monocotyledon

/mon'euh kot'l eed"n/, n. Bot.

an angiospermous plant of the class Monocotyledones, characterized by producing seeds with one cotyledon and an endogenous manner of growth. Cf. dicotyledon.

[1720-30; < NL; see MONO-, COTYLEDON]

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byname  monocot 

 one of the two great groups of flowering plants, or angiosperms, the other being the dicotyledons (dicots). There are approximately 60,000 species of monocots, including the most economically important of all plant families, Poaceae (true grasses), and the largest of all plant families, Orchidaceae (orchids). Other prominent monocot families include Liliaceae (lilies), Arecaceae (palms), and Iridaceae (irises). Most of them are distinguished by the presence of only one seed leaf, or cotyledon, in the embryo contained in the seed. Dicotyledons, in contrast, ordinarily have two cotyledons.

      It is now widely believed that the monocots were derived from primitive dicots. The single cotyledon, the parallel-veined leaves, the scattered vascular bundles in the stem, the absence of a typical cambium, and the adventitious root system of monocots are all regarded as derived characteristics within the angiosperms, and any plant more primitive than the monocots in these several respects would certainly be a dicot.

      The roots of a monocot lack a vascular cambium (the area of secondary xylem and phloem, or secondary vascular tissue, development) and therefore have no means of secondary thickening. In other structural respects, monocot roots are essentially similar to those of dicots. Many dicots have a taproot or several strong roots, with several orders of branch roots, all originating eventually from the embryonic root (radicle). The taproot or primary roots in such a system have a vascular cambium and are thickened by secondary growth. This kind of root system is not available to monocots. Instead, the primary root that originates from the radicle of the embryo soon aborts or is undeveloped so that no primary root is produced. The root system of monocots is thus wholly adventitious—i.e., the roots originate laterally from the stem or from the hypocotyl (the region of transition between the root and the stem in the embryo). The roots are all slender, and the plant is said to be fibrous-rooted.

      Flowers of monocots differ from those of dicots mainly in the number of parts of each kind. Monocot flowers most often have the parts in sets of three, occasionally four, but almost never five. The numbers are especially characteristic of the sepals and petals. The stamens and pistils may be numerous even when the perianth is trimerous (in sets of three), or the single ovary may have only two carpels instead of three. Often there are six stamens, representing two whorls of three.

      Evolutionary diversification among the monocotyledons appears to have been constrained by a number of fundamental features of the group, most notably the absence of a typical vascular cambium and the parallel-veined rather than net-veined leaves. Within these constraints, the monocots show a wide range of diversity of structure and habitat. They are cosmopolitan in their distribution on land. They also grow in lakes, ponds, and rivers, sometimes free-floating but more often rooted to the bottom. Some of them grow in the intertidal zone along the seashore, and a few are submerged marine plants rooted to the bottom in fairly shallow water along the shore.

Additional Reading

General overviews of the monocotyledons include R.M.T. Dahlgren, H.T. Clifford, and P.F. Yeo, The Families of Monocotyledons: Structure, Evolution, and Taxonomy (1985); R.M.T. Dahlgren and H. Trevor Clifford, The Monocotyledons: A Comparative Study (1982); C.R. Metcalfe, Anatomy of the Monocotyledons (1960– ); J.W. Purseglove, Tropical Crops: Monocotyledons, 2 vol. (1972, reprinted 1988); C.P. Daghlian, “A Review of the Fossil Record of Monocotyledons,” The Botanical Review, 47:517–666 (1981); R.M.T. Dahlgren and Finn M. Rasmussen, “Monocotyledon Evolution: Characters and Phylogenetic Estimation,” Evolutionary Biology, 16:255–395 (1983); and M.S. Zavada, “Comparative Morphology of Monocot Pollen and Evolutionary Trends of Apertures and Wall Structures,” The Botanical Review, 49:331–371 (1983).

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