Grasses – the ubiquitous plants

Of all the families of flowering plants, the Gramineac, the grass family, must rank above all others, exceeding as it docs in amount, variety and value of its products, and in the number of its individuals. There are about 620 genera and 10,000 species of grasses, varying in size from the lilliputian to the gigantic, and their products form the staple diet for mankind in many parts of the world; the genera Triticum (wheat), Avena (oats), Hordeum (barley), Sccak (rye), Zea (maize) and Oryza (rice) have been cultivated for thousands of Grasses are distributed through almost every possible kind of environment. There are species thriving in and around water, others which grow in deserts, and yet other species can be found surviving the intense cold of the polar regions. Great adaptability is shown even within a species; the sheep’s fescue Festuca ovina occurs both at sea-level in Britain and at 5,500m in the Himalayas, whilst sweet vernal grass, Anthoxanthum odoratum flourishes on sand, loam or clay years.

Marram grass and the sand dune system

A typical seashore sand dune system, itself often based on shingle bars, can reach 40m in height in Britain, and can advance remorselessly under the influence of wind to cover whole villages and alter the course of river mouths in the space of a century or two. Dunes are eventually halted when stabilised by plant colonisation, as seen at Studland, Dorset. The first major coloniser is often sand couch grass, Agropyron junceiforme, which tolerates occasional submergence in sea water. This is typically followed by marram grass, Ammophila arenaria , before it can support less tenacious plants. At Studland the beach and dunes are backed by heath, where plants like the ling and various sedges and rushes gradually supersede the marram grass. Eventually the heath gives way to shrubs. Many flowering plants exist within these frameworks. Marram grass thrives in fresh sand and moving dunes, its powerful horizontal shoots anchored by roots which can penetrate 18m (6oft) down. Once dunes become static, marram grass tends to disappear. soils, and can be found in countries as different in their climates as North Africa and Siberia. Strains resistant to certain toxic metal wastes have recently appeared with the result that grasses have even become established on mine spoil heaps.

Natural grassland, the home of the fastest animal runners in the world, occurs on all continents, principally in their interiors, as in the prairies of North America, the Argentine pampas, the steppes of the Old World and the South African veld. Here grass species have come to dominate the flora over vast stretches of land. This is mainly because of the climate: rainfall is insufficient to support trees but high enough to keep deserts from forming. In these regions, tough coarse grasses of the genera Spowbolus, Stipa, Elymus and Agropyron in particular grow in thick profusion. Their matted roots and rhizomes form a thick turf, often a quarter of a metre or more in depth, which traps and conserves the scant moisture well. These grasses may grow as tall as 3m (10ft) in 500-625mm (20-25in) of annual rain.

Grassland and evolution of grazing mammals

Once natural grassland covered 42 percent of the earth’s surface, providing a habitat for the evolution and support of large herds of grazing mammals and colonies of rodents like the prairie-dog. Early hoofed mammals similar to our present-day grazing animals existed some 70 million years ago, but teeth studies show that they were largely browsing animals, cropping forest leaves. Fossil evidence indicates that around some 40 million years ago, at the opening of the Miocene period, a drastic change in the climate occurred. Rainfall decreased, so that forest cover could no longer be supported, and consequently grasses along with other low-growing plants became dominant over vast plains, both in the Old and the New World. In this new habitat, the true grazing animals evolved. Antelopes and sheep are recognizable from the Upper Miocene, and oxen, goats and horses from the following Pliocene period.

Nowadays much of this natural grassland has disappeared. Unwise cultivation practices and excessive grazing have caused this in part, for once the soil’s protective covering of leaves and stems and the binding network of roots and rhizomes are destroyed, wind and rain soon erode the soil and dust bowls result. However, in tropical and other regions normally capable of supporting taller vegetation, grassland exists chiefly through man’s activities, be these burning or grazing practices. Should these activities stop, the vegetation would revert to its natural tree-covered state.

Structure and growth pattern

Grassland today is principally used for grazing domestic animals and for providing cereal crops for mankind, but to see how grass can be of such value to man, we must look at its form and growth patterns.

Most grasses are annual or perennial herbs with occasional woody forms. They generally have fibrous root systems and can reproduce themselves vegetatively by means of underground stems as in the couch grass (Agropyron repens) or by runners like the common bent (Agrostis tenuis), for example. Grass leaves are characteristically linear and are arranged in two ranks on the stem, each leaf having a sheathing base which surrounds the stem. At the top of the sheath, there is a little membrane called a ligule where the leaf blade starts.

Grass flowers are usually hermaphrodite and are carried in groups called spikelets, with enclosing bracts called glumes. Each flower has two protective bracts called the palea and the lemma, with sometimes a long bristly awn attached, as in oats, barley and bearded wheat. Being wind-pollinated, grass flowers tend to be very inconspicuous, lacking petals and sepals (though in the majority of cereals self-pollination occurs). The stamens, usually three in number, are long and hang out of the flower, exposing the pollen to the wind. The single carpel has two feathery styles for capturing airborne pollen. Upon fertilization, the single ovule develops into a caryopsis which, being light, is generally dispersed by the wind and, being buoyant too, sometimes by water. Whole plants are sometimes transported by the sea. The sugar cane on Cocos Keeling Island was derived from a clump from Java, 1,126km (700 miles) away, and bamboos have also been moved from place to place. In Europe, Puccinel-lia maritima, Elymus arenarius and Ammophila arenaria are dispersed by drifting rhizomes in rivers and the sea.

