There are over 12,000 species of grasses and they have colonized almost every area of the world. One of the younger groups of plants, they are more closely related to lilies, orchids and iris than roses and sunflowers. Grasses such as rice, wheat, barley and corn are important food plants for humans while others provide forage for grazing animals.
Grasses have round hollow stems that grow vertically from a crown, the point at ground level, or slightly below, that sprouts new growth. Unlike most plants, grass stems keep extending themselves from the bottom. This allows them to survive grazing, fire and other processes that destroy the tops of the leaves.
Some grasses have stems that grow horizontally and sprout new plants from their tips. If these run along the top of the ground they are called stolons, if they grow through the soil they are called rhizomes. These modified stems allow the plant to colonize new ground quickly.
Grasses have dense, fibrous root systems that resist erosion and add large amounts of organic matter to the soil. Prairie soils, where grasses have grown for centuries, are some of the best in the world, rich, deep and dark in color. Roots grow from the crown, like stems, but elongate from the tip rather than the base. As part of the natural growth cycle, these roots die and then are renewed from the crown.
Grass leaves are long and narrow, composed of two parts, the blade and the sheath. If you look closely at any grass, you'll see that the base of the leaf is rolled around the stem. This is the sheath. The blade, the part that we usually think of as the leaf, has veins that run parallel to each other up the blade, rather than forming a network.
Grass flowers do not have petals. Each group is arranged in a spike made up of spikelets that have one or more flowers. Each spikelet is protected by two bracts called glumes. If you look closely, you can often see the stamens, the male parts of the flower covered with pollen, protruding from the spikelets. The feathery stigmas, the female part that lead to the ovary, may also be seen. Most species are wind pollinated.
The seed of a grass is a dry fruit called a caryopsis. Think of a grain of rice or barley as a large caryopsis. Most grass seeds are much smaller, and may have feathery extensions that act as parachutes to help with seed dispersal.