Grass Family


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Grass Family

 

(Poaceae, or Gramineae), a family of monocotyledonous plants; annual, biennial, or perennial grasses, more rarely shrublike or treelike forms. The stem of grass, called the culm, is usually cylindrical, sometimes flattened on the sides, and erect, ascending, lodging, or recumbent (and then it often takes root in the nodes). The culm has transverse, mostly swollen nodes and is divided into inter-nodes, which are usually hollow inside (except for the base and the upper part of the inflorescence); less frequently they are filled with loose tissue. The stem is 1 centimeter (cm) to several dozen m high; in countries with a temperate climate, it is generally 0.3–1.2 m high with a diameter of 3–5 mm. In some grasses, the stem lignifies and becomes very tall (in bamboo, up to 40 m, with a diameter of 30 cm). The stem is characterized by intercalary growth from the meristem in the lower part of the internode.

The leaves proceeding from the nodes of the stem are alternate, are usually arranged in two rows, and consist as a rule of a sheath, a blade, and a ligule. The sheath is tubular and it holds the stem tightly; it is usually open and only rarely grown together, or closed; at its base there is usually a perceptible ringlike thickening called the leaf, or sheath, node. The leaf blade is elongated, linear, or linear-lanceolate; infrequently it is broad. The blade usually has parallel venation and is often folded lengthwise, or its edges are convolute. The ligule is a semitransparent, membranous projection between the sheath and the blade. It is whole or dissected and is sometimes reduced to several filaments or cilia, or is occasionally even absent.

The root system of grasses generally consists of numerous fine adventitious roots collected in a fascicle (fibril); the primary roots die off early. The roots of many meadow and steppe grasses often occur only in the top soil layer; others go down into the soil 1–1.5 m or more. The total length of all the roots of a single plant may be 10 km, which is much more than the roots of many other plants.

Grass flowers are small and unattractive, monoclinous or (less frequently) diclinous, and gathered in a simple inflorescence called a spikelet. Spikelets, in turn, form complex second-order inflorescences—with compound spike, tassel, panicle, and so forth. An individual spikelet consists of an axis (rachis) bearing one or several, and sometimes even many, flowers. At the base of the axis are the so-called spikelet glumes, of which there are usually two: the lower and, opposite it and generally a little higher, the upper. Sometimes there are more than two spikelet glumes or there is only one; less often they are absent. Close together above the spikelet glumes are the so-called flowering glumes, of which the lower one, the so-called lemma, is larger than the upper one and is often furnished with an awn. On the other hand, an awn is rare in spikelet glumes. Between the lemma and the upper flowering glume there are two (less frequently, one or three) delicate perianth scales or floral membranes, the so-called lodicules; they are sometimes absent, for example, in cleistogamous flowers.

There are usually three stamens, less commonly one, two, or six, arranged in two circles. Sometimes the stamens are split up and there are more of them (occasionally as many as 120). The filaments are delicate. The anthers are generally versatile, and they open in a longitudinal slit. The branches of the stigma (two, less often one or three) are generally plumose or penicillate, and the ovary is superior and unilocular and has one ovule.

Cross-pollination by wind is typical of grasses; some grasses, particularly cultivated ones, are characterized by self-pollination. The fruit of most grasses is called a caryopsis. Occasionally it resembles a drupe, nut, or berry. The seed has an abundance of starchy endosperm.

In most grasses, especially those that grow in the temperate zone, the aerial part of the stem branches only at the very top (near the common inflorescence) and at the bottom (near the soil surface). In these places the nodes are very close together and they form so-called tillering nodes from whose buds grow new aerial shoots that produce, in turn, new tillering nodes. Thus, one plant may have several dozen stems (as do many meadow grasses). Emerging from the tillering nodes, the shoots either pierce the sheath (so-called extra-vaginal shoots) or they grow inside it (so-called intravaginal shoots).

