Grasses

Grasses, the Poaceae, are a family of flowering plants of great economic and ecological importance. The legacy name of the grass family is Gramineae.

Directly and indirectly, grasses are the majority of the human diet. Occurring in many habitats, grasses are the only family that occurs naturally on all continents, and for which is named a world biome, the Grassland Biome.

There are over 10,000 species of grasses. Grasses dominate extensive ecological areas, urban lawns, steppes, and prairies, such as the 1.1 million-square-mile Great Plains of North America. The species Stenotaphrum secundatum, St. Augustinegrass, is the main landscape plant of Florida.

The origin and diversity of a cultivated grass species, Zea mays (maize), puzzled scientists for generations. The species was the basis of Barbara McClintock’s discovery of jumping genes. Some of the most productive biomass crops such as Saccharum officinarum are grasses. The grass family includes a woody group, the subfamily Bambusoideae (bamboos). The grass family includes many of the world’s worst weeds.

Grasslands are the genesis of the richest and most complex soil order, Mollisols, including Chernozems, Kastanozems, and Phaeozems of the world’s most productive breadbaskets such as Ukraine, Iowa, and southwest Brazil.

The wellspring of history

Grasslands support diverse wildlife including large mammals, mostly ungulates in herds.

Nomadic peoples of Central Asia lived by herding. Across 5,000 miles of Eurasian Steppe from Hungary to Manchuria, grassland herders, mostly Turkic peoples riding horses, ruled world trade while periodically conquering the empires of China, Europe, India, and the Middle East. Through the Silk Road they enabled the world’s greatest leap of cultural and technological exchange.

Reproductive biology

Grass flowers have no visible perianth, thus no showy petals to attract insects for pollination. Grass pollen grains have no appendages for attachment to insects.

Consequently, grasses pollinate by wind that carries their pollen. The grass pollen is nearly spherical, about 25 microns in diameter for most species, and so similar across the grass family that grass pollen is not very helpful in identification.

The spikelet is the basic unit of the grass inflorescence and the main structure for identifying grasses. The spikelet contains one or more florets, each floret with one flower held between a boat-hull-shaped lemma and deck-shaped palea. The spikelet contains as many as two glumes that are proximal, that is, at the base below the florets, or floret if there is only one. If there is more than one floret, they are attached to a central axis within the spikelet, called the rachilla.

The female receptive organ of grasses, the stigma, has two (rarely three) feathery lobes. The pollen-laden anthers are generally three per floret. After fertilization, the grass ovary develops into a dry, one-seeded fruit, the caryopsis or “grain” (which is edible), with a normally fused fruit coat. The “chaff” of grasses consists of accessory paleas, lemmas, glumes, and other accessories, which are removed in threshing. Additional structures help protect grass seeds from herbivores and help in their dispersal. For example, twisted awns can attach to the fur of animals, and bristles can act as minute parachutes for dispersal by wind.

Vegetative structure

The leaves of grasses are separated into two regions. The lower or proximal portion is called the sheath. It clasps the stem, which is called a culm if it is upright and aboveground.

The upper or distal portion of the leaf is called the blade and it generally extends away from the stem. Leaf blades may be flat, folded, or rolled.

Between the blade and the sheath of the grass leaf is a usually lighter region called the collar, which may have at its top a ligule which may be a membrane or a crown of hairs. At the sides of the collars there may be extended flaps called auricles. The stems of grasses are nearly circular in cross-section and may be solid or hollow. (For more on grass morphology see Earth at Home.)

Growth and ecology

Grass leaves and stems elongate from intercalary or basal meristems, which allows them to be grazed by animals or mown by machines, without much damage.

Many grasses are sod forming, especially if they can spread by horizontal stems, either rhizomes belowground or stolons above. The combination of grass growth habit, intercalary meristems, and fibrous root systems enable grasses to resist or recover from soil erosion, grazing, traffic, and fire.