(Return to Family List)


Grass Family

The Poaceae, or grass family, is a huge family, cosmopolitan in distribution, occurring from desert to freshwater and marine habitats and at all but the highest elevations. Native grasslands develop where there are periodic droughts, level to gently rolling topography, frequent fires, and in some instances grazing and certain soil conditions. Grass-dominated communities cover about 24% of the land surface. Woody bamboos are important in the forest ecology of tropical and temperate Asia. Grasses are our most important food sources: about 70% of the world's cropland is planted to grasses and over 50% of humanity's calories come from grasses. People have cultivated cereal grasses for at least 10,000 yrs. From the beginning of their domestication, wheat (Triticum aestivum), barley (Hordeum vulgare), and oats (Avena sativa) in the Near East, sorghum (Sorghum bicolor) and pearl millet (Pennisetum americanum) in Africa, rice (Oryza sativa) in southeast Asia, and maize or corn (Zea mays) in Meso-America have made possible the rise of civilization. In terms of global production, the highest four crops are grasses: sugarcane (Saccharum officianale), wheat, rice, and maize. Grasses are also used for livestock food, erosion control turf production, and as a sugar source for the fermentation of alcoholic beverages, such as beer and whiskey. Bamboos are economically important in many tropical areas for their edible young shoots, fiber for paper, pulp for rayon, and strong stems for construction. That's not bad for a single family.

Our grasses are all herbaceous and often rhizomatous. Their leaves are alternate and 2-ranked, and consist of a sheath, a ligule, and a blade. The sheath tightly encircles the stem, the margins overlapping but not fused or, occasionally, united to form a tube. The ligule is a membraneous flange or fringe of hairs at the apex of sheath. The blade is simple, usually linear, usually with parallel venation, flat or sometimes rolled into a tube, and continuous with the sheath. The inflorescence a spike, panicle, cyme, or raceme of spikelets. Each spikelet is composed of an axis bearing 2-ranked and closely overlapping basal bracts, called glumes, and florets. The usually 2 glumes can be equal or unequal in size. There are 1 to many florets per spikelet, each composed of a bract (the lemma) subtending a flower, and another bract (the palea) lying between the flower and the spikelet axis. The lemmas sometimes bear 1 or more needle-like, straight or bent awns. The flowers are small, bisexual or unisexual, usually wind-pollinated, and greatly reduced. There are usually 3 stamens and 3 carpels, but often only 2 carpels with their plumose stigmas are apparent. The fruit is a single-seeded caryopsis (grain) often associated with parts of the spikelet for dispersal. Spikelet characters that are useful in identification include size, plane of compression, presence or absence of glumes, number of florets presence of sterile or incomplete florets, number of veins on glumes and lemmas, presence or absence of awns, and aggregation of spikelets in secondary inflorescences.

Dactylis glomerata
Holcus lanatus

Lolium perenne
Poa bulbosa
Poa annua