The vegetative shoot of the grass grows in a rather peculiar fashion. The very closely-noded stem constantly produces new leaves at its tip, thus producing a continuous sequence of growth, whilst remaining extremely short. New short-stemmed leafy shoots branch out from the axils of the leaves and these shoots in turn produce further shoots from their leafy axils, so that a large number of shoots can be produced without any marked lengthening of the stem. This process is called ‘tillering’ and is particularly noticeable in young cereal plants where it is a valuable asset in that less seed per hectare is needed to produce a good number of shoots.

Food-source for livestock

A typical grass turf, about 30cm (i2in) high consists almost entirely of leaf, with the stems very short and hidden from view and lying, with the buds, within a centimetre of the ground. (Of course with exceptionally tall-growing grasses like the bamboos, the vegetative stems are much longer.) This characteristic is why grasses form such a valuable food for grazing animals, since in this vegetative state they can be repeatedly grazed without damaging the stems and buds, and a constant regrowth of leaves is ensured. Few other plants can survive such treatment. Of course, if grazing becomes over-intensive, and the grass is nibbled right down, the grass plants will become damaged. When flowering time arrives, the mature shoots elongate to bring the flowers up above the level of the leaves, tillering and leaf production ceases and, following the shedding of the fruit, the shoot dies. Shoots remaining in the vegetative state then further the growth of the plant.

This characteristic growth of grasses also permits mowing to provide winter fodder. Grass is at its peak in nutritional value when it is young and short, but mowing is usually left until the flowering heads appear, as prior to this it is difficult to handle the crop efficiently enough for natural drying (the cheapest method). At this stage too, the maximum weight of herbage per hectare is obtained. Any later and the plants become coarse and stemmy following the setting of the seed. It is important to have a leafy crop rather than a stemmy one because the leaves are more palatable and contain more protein than the stalks.

Species of grass used for feeding to livestock are grown for their vegetative parts; some other members of the grass family, the cereals, are cultivated for their grain.

Food-source for man

One or more of the cereals has formed the staple food for mankind in all the important civilizations of the world. They were originally derived from wild grasses and have arrived at their present highly cultivated state through ages of selective breeding. Some of these ‘wild progenitors’ still exist today, and so the hereditary pathways which our crops followed can still be traced.

The cereals are very adaptable plants, which is why they are cultivated all over the world. They tolerate a wide range of soils and climates and are straight forward to cultivate. Grain stores well in large quantities so that it can be carried over from one season to the next, and is easy to handle for transportation. Also cereals breed easily; most are self-pollinating, with the exception of rye. Most cereals are annuals, enabling harvesting in the same growing season as sowing. Some varieties are winter-hardy too.

If the three most important food plants in the world had to be picked, the lots would almost inevitably fall to three grasses: rice, wheat and corn (maize). Just over one-half of all the harvested land of the world is devoted to them and together they account for well over three-quarter billion tonnes of grain annually. The remainder of the world’s crop of about one billion tonnes is made up of other grains: barley, oats, rye, millet and sorghum. Just over half the world’s production of these grains comes from the USA, the USSR and western Europe.

Today’s high-yield grains contain about 5-13 percent vegetable protein, which is rich compared to the only other staple crop that approaches them on a worldwide basis: potatoes. Wheat and rice together supply about 40 percent of man’s food energy. Rice

Half the world’s population subsists wholly or partly on one grass—rice. This annual with its smooth narrow leaves and stems from 50— 100cm (20-4oin) long, has its origins as a swamp grass and may well have been the first crop to have been raised in the Far East. Today over 2,000 varieties of rice exist so that the crop may be cultivated in a wide range of soil and climate conditions. Rice grows best in tropical and subtropical flood plains and river valleys where the young shoots are transplanted from their seed beds into the io-20cm (4-8in) water they need to attain maximum growth. When the grains in the upper spikelets are just ripe, the rice fields are drained and about two weeks later the crop is ready for harvesting. Fol- lowing threshing, the grain, like barley and oats but unlike wheat, retains its husk. This is removed, giving brown rice; modern taste demands further milling (polishing) which strips the grain of most of its oil, protein, minerals and vitamins, leaving roughly 90-94 percent starch. Previously the disease beri-beri (weakness) was common amongst people relying heavily on rice for their food, but now the missing elements can be preserved in milling or added to the diet in other ways. Wheat

Wheat (Triticum) is grown on nearly onethird of the land devoted to grain production. Unlike rice, it does not grow well in the tropics, where one of its major diseases, the wheat rust fungus, flourishes. Wheat is grown where summers are hot and dry, and winters cold and wet. It is one of the oldest of the cereals and many subspecies and varieties have been bred to suit different conditions. Maize

Maize (Zea mays), or Indian corn, was first domesticated from the wild maize by the Indians of tropical America over 7,000 years ago. The cultivation of maize progressed and spread rapidly for the Indian was skilled in the art of plant breeding. Hybrid plants of more vigorous and prolific growth were produced by breeding maize with a wild grass relative, Tripsacum dactyloides, giving teosinte (Euchlaena mexicatm), which was bred back with true maize, giving the hybrid plants. Today’s maize cobs carry 500-1,000 kernels whereas the earliest known cultivated cobs bore only 48 kernels.

