Monocotyledons belong to one of the two groups into which the flowering plants (angio-sperms) are divided. They are so called because their seedlings have only one seed leaf, or cotyledon, compared with the two seed leaves of the dicotyledons. Monocotyledons are sometimes considered to be more advanced than dicotyledons. They were originally thought to be derived from dicotyledons. Fossil leaves of both groups, however, have been found in rocks of the same age; the earliest that have been positively identified come from the Lower Cretaceous epoch, making them about 130 million years old. As these plants can be placed in families living today, it is reasonable to assume that they must have evolved at a much earlier time. It is more probable that they both evolved from the same unknown seeded ancestor some time before or during the Jurassic period, from 180 million to 130 million years ago.
Monocotyledons are classified into 9 orders and about 30 families and make up about one-fourth of all flowering plants. They range from small plants, such as snowdrops (Galan-thus nivalis), to trees, such as the date palm (Phoenix dactyiifera), and include the grasses (Gramineae) and the orchids (Orchidaceae). Monocotyledons grow throughout the world, and many are popular garden plants.
One group, the bromeliads (Bromeliaceae), the group to which the pineapple belongs, mostly exists as epiphytes. They grow upon the branches of other trees—not parasitizing them, merely using them as support. They have aerial roots that hang down and soak up the moisture of the humid forest air.
Grasses are particularly successful. Grasslands will develop wherever there is little scrub or forest cover, provided there is enough moisture.
Stems and roots
The stem structure of monocotyledons is simpler than that of dicotyledons. It is made up of a large number of small parallel, vascular bundles that are closed and are scattered within the stem tissue. Unlike dicotyledons, no true thickening of the stem takes place, although in a few genera dividing cells (cambium) differentiate in the outer part of the stem, forming additional vascular bundles and tissues in which they are embedded.
The core of the trunk of a monocotyiedonous “tree” is usually a spongy, fibrous mass of tissue rather than hard wood. The trunks of palms (Palmae) are an exception, their cores being very hard. There are only a few monocotyledon families with species that grow as shrubs or trees. Most belong to the palm family; other woody monocotyledons include screw-pines (Pandanus spp.), dragon trees [Dracaena spp.), yuccas (Yucca spp.) and some of the aloes (Aloe spp.). The banana (Musa sp.) is not a true tree because the “stem” is formed out of tightly rolled leaf sheaths and is not woody.
In many monocotyledons, the largest area of growth is underground. The stems grow as rhizomes, corms, bulbs, and tubers, and the aerial projections are only lateral branches or flowering shoots. The roots of monocotyledons differ widely from those of dicotyledons. The primary root, which in a dicotyledon would develop as a taproot, rarely passes the seedling stage before it aborts. In the grasses, the main root disappears soon after germination. The root system of monocotyledons develops from roots growing from the stem. These are simple in structure and do not show any form of secondary thickening. In most species, roots form a fibrous mass, but some large trees, such as several of the palms, have a relatively small root system compared with dicotyledon trees. In some orchids (Orchi-daceae), the roots do not branch but remain as simple linear organs. In monocotyledons that have underground stems, such as rhizomes or corms, the roots tend to be annual, dying off at the end of the growing season to be replaced by new ones the following year.
The most common type of monocotyledonous leaves are linear or sword-shaped. They have parallel veins—a characteristic of monocotyledons. There are, however, some monocotyledons that have cordate (heart-shaped), ovate, or arrow-shaped leaves, and they generally have a network of veins (reticulate) or ladderlike veined leaves (scalariform). These types of leaves are found mostly in the water plantain family (Alismataceae), yams (Dioscoreaceae), arums (Araceae), and the smilax family (Smila-caceae). The leaves may grow very large, those of the banana (Musa paradisiacal reaching 6 to 10 feet (2 to 3 meters) in length. Generally, the base of the leaf is sheathed, and the leaf stalk (petiole) is frequently absent. But in palms, the petiole is extremely well developed and supports the huge, spreading, hand-shaped (palmate) or feather-shaped (pinnate) leaf.
The leaves of most monocotyledons sheathe the stem and grow alternately on it. The leaf sheath is usually thick and in palms clasps the stem to give the trunk its distinctive ridged appearance. The leaves of aquatic monocotyledons are generally ovate, but the emergent marsh plants may show a variety of leaf shapes. In the arrow-head (Sagittaria sagit-tifolialxhe submerged leaves are linear, the floating leaves are ovate, and the aerial leaves are arrow-shaped.
In monocotyledon flowers, there are usually five whorls of floral organs: two whorls of perianth segments (sepals and petals), two whorls of stamens, and one whorl of carpels. There are normally three parts to each whorl compared with a varied number in dicotyledons. In many species, especially those of the lily family (Liliaceae) and iris family (Iridaceae), the perianth whorls are brightly colored, and flowers in these groups tend to be radially symmetrical (actinomorphic), although there are exceptions. Gladioli (Gladiolus spp.) have unequal (zygomorphic) flowers, as do bananas, gingers (Zingiberaceae), cannas (Can-naceae), and orchids.
