Grasses arose seventy million to eighty million years ago, in the Late Cretaceous period of the Mesozoic era. They succeeded partly because they concentrate their growth lower down on the leaf and stem than other plants do and thus can regenerate quickly when fire or herbivores remove the top part of the plant.
This makes them ideal for human uses such as lawns and pasture for domestic livestock. Grasses are a very important food source for humans. Grasses provide all cereal grains—barley, corn, millet, oats, rice, rye, and wheat. Other grasses used for food include sorghum, sugarcane, and bamboo.
The grass family is important botanically and ecologically, too. With about 650 genera and 7,500 to 10,000 species, it is the fourth largest angiosperm family. In number of individual plants, it far outranks any other flowering plant family, composing the natural vegetation of about one-quarter of the earth’s land surface.
The family is especially abundant in semiarid climates and has the widest range of all the angiosperm families. Grasses provide a vital food source for many grazing, wild animals. By binding the soil with their roots, grasses protect it from erosion. In addition, grasses build up the soil when they die and decompose.
Stems, Roots, and Leaves
|Stems, Roots, and Leaves|
Grass culms are divided into nodes, or joints, which are solid, and internodes (the regions between the nodes), which are usually hollow. Elongation of the culm occurs mainly at the bases of the internodes, rather than at the stem tip as in most plants.
Grasses range in height from only a few centimeters, in annual bluegrass, to 30 or 40 meters, in treelike bamboos. Grasses may have underground stems, or rhizomes. The roots are slender and fibrous and for man extensive system that may compose a large proportion of the plant’s total biomass. This large root system helps grasses obtain water in dry regions.
The leaves of grasses are generally arranged alternately on the stem and have no petiole, or stalk. The lower part of the leaf, the sheath, is wrapped around the culmlike a split tube and is attached at its base to a node. The upper part of the leaf, the blade, diverges from the culm and is slender and elongated, tapering to a point at the tip.
Flowers and Fruits
Many grass flowers have three stamens and a single pistil that has two stigmas. Long, dangling anthers in the stamens and long, feathery stigmas help the wind transfer pollen efficiently from stamen to pistil.
Identification of grass species is based to a considerable extent on flower arrangement and characteristics of modified leaves, called bracts, that surround the flowers.
|Flowers and Fruits|
The grass fruit, called a caryopsis, or grain, consists of a single seed with its seed coat firmly adherent to the thin fruit wall. Wind plays a major role in dissemination of the fruits. Grasses may germinate, grow, set seed, and die in the same year (annuals)or live and produce seed for many years (perennials).
The bamboos are perennial, often treelike, grasses belonging to the Bambusoideae, a subfamily that is thought to be an early offshoot in the grass family lineage. Bamboo taxonomy is poorly understood. One estimate holds that there are roughly 45 genera and 480 species.
Like other grasses, bamboos have jointed culms, which are hollow except at the nodes, where there are partitions. The culms, which originate from rhizomes, are often called canes. The canes are light and elastic. Their hardness is due not to secondary xylem, or wood, as in most trees and shrubs, but rather to scattered fibers in the outer walls of the cane internodes.
Bamboos are distributed mainly in tropical and subtropical regions, with large concentrations in Asia and South America. A few species reach mild temperate areas. In the United States, there is a single native species, Arundinaria gigantea, called cane. It forms canebrakes in southern bottom lands. Bamboos are grown as ornamentals in many parts of the world.
Dense bamboo thickets are sometimes planted as living fences or barricades. In Asia, bamboos are very significant economically, providing materials for building, matting, and many other purposes. The young shoots are popular as food in eastern Asia.