Cobs develop in the axils of the middle leaves of the main stem which is stout, solid and bears fleshy broad-bladed leaves. The female flowers, borne on separate inflorescences, possess long delicate pink or orchid-coloured styles called ‘silks’ which wither later as the ripening grains pass from their early soft ‘milk’ stage into the mature hardened state.

Maize is indeed a useful plant. Not only is the grain a popular vegetable, but it is ground into flour, used to make alcoholic drinks and sweet syrups, and, when dry, as poultry feed; leaves are used as fodder; the spathes surrounding the unopened spikes are used in paper-making; the dry cobs are used for firing. Even the stalks are used, as the fences, walls and roofs of whole villages in Guatemala can testify. In countries where amounts of sunshine are not sufficient to fully ripen the grain, maize provides green fodder or silage. Other cereals

Other cereals important to man include: oats (Avena saliva), the ancestors of which originally occurred as weeds in other crops; rye (Secale cereale), which can survive soil acidity, poor hungry land and dry light soils; and barley (Hordeum vulgare), which can be grown as far north as the Arctic Circle and was formerly used for making bread and fermented drinks in the most ancient civilizations of the Old World until wheat replaced it. Sugar cane

A grass prized for its sweet juice since ancient times is the sugar cane, Saccharum qfficinarum, which today accounts for twothirds of the world’s annual sugar production. Originating in India and New Guinea, it is a plant of tropical and subtropical countries, requiring a moist rich soil, preferably near the sea. It is in fact one of the few perennial grasses which has its vegetative parts used for human food, and unlike other grasses, it is selected for sterility. Many canes never flower; those that do rarely produce fertile seeds, and the plant is normally propagated vegetatively. The plant grows to a height of 4-5111 (13—16ft) in 8-15 months. At maturity, it has stout solid stems or canes, varying in diameter from [—6cm (0.4-2.4111), from which the sugar is extracted by crushing. The residue, known as bagasse, is used in the manufacture of wallboards, plastics and paper, and in oil-refining. Molasses, another by-product, is used both as cattle feed and in chemical manufacture, including boot-blacking and rum. Bamboo

The bamboos (Bambuseae) are the largest grasses. Their strong woody stems (culms) are hollow, jointed and glossy, and of variable colour. Strange legends surround many of the unusually-coloured specimens.

Bamboo shoots, with cells rich in protein, are amongst the fastest growing stems. At Kew Gardens, a culm of Bambusa arundinacea grew about 91cm (36m) in 24 hours! This is caused by a combination of active cell division and cell elongation in as much as one metre of the stem tip. Typically a height of 30m (98ft) can be attained within a matter of months, as compared to decades in the case of trees. In fact the individual bamboo cells do not grow any faster, there is just more of them doing the work.

The uses of bamboo for mankind are infinite: house, boat and bridge building, fishing and farming gear, musical instruments, paper, and food are just a few amongst them. They are generally natives of tropical and subtropical regions, where litter from the dense vegetation provides the rich deep loamy soil that bamboos prefer. However diversity amongst grass species is typical, and the bamboos are no exception; some species can survive at temperatures below freezing point and can be found on the snow-line of the Andes (4,500m—1 5,000ft), and at 3,000m (10,000ft) and more in the Himalayas.

Many species of bamboo are notably long-lived, and when they die it is in rather a strange fashion. In effect, they commit suicide every 33 to 66 years. At these times, the giant bamboos spontaneously burst into flower with great flowering spikes taking the place of the ordinary leafy shoots. In so doing, the plant uses up all its energy reserves without replenishing them in any way, and thus the plant dies. Strangely, all bamboos of the same species flower in the same year, wherever they may be growing. A Javanese bamboo transplanted to Jamaica will still flower at the same time as the rest of its Javanese relatives. Obviously, the year of the bamboo flower can be a calamitous one for the natives of tropical regions where a particular species is heavily relied upon to supply the bulk of local building materials. Lawns

Many grasses make beautiful ornamental plants. Lawns are an outstanding example of the collective beauty of grasses. In establishing a lawn, however, only certain species can be used if the desired effect is to be obtained. For temperate climates, the genera Agrostis and Festuca provide the best lawn grasses, meeting the requirements of a dwarf habit with firm leaves and persistent growth to withstand regular hard cutting by a mowing machine. Of the two genera, Agrostis tends to be the more aggressive and enduring. Under normal conditions, on a well-drained soil, a mixture of 70 percent chewings fescue (Festuca rubra subsp. commutata) and 30 percent New Zealand browntop (Agrostis tenuis), is a popular lawn seed mixture.

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