Monocots

The monocotyledons (or, in abbreviated form, the monocots), class Liliopsida, are one of the major groups of flowering plants (angiosperms). There are about 100 families and 67,000 species of monocots, and the monocots consequently represent about one-fourth of the approximately 250,000 species of flowering plants. Some of the larger families of monocots are the grass family (Poaceae, or Gramineae), palm family (Arecaceae, or Palmae), and orchid family (Orchidaceae).

Economic and Ecologic Importance

Many of the most important plant species grown for human consumption are in the grass family, which includes rice, corn (maize), wheat, rye, barley, teff, millet, and other species. Many species of the grass family are also grown for animal consumption or as lawn grasses; examples include timothy, fescue, and bluegrass. Another group of great economic importance is the palm family, which includes coconuts, dates, and the oil palm. In addition to these foods, the palm family provides construction materials for housing, thatching, and a variety of tools and implements in many parts of the world. The largest family of monocots, in terms of number of species, is the orchid family. Although orchids are widely grown as ornamentals, only one species, the vanilla orchid, is grown as a food plant. The flavoring agent vanilla is extracted from the podlike fruits of this species.

Apart from their obvious economic importance as sources of foods and other materials of use to humanity, various monocots are of great significance as dominant elements in a variety of habitats, such as prairies (many grasses), marshes, bogs, and other wetlands (many members of the sedge family, or Cyperaceae), and ponds and streams (various members of the frog's-bit family, Hydrocharitaceae, and related aquatic families). Members of the orchid family and the pineapple family (Bromeliaceae) are important epiphytes in tropical forests, where they provide food to pollinating insects and birds and habitat for insects, fungi, and other kinds of organisms in the forest canopy.

Anatomy

One of the distinctive characteristics of monocotyledons is the feature that gives the group its name, the presence of a single cotyledon, or seed leaf, in the embryo (as opposed to two in dicotyledons). Another important characteristic of monocots is the early death of its primary root, the initial root that emerges when a seed germinates. Thus, there is no taproot, and the entire root system of an older plant consists entirely of roots that emerged from stems. Another characteristic of monocots is the presence of scattered vascular bundles in the stems, as observed in cross-section, in contrast with the characteristic arrangement of the vascular bundles in a ring, as occurs in dicots and gymnosperms . Secondary growth, the process by which a stem or root continues to increase in girth through the development of additional cell layers, occurs in only a few monocots, such as the Dracaena. True wood (as occurs in gymnosperms and many dicots) is the result of secondary growth, and because this form of development is absent in most monocots, almost all of them are herbaceous plants.

Monocots nonetheless exhibit a variety of growth forms. Most are perennial herbs, often with specialized organs such as bulbs, corms, tubers, and rhizomes, which store food resources. These structures, which are specialized stems with or without specialized leaves, are seen in many perennial herbs such as crocuses, daffodils, irises, and onions. The aboveground parts of these plants die back each year when a cold or dry season approaches and are regenerated from the various belowground structures when suitable growing conditions return. Although they are often called trees, banana plants are actually large herbaceous perennials that lack wood as well as a vertical trunk. The actual stem of a banana plant extends only a short distance above the base of the plant, and what appears superficially to be the main stem is actually a tight aggregation of the lower parts of the leaves. Most monocots that are woody in texture, such as bamboos and palm trees, lack secondary growth, and their stems are relatively uniform in diameter from the base to the top of the plant. Several families of monocots are floating or rooted aquatics in fresh and salt water. These plants often have ribbonlike stems and leaves, and can be mistaken for algae if their flowers and fruits are overlooked.

Many species of monocots have leaf bases that completely encircle the stem, thus forming a sheath. The layers of an onion bulb (members of the Alliaceae family) are leaves of this type. In the leaf blades of most monocots the major strands of vascular tissue (the veins) are parallel to each other. In this manner they differ from the typically reticulate or netlike system of veins that occurs in most dicots, where the major veins branch and diverge, with many of the branches meeting. There are exceptions, and a reticulate leaf venation system occurs in some groups of monocots, such as the aroid family (Araceae), which includes skunk cabbage, Jack-in-the-pulpit, and philodendron, the latter of which is frequently grown as a houseplant. An unusual variant form of parallel leaf venation occurs in a group of mono-cots that includes the ginger family (Zingiberaceae) and the banana family (Musaceae). In these families, as exemplified by the leaf of the banana plant, there is a bundle of parallel veins along the midrib of the leaf, and these diverge in succession toward the margin of the leaf, the result being a characteristic pinnate-parallel leaf venation pattern.

