THE GRASS AND REED FAMILY (Poaceae, syn. Gramineae)

The Poaceae (also called Gramineae or true grasses) are a large family of monocotyledonous flowering plants with more than 10,000 domesticated and wild species. Poaceae represent the fifth-largest plant family, following the Orchidaceae (orchids), Asteraceae (daisies), Fabaceae (peas), and Rubiaceae (bedstraws). Though commonly called "grasses", seagrasses, rushes, and sedges fall outside this family. Grasslands are estimated to compose 20% of the vegetation cover of the Earth. Poaceae thrive in many other habitats, including wetlands, forests, and tundra. The grass family is one of the most widely distributed and abundant groups of plants on Earth. Grasses are found on every continent, and are absent only from central Greenland and much of Antarctica. The evolution of large grazing animals in the Cenozoic (beginning 66 million years ago) contributed largely to the spread of grasses. Without large grazers, fire-cleared areas are quickly colonized by grasses but with enough rain, tree seedlings soon develop and eventually shade out and kill most of them. Trampling grazers kill seedling trees but not grasses.

Domestication of poaceous cereal crops such as maize (corn), wheat, rice, barley, and millet lies at the foundation of sedentary living and civilization around the world, and the Poaceae still constitute the most economically important plant family in modern times, providing forage, building materials (bamboo, thatch) and fuel (ethanol), as well as food.

Members of the grass family typically have hollow stems called culms, plugged at intervals by solid leaf-bearing nodes. Grass leaves are nearly always parallel veins and arranged alternately in two opposite rows. Each leaf is differentiated into a lower sheath hugging the stem and a blade with entire (i.e., smooth) margins. The leaf blades of many grasses are hardened with silica phytoliths, which discourage grazing animals; some, such as sword grass, are sharp enough to cut human skin. A membranous appendage or fringe of hairs called the ligule lies at the junction between sheath and blade, preventing water or insects from penetrating into the sheath. Grass blades grow at the base of the blade and not from elongated stem tips. This low growth point evolved in response to grazing animals and allows grasses to be grazed or mown regularly without severe damage to the plant.

Flowers of Poaceae are characteristically arranged in spikelets, each spikelet having one or more florets. The spikelets are further grouped into clusters. A spikelet consists of two (or sometimes fewer) bracts at the base, called glumes, followed by one or more florets. A floret consists of the flower surrounded by two bracts, one external—the lemma—and one internal—the palea. The flowers are usually hermaphroditic-- maize being an important exception—and wind-pollinated. The perianth is reduced to two scales called lodicules that expand and contract to spread the lemma and palea; these are generally interpreted to be modified sepals. The fruit is a caryopsis, a type of grain in which the seed coat is fused to the fruit wall.