In all, there are some 10,000 different species of grass. Most are perennials that regrow from the rootstock each year. Except for the woody-stemmed bamboos, grasses are generally herbaceous plants. Each grass plant consists of slender jointed stems bearing long narrow parallel-veined leaves. The leaves are divided into a flattened blade and tubular sheath. The plant’s delicate emerging stems are protected by layers of leaf sheaths. At flowering, they elongate rapidly bearing their clusters of tiny flowers aloft.

Blades of grass

Each grass leaf has two parts – a long narrow folded, rolled or flat blade extending from a cylindrical leaf sheath. Characteristically, a small membrane or row of hairs, known as the ligule, marks the blade-sheath junction. Concentric layers of leaf sheaths support the weak immature stems and protect the growing points.

Like all green plants, grasses manufacture their own food from water absorbed via the roots and carbon dioxide from the air. This process, called photosynthesis, is powered by sunlight and takes place mainly in the leaves. Temperate grasses, such as Bromus and Poa, fix carbon dioxide and convert it to starch in a single process in the same cells (C3 pathway). Andropogon, Panicum and other tropical grasses have an extra carbon dioxide capture stage that takes place separately from starch production (C4 pathway). This division of responsibilities allows them to benefit from the high tropical temperatures and light levels and to continue to photosynthesise even when water is short.

Grass leaves grow from an area just above the ligule. If the tips are grazed or cut, the blade will continue to lengthen.

Covering the ground

Grasses cover the ground very effectively, but different grasses have different ways of producing a dense turf. All grasses produce branches, known as tillers, from buds very close to the soil surface. Some tillers simply grow upwards, within the encircling leaf sheath, producing a tufted or tussocky plant such as pampas grass (Cortaderia selloana) or cocksfoot (Dactylis glomerata). Others creep across the ground by means of stems that sprout new plants at intervals, each bearing their own fibrous roots. The sub-tropical Bermuda grass (Cynodon dactylon) spreads by means of stems (stolons) that extend out over the soil surface. Couch grass (Agropyron repens) has tough underground stems (rhizomes) that worm their way through the soil. It is dreaded by gardeners because, however carefully they weed, a small piece of rhizome always gets left behind, ready to produce a new invader.

A single plant of red fescue (Festuca rubra), which produces long rhizomes, has been estimated to cover an area over 250 m wide.