In most monocots, the floral parts occur in multiples of three. One example is the tulip, which has six petals (often called tepals, since there is no clear differentiation of sepals and petals), six stamens, and a pistil with three chambers or locules, representing the three carpels . The pollen grains of monocots also differ from those of most dicots. In monocots, each pollen grain has just one thin-walled region, the colpus, which is the area from which the pollen tube emerges when the pollen grain germinates. Most dicots, in contrast, have three such regions. This thin area of the pollen wall often takes the form of a single elongate furrow, or sulcus, that extends most of the length of the pollen grain.

see also Alliaceae; Bamboo; Dicots; Evolution of Plants; Grasses; Orchidaceae; Palms; Systematics, Plant.

Jerrold I. Davis

Bibliography

Bailey, L. H. Manual of Cultivated Plants. New York: Macmillan, 1949.

Dahlgren, R. M. T., H. T. Clifford, and P. F. Yeo. The Families of Monocotyledons. New York: Springer-Verlag.

Heywood, V.H., ed. Flowering Plants of the World. Oxford: Oxford University Press, 1993.

Wilson, K. L., and D. A. Morrison, eds. Monocots: Systematics and Evolution. Victoria, Australia: CSIRO Publications, 2000.

Monocots

Monocots, or monocotyledons, are a class of the flowering plants, or angiosperms. Monocots are named for and recognized by the single cotyledon , or seed leaf, within the seed. The first green blade emerging from the seed upon germination is the cotyledon, which contains sugars and other nutrients for growth until the leaf is able to photosynthesize.

Monocots comprise about 67,000 species, or one-quarter of all flowering plants. They include not only the very large grass family (Poaceae, 9,000 species), but also the orchid family (Orchidaceae, 20,000 species), and the sedge family (Cyperaceae, 5,000 species), as well as palms, lilies, bromeliads (including pineapple), and the Araceae, which includes skunk cabbage and philodendron. The angiosperms have traditionally been divided into monocots and dicots alone, but recent work has shown that while monocots form a natural evolutionary group, dicots do not, and so the angiosperms are now grouped into monocots, eudicots , and basal angiosperms.

In addition to the single cotyledon in the seed, monocots can be recognized by the arrangement of vascular tissue in the stem. Vascular tissue includes xylem , used for water transport from the roots, and phloem , which carries sugars and other nutrients from the leaves to other tissues throughout the plant. Unlike other angiosperms, whose vascular tissue is arranged in rings around the periphery, the vascular bundles of monocots are scattered throughout the stem. One consequence of this is that monocots cannot form annual rings of hardened tissue—wood—and so are limited in the strength of their stems. Nonetheless, some monocots, notably the palms, do attain significant height. Leaves of monocots have parallel veins, as seen in grass.

The roots of monocots also differ from other flowering plants. In monocots, the first root to emerge from the seed dies off, and so no strong, central tap root forms. Instead, monocots sprout roots from shoot tissue near the base, called adventitious roots. The familiar fibrous root system of grasses is an example of this rooting pattern. Many monocots form bulbs, such as onion, gladiolus, and tulips. These are not root structures, but rather modified stems, made of compact leaves. This can be easily seen in the layers of the onion.

Most monocot flowers have flower parts in sets of three, so that there may be three or six petals, for instance, along with three egg-bearing carpels and pollen-bearing stamens in some multiple of three. The pollen grains of monocots have a single slit, or aperture, which splits open to allow the pollen tube to grow during fertilization . In contrast, the pollen grain of eudicots has three apertures.

Orchid flowers are among the most beautiful and complex of all flowers, due in part to their long and specialized relationship with specific pollinators. Some orchid flowers have evolved to resemble the female of the bee species that pollinates them, luring the male in to attempt copulation. During this process, the pollen, all of which is retained in a single, sticky mass, is transferred to the male bee, who will carry it to the next flower in another fruitless attempt to find a mate.

In contrast to the showy orchids, grass flowers are rather simple and dull, in keeping with the absence of any need to attract insects. Grass flowers are suspended at the tip of the plant, where wind can carry the pollen away to land on the female flower of a neighboring plant. Three grasses—corn, wheat, and rice—provide the vast majority of calories consumed by humans throughout the world. Their seeds, called grain, are rich in carbohydrates and contain some protein and vitamins as well.

see also Angiosperms; Eudicots; Evolution of Plants; Flowers; Grain; Grasses; Leaves; Roots; Seeds; Shoots

Richard Robinson

Bibliography

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.