Compared with lacy ferns, showy flowers, and towering trees, the grasses seem hardly worth noticing. Yet these modest plants flourish in harsh conditions, cover much of the land masses of the world, support huge populations of grazing animals, and produce prodigious amounts of seeds – the grains that feed our livestock and us. Grasses have fascinating and unique adaptations that make them extremely resilient and set them apart from other kinds of plants.
Grasses grow just about anywhere – in fields, wetlands, saltmarshes, mountaintops, deserts, and even in shady forests. There are ten thousand species worldwide, and – in prairies, savannahs, pampas, and steppes – they cover a quarter of the earth’s land. Grasses dominate in conditions that are challenging for most other plants. They thrive in open plains that are dry and windy, lacking in shade, exposed to rain and snow, blizzards, and tornados. They are also built to survive fire and grazing by hordes of animals, from swarms of insects to herds of elephants.
A grass plant has narrow leaves, a slender stalk, and a large mass of roots. The stems of grasses are unique in having joints or nodes, bulges where the leaves attach. Round in cross-section, the stem is solid at the joints and usually hollow between them. Strong and flexible, grass stems can be bent over by strong wind or an animal’s tread and then spring back to stand upright. Two other plant families with long, thin leaves are often confused with grasses. Sedges have stems that are triangular in cross-section. Rushes have round stems, but lack the telltale joints. Students of grasses remember the differences with this simple rhyme: “Sedges have edges, and rushes are round, while grasses have joints from their tops to the ground.” Plants like cattails, irises, lilies, and plantain also lack joints on their stems, and the arrangement of their leaves and flowers are clearly different from grasses.
Besides the main stalk, many grasses grow special horizontal stems from their bases. They can grow above or below the ground, called stolons or rhizomes respectively. Rhizomes are usually white and push up to the surface at intervals, while stolons are green and spread outwards over the ground, forming circular patches. Both have nodes from which they can send roots downwards and shoots upwards to form new plants. Crabgrass, the bane of gardeners, has stolons that crawl sideways from the main plant like so many jointed legs, starting new plants as they creep along. Perennial grasses like Kentucky bluegrass spread by means of rhizomes, creating dense mats of sod that crowd out other plants – one reason they are favored for lawns and athletic fields.
Another way that grasses spread is by forming tillers, small new plants that grow from buds near their bases. Tillers grow roots, stems and leaves of their own, and can become independent of the parent plant. Some grasses, like timothy and orchard grass, form clumps or tussocks because of the large number of tillers they produce. Common in pastures and hayfields, they make good hay and provide habitat for grassland nesting birds and other animals.
Most plants grow from the tips of shoots; when these are broken off, they can’t grow from these points anymore. But in grasses the growing points are at the base of the plant, often below ground. This means that when they are grazed by herbivores, burned by prairie fires, or cut for hay, they can usually continue to grow unharmed. This is an important adaptation for life in grasslands where both frequent fires and large herds of grazing animals are common.
The roots of grasses contribute to their resilience as well. Grasses have large amounts of tangled, fibrous, branching roots that anchor the plant and hold onto the soil. The root system of a grass can make up ninety percent of its weight. This dense mass of roots can hold onto loose sand and gravel, penetrate cracks and crevices, and grow a long distance for water. In a drought or during cold weather, grasses die back to the ground. Perennial grasses can grow anew when conditions improve, using energy and water stored in the roots.
A grass leaf has two parts. Starting at a node, the leaf first grows as a tube or sheath, encircling the stem, and then extends outward in a long, thin, tapered blade. A close look shows parallel veins running lengthwise through the leaves. One leaf forms at each node, and alternating leaves usually branch out in opposite directions, though sometimes this is hard to see. Grass leaves make good whistles, but they aren’t easy to eat. Most grass leaves contain microscopic crystals of silica, making them extremely tough and hard to chew – an adaptation to deter grazing by herbivores large and small. In turn, many grazing animals have evolved strong grinding molars and special digestive systems that help them break down the tough fibers. Grass leaves and stems are also covered with fine hairs to discourage feeding by insects.
Many people are surprised to learn that grasses are flowering plants, for their flowers are tiny and inconspicuous. The tiny flowers form at the top of the stem, clustered together in a flower head, or inflorescence. These differ in their design and are a good starting place for distinguishing different kinds of grasses. The flower head can be lacy or compact, upright or drooping, branched or bristled, cylindrical or conical, and many other forms. The common names of some grasses reflect the form of the inflorescence, like squirreltail grass with long silky hairs on its flower head and foxtail grasses with soft bristly seed heads. Bottle brush grass looks just like – a bottle brush! The long pointed bristles on these grasses are called awns. There are many related species called panic grasses. They are named for the arrangement of their flowers in an open, loosely branched head called a panicle, and not for a tendency to take alarm!
Grass flowers are mostly green, lacking showy petals, nectar, or fragrances that attract pollinators to other flowering plants. Grasses depend on wind for pollination. When grasses are in bloom, you may notice the dangling stamens, releasing clouds of fine pollen at the slightest touch. With a hand lens, you can see the feathery and often brightly colored pistils, out to catch a flying pollen grain. They may be small, but grass flowers add subtle colors to the landscape. Covered with tiny blossoms, the flower heads of blue-joint give a rosy blush to wet meadows in summer. The dried leaves and seed heads of many grasses lend a golden haze to the landscape in the fall.
In each tiny flower a seed forms – the grains that we and so many other animals depend upon for food. Rice, oats, wheat, barley, corn, rye, and millet are all the seeds of grasses, foods rich in starch and protein, easy to store and plentiful. Grasses produce vast numbers of seeds, a way of assuring that at least a few will escape uneaten to start a new plant. Grass seeds may be gathered and planted by ants, blown by wind, hitchhike on traveling animals, or even go right through the digestive system of an herbivore and land in a pile of fertilizer.
All over the world, in places too dry for forests, grasses dominate the landscape, countless stems standing side by side, forming a carpet of green. Even in woods and wet places, we can find grasses thriving. Without fanfare, these unassuming plants go about their business – growing leaves and flowers and seeds – and, in the process, feed the whole world.
Suggested Reading
Brown, Lauren. Grasses: An Identification Guide. Boston: Houghton Mifflin, 1979.
Eastman, John. The Book of Field and Roadside: Open Country Weeds, Trees, and Wildflowers of Eastern North America. Illustrated by Amelia Hansen. Mechanicsburg, PA: Stackpole, 2003.
Harrington, H. D. How to Identify Grasses and Grasslike Plants. Athens, OH: Swallow P, 1977.
http://plantscience.psu.edu/research/centers/turf/extension/factsheets/cool-season
http://www.plantsciences.ucdavis.edu/gmcourse/module_resources/module1/resources/howgrassgrows.swf
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