Monocotyledons


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Monocotyledons

This group of flowering plants (angiosperms), with one seed leaf, was previously thought to be one of the two major categories of flowering plants (the other group is dicotyledons). However, deoxyribonucleic acid (DNA) studies have revealed that, although they do constitute a group of closely related families, they are closely related to the magnoliids, with which they share a pollen type with a single aperture. The eudicots are much more distantly related. In general, monocots can also be recognized by their parallel-veined leaves and three-part flowers. Their roots have disorganized vascular bundles, and if they are treelike (yuccas, aloes, dracaenas) their wood is unusually structured. Among the important monocots are grasses (including corn, rice, and wheat), lilies, orchids, palms, and sedges. See Dicotyledons, Eudicotyledons, Flower, Magnoliophyta

Monocotyledons

 

(Monocotyledoneae, or Liliatae), a class of angiospermous plants having only one cotyledon in the embryo.

In monocotyledons, in contrast to dicotyledons, the vascular bundles are usually scattered randomly, and the stems and roots do not increase in thickness owing to the absence of a cambium. (In a few monocotyledons, secondary growth does occur but only as the result of the appearance of secondary formative tissue on the periphery of an organ.) The leaves usually have parallel or longitudinal venation, and the floral parts occur in multiples of three. The embryo rootlet generally atrophies, and a system of adventitious roots develops instead of a primary root. The stems are only slightly branched. The leaf blade often is not divided, and, therefore, the leaves are generally entire. A few species have a cambium, but it quickly ceases to function. Some species have netted-veined leaves, and two-parted and four-parted flowers are sometimes encountered.

It was formerly believed that the monocotyledons constitute the most primitive flowering plants extant. Few botanists maintain this view today. The most common hypothesis is that among the ancestors of the class were primitive dicotyledons of the subclass Magnoliaformes. Another hypothesis maintains that the monocotyledons and the dicotyledons underwent parallel development.

Monocotyledons comprise more than 65,000 species, which make up between 65 and 70 families. There are more than 2,600 species in the USSR, which constitute 15 percent of all higher plants found in the country. The plants are ubiquitous. In the arctic and temperate regions they are primarily grasses; arboreal species, especially those with secondary stem growth, are found in subtropical and, more commonly, tropical regions. Monocotyledons include rhizomatous perennials, as well as tuberous and bulbous plants. They form the largest component of meadow, steppe, and savanna grasses.

Monocotyledons play a major role in human life, since they include such important food crops as wheat, corn, rice, barley, rye, and sugarcane. There are numerous forage grasses, such as timothy, fescue, and meadow grass. Coconut palms and date palms yield edible fruits, oil, and other products. Several plants, such as tulips, lilies, and hyacinths, are valuable ornamentals.

Among the monocotyledons are numerous medicinal and fiber plants that are widely used by man.

REFERENCES

Flora SSSR, vols. 1—4. Moscow-Leningrad, 1934–35.
Kozo-Polianskii, B. M. Kurs sistematiki vysshikh rastenii. Voronezh, 1965.
Takhtadzhian, A. L., Sistema i filogeniia tsvetkovykh rastenii. Moscow-Leningrad, 1966.
Engler, A. Syllabus der Pflanzenfamilien, 12th ed., vol. 2. Berlin, 1964.
Cronquist, A. The Evolution and Classification of Flowering Plants. London, 1968.

M. E. KIRPICHNIKOV

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