Many species of monocotyledons show a reduction in flower parts. The flowers of the sweet flag (Acorus calmus) have perianth whorls, but these are so reduced as to be inconspicuous; the other flower parts are typically arranged in threes. No perianth whorls are found in the bog arum ICalla palustrisl, and the flowers are surrounded by a white sheath or spathe (present also in the sweet flag but it is green and does not surround the flower). The number of stamens in the cuckoo pint (Arum maculatum) are reduced to three or four, and each ovary contains only one ovule. There are no perianth whorls, but the flowers are again enclosed in a spathe.
In aquatic plants there is a greater reduction in flower parts. Duckweeds (Lemna spp.) have minute flowers that float on the surface of the water. Each flower comprises a spathe surrounding a single stamen. Species of eel-grass (Zostera sp.) have further reduced flowers; the male has only one anther and the female one carpel.
Monocotyledons are pollinated in a number of ways. Zygomorphic flowers are generally pollinated by animals, such as those of bananas, which rely on bats and birds for the distribution of pollen. The orchids, however, are pollinated by insects, and their flowers show a wide range of adaptations to ensure that the correct insects are attracted to them. In some orchids, such as Cymbidium, the flowers twist through 180° as they develop so that they hang upside down when mature, providing a landing place for insects.
Many aquatic monocotyledons are pollinated by water or wind. Canadian pondweed (Elodea canadensis) has female flowers that grow at the surface of the water and are pollinated by the submerged male flowers, which break off and float to the surface. In other species of pondweed, submerged flowers are pollinated by water or, if the flowers float on the surface, their pollen is distributed by the wind.
Germination and seeds
The food supply of monocotyledon seeds is generally in the form of endosperm, which is nutritive tissue formed from the embryo sac. The storage material in the seed may be starch or oil. In some families, notably the orchids, the seeds have neither endosperm nor other stored food because they are too small. In this case, the seeds rely on symbiotic fungi, which enter their tissues and supply the seedlings with the necessary nutrients during the early stages of their development. During the germination of a monocotyledonous seed, the radicle (embryo root) emerges from the lower part of the seed followed by a bud called the plumule (upper embryo shoot), which is surrounded by the cotyledon. In some species, the cotyledon remains in the seed where the plumule may be surrounded by a special sheath. In grasses, the plumule is also surrounded by a sheath (coleoptile), a similar sheath (coleorhiza) enclosing the radicle. The plumule develops into an aerial shoot, which is usually herbaceous and which may reach a considerable height.
Many different types of fruit are found in monocotyledon families. In the lily family, plants such as tulips (Tulipa spp.) bear capsules containing a number of seeds, whereas Solomon’s seal (Polygonatum sp.) carries its seeds in juicy berries. The sedges (Cype-raceae) produce fruit in the form of nutlets, and the palm tree Cocos nucifera bears one-seeded drupes known as coconuts. Flowering rushes (Butomaceae) contain their seeds in a follicle.
Man uses monocotyledons for many purposes but by far the most important is their cultivation as a food source. Cereals are grown throughout the world, and their grain provides the staple diet in many countries. Wheat (Triticum spp.), rye (Secale cereale), barley (Hordeum spp.), oats (Avena sativa), millet (such as Panicum spp., Pennisetum spp., Se-taria sp.), corn (Zea mays), rice (Oryza sativa), and sorghum (Sorghum spp.) are notable cereals. Bamboos are grasses whose woody stems have been used for construction for centuries, as have rattans, the flexible stems of climbing palms. The lily family contains many edible species, including onions (Allium cepa), leeks (A. porrum), garlic (A. sativum), and shallots (A. ascalonicum). In parts of Africa and Asia, the root tubers of yams provide an important food source. Some trees of the palm family produce edible fruits, the more familiar of which include the coconut and the date.
Other palms provide valuable fibers. These are extracted from the leaves and are used for brushes and brooms. Another fiber comes from the outer husk of the coconut and is used for matting.
A number of monocotyledon species are cultivated for the fiber that can be extracted from them. Sisal is a strong fiber found in the leaves of some agaves (Agave spp.) and when twisted can be made into rope. Other notable fibers obtained from monocotyledons include New Zealand flax (Phormium tenax), bowstring hemp (Sansevieria spp.), and yuccas.
Many monocotyledons are grown as garden plants, particularly members of the Ama-ryllidaceae family, including snowdrops and daffodils (Narcissus spp.). Irises, freesias, gladioli, and crocuses all belong to the iris family and are popular garden plants.