Grasses - biology.

Grasses - biology. I INTRODUCTION Grasses, common name for a large family of flowering plants that is economically and ecologically the most important in the world. The grass family contains about 635 genera and 9000 species, making it the fourth largest family after the legume, orchid, and composite families (see Composite Flowers). II IMPORTANCE All the world's cereal crops are grasses, and thus the grass family is economically very important. The world's 5 top crops produce more tonnage than the next 25 combined, and 4 of the top 5 are the cereals rice, wheat, corn, and barley. Human well-being depends on these few grasses, so even small crop failures of any one of them can produce widespread hunger and economic disruption. In addition, the family provides most of the world's sugar (see Sugarcane). Another member of the family, bamboo, is an important construction material as well as a food source; it also has been used in papermaking. Citronella, used both in perfumery and as an insect repellent, is an oil distilled from the leaves of certain grasses. Grasses are the primary source of food for domestic and wild grazing animals, which feed on pastures and grasslands and which are fed hay and silage harvested from them. The total land area devoted to these kinds of croplands is greater than the land area for all other kinds of croplands combined. Another economically significant use of grasses is for the lawns maintained in many parts of the world. Perennial grasses are well adapted for use in lawns because their basal meristems (growing points) are not lost with mowing. Widely grown in the northern United States are Kentucky bluegrass (actually a native of Europe), bent grasses, and fescue. Bermuda grass and zoysia are better adapted to the warm conditions of the southern states. Often special grasses are used where particular conditions of soil or exposure make the more common species unsuitable. For example, buffalo grass is used where watering is restricted, as in the Great Plains region, and carpet grass may be used on poorly drained soils. Occasionally, annual grasses such as redtop are used to establish a lawn in a short time, but they often persist, compete with the perennial grasses, and become unsightly. In fact, not all grasses are beneficial, and some are considered pests. Most of these are weeds that occur in croplands and decrease overall production by competing with the cultivated crop, interfere with harvesting, or lower the overall food or other value of the crop by contamination. Some tropical forage grasses produce lethal concentrations of hydrocyanic acid under certain conditions. III DISTRIBUTION AND SIZE RANGE Grasses are the most widely distributed group of flowering plants, occurring from well above the Arctic Circle through the temperate and tropical regions to Antarctica. Indeed, grasses are the only flowering plants to grow on the Antarctic continent. This huge geographical range is paralleled by great ecological amplitude and variation in size. Grasses are most abundant in open habitats such as prairies, tundra, steppes, savannas, and paramos (see Grassland), but many species occur in forests, particularly in the Tropics. Some are adapted to aquatic habitats, including salt water, fresh water, and still or running water. A few species float on the water's surface and are not attached to the ground. Grasses also occur in desert regions. In size, grass plants range from tropical giant bamboos that reach a height of more than 100 m (more than 330 ft) to annual bluegrass, only a few centimeters tall. Some grasses lack leaf blades, but the blades of others may attain a length of 5 m (16 ft). IV GENERAL CHARACTERISTICS Grasses are rather uniform in basic vegetative structure, and several features are characteristic of the group. The main roots are usually fibrous; secondary roots, called adventitious roots, often arise from the nodes (joints) of the stems, as in the prop roots of corn. The stems are usually herbaceous (lawn grasses) or hollow (bamboo), but exceptions occur, such as the pithy stems of corn and the woody stems of some bamboo. The leaves, which are borne at the nodes along the stem, are in two rows and consist of two parts: the sheath and the blade. The sheath, a distinctive feature of the grasses, encircles the stem and gives support to the area just above each node. This area needs support because it is composed of soft growth tissue called meristem. Indeed, the grass stem increases in length not from the tip, as do most plants, but by growth all along the stem above each node. Another distinctive feature of grasses is the ligule, a short hairy or membranous projection, at the point where the leaf sheath joins the leaf blade. The function of the ligule is still unknown, but it may keep moisture from entering the region between the stem and the sheath. The leaf blade is typically long and narrow, with parallel veins, but great variations in shape and size occur. The leaf blade also has a meristematic area, which is located at its base above the place where the blade joins the sheath. Growth occurs in this area rather than at the leaf tip, as in most plants. Therefore, even if the upper end of the leaf is cut off, the blade can continue to grow. This feature, together with the presence of meristem tissue in the stems and the fact that grasses branch near the ground, enables grasses to withstand the rigors of many natural and artificial environments in areas where other plants cannot grow. The usefulness of grasses as lawn plants is also derived from these features, because grasses continue to grow after mowing. In addition, grasses can withstand burning, grazing, and trampling and now dominate large areas where such events occur. V FLOWERS AND FRUIT The flowers of grasses are usually individually inconspicuous, but they are often aggregated into large, sometimes showy clusters (inflorescences). For example, in the corn plant, the young ears are clusters of the female flowers, and the tassels are clusters of the male flowers. Most grasses are pollinated by wind, so that their flowers are highly reduced and very simple, as are most wind-pollinated flowers. The individual flowers of grasses are called florets because of their simplicity and small size. They have no sepals or petals (floral whorls); rather, these structures are represented by two tiny scales, called lodicules, at the base of the floret, outside the usually three stamens (male flower parts). The single ovary (female flower part) is superior--that is, borne above the other flower parts--and is topped by two feathery stigmas. The ovary matures into a distinctive, single-seeded fruit called a caryopsis, characterized by the fusion of the seed to the ovary wall. Grass florets are grouped into units termed spikelets. A spikelet consists of an axis, called a rachilla, along which are borne the florets. Each floret is enclosed by a scale--called a lemma--on the outside, and another--called a palea--on the inside. At the base of the rachilla are two scales called glumes, which are empty. The glumes and lemmas are usually hard in texture and often extend into a bristle or awn; the palea is delicate and membranous. Although their basic parts are simple and few, great variation exists among grasses in details of the structure of spikelets and their aggregation into flower clusters. This, together with details of overall structure and less easily observed characteristics of anatomy, cytology, and chemistry, accounts for the tremendous number of species of grasses. VI OTHER GRASSLIKE PLANTS Plants of two other groups, the sedges and the rushes, are superficially similar to grasses and are often confused with them. The three groups are easily distinguished, however, by examining their flowers, stems, and leaves. Rushes have flowers with six-part perianths (floral whorls) and from three- to many-seeded fruits, whereas the flowers of sedges and grasses have no perianth and have single-seeded fruits. Sedges usually occur in moist habitats and have solid stems that are triangular and bear three rows of leaves. Grasses usually occur in dry habitats and have round, hollow stems with two rows of leaves. Scientific classification: Grasses make up the family Poaceae (formerly Gramineae). Citronella is distilled from grasses of the genus Cymbopogon. Kentucky bluegrass is classified as Poa pratensis. Bent grasses are classified in the genus Agrostis. Fescue grasses are classified in the genus Festuca. Bermuda grass is classified as Cynodon dactylon. Zoysia grasses are classified in the genus Zoysia. Buffalo grass is classified as Buchloë dactyloïdes, carpet grass as Axonopus affinis, redtop as Agrostis gigantea, and annual bluegrass as Poa annua. Contributed By: Marshall R. Crosby Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.