Grasses

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 9,000 species, making it the fourth-largest family after the legume, orchid, and daisy families.

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, maize, 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 produced from sugar cane. Another member of the family, bamboo, is an important construction material as well as a food source; it also has been used in paper-making. 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 wild grazing animals and domestic stock, 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 crops is greater than the land area for all other kinds of crops combined.

Another economically significant use of grasses is for the lawns, parks, and playing fields 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 temperate regions are bent grasses, fescue, and Kentucky bluegrass (actually a native of Europe where it is known as smooth meadow grass). Bermuda grass and zoysia are better adapted to hotter climates. Often special grasses are used where particular conditions of soil or exposure make the more common species unsuitable. Occasionally, annual grasses are used to establish a lawn in a short time, but they may 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 grow among other crops 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, savannahs, 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 soil. Grasses also occur in desert regions.

In size, grass plants range from tropical giant bamboos that reach a height of more than 40 m (130 ft) to annuals, only a few centimetres 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 maize. The stems are usually herbaceous (lawn grasses) or hollow (bamboo), but exceptions occur, such as the pithy stems of maize and the woody stems of some bamboos.

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 rigours of many natural and artificial environments in areas where other plants cannot grow. Grasses are adapted to withstand burning, grazing, and trampling, and now dominate large areas where such events exert a major influence on the habitat. The usefulness of grasses as lawn plants is also derived from these features, because grasses continue to grow afterwards.