , straw ) and fuel (ethanol). Though they are commonly called " grasses ", seagrasses, rushes, and sedges fall outside this family . The rushes and sedges are related to the Poaceae, being members of the order Poales , but the seagrasses are members of order Alismatales .