Depending on the way the shoots are formed, grasses are divided into long-rhizome (or rhizome), compact tufted, and loose tufted. Long-rhizome grasses form, far from the mother plant, aerial daughter shoots that develop from buds in the long rhizomes found not too deep in the earth. This type of grass—for example, bushgrass, slender foxtail, and meadow bluegrass—grows mostly on well-aerated mellow soils, frequently settling in places not yet occupied by other plants (on fresh alluvium in riverbeds, on the sites of slash fires, and so forth). Compact tufted grasses form new shoots next to the mother plant from tillering nodes found above the soil or near the surface. New tillering nodes develop on the young shoots. The result of the development of buds from the tillering nodes of all orders (the secondary and those following) is a compact, fascicular mass of new shoots forming a “bush” or “sod.” Compact tufted grasses propagate themselves by seeds, usually on soils that are not too well aerated. Tufted hair grass and matgrass are typical of compact tufted grasses. In loose tufted grasses, new shoots emerge diagonally upward from tillering nodes that are present in the surface layer of soil (usually about 5 cm thick) and form a loose bushy mass. The loose tufted grasses include meadow fescue, timothy, and orchard grass. Some grasses, like velvet bent, are capable of producing aerial shoots (runners, stolons) that take root in nodes and form small mats. Grasses live from one year to several decades.

Grasses constitute one of the largest families of angiosperms, with more than 600 genera and up to 10,000 species; there are almost 150 genera (and about 1,000 species) in the USSR. The grass family is usually subdivided into two or more (up to 12) subfamilies and more than 25 (up to 60) tribes. Grasses are found wherever angiosperms can grow, frequently occupying vast areas of land. Grasses are especially important in meadows, prairies, steppes, and savannas. They also play a major part in the life of man because they include the most important cereal plants, such as wheat, rice, corn, oats, rye, barley, millet, and sorghum, as well as sugarcane. Wild and cultivated grasses serve as forage for animals. Grasses are also important in industry as sources of starch, alcohol, paper, vegetable and aromatic oils, building materials, and raw materials for rope, mats, and bristles. Grasses are used in the liqueur and vodka industry and as medicinal and ornamental plants. Many grasses, such as perennial ryegrass, Bermuda grass, meadow grass species, couch grass, and bentgrass are used to stabilize loose soils and sands and to sod airports. Some grasses, such as wild oats, couch grass, and Bermuda grass, are considered aggressive weeds.

REFERENCES

Avdulov, N. P. “Kariosistematicheskoe issledovanie semeistva zlakov.” In Trudy po prikladnoi botanike, genetike i selektsii: Prilozhenie 44. Leningrad, 1931.
Rozhevits, R. lu. Zlaki: Vvedenie v izuchenie kormovykh i khlebnykh zlakov. Moscow-Leningrad, 1937.
Takhtadzhian, A. L. Sistema i filogenlia tsvetkovykh rastenii. Moscow-Leningrad, 1966.
Flora SSSR, vol. 2. Leningrad, 1934.
Kormovye rasteniia senokosov i pastbishch SSSR, vol. 1. Moscow-Leningrad, 1950.
Iakovlev, M. S. “Struktura endosperma i zarodysha zlakov kak sistematicheskii priznak.” Trudy Botanicheskogo in-ta AN SSSR: Seriia 7, 1950, no. 1.
Tsvelev, N. N. “Sistema zlakov (Poaceae) flory SSSR.” Botanicheskii zhurnal, 1968, vol. 53, no. 3.
Serebriakova, T. I. Morfogenez pobegov i evoliutsiia zhiznennykh form zlakov. Moscow, 1971.
Arber, A. The Gramineae: A Study of Cereal, Bamboo and Grass. Cambridge, 1934.
Pilgar, R. “Das System der Gramineae.” Botanische Jahrbucher, 1954, vol. 76.
Stebbins, G. L. “Cytogenetics and Evolution of the Grass Family.” American Journal of Botany, 1956, vol. 43, no. 10.
Chase, A., and C. D. Niles. Index to Grass Species, vols. 1–3. Boston, 1963.
Gould, F. W. Grass Systematics. New York, 1968. (Contains a bibliography.)
Hubbard, C. E. Grasses. Rev. ed. Harmondsworth, 1968.

M. E. KIRPICHNIKOV

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