MAGNOLIOPSIDA

also called dicotyledons, or dicots, one of two classes of the division Magnoliophyta, or angiosperms, the other class being the Liliopsida (monocotyledons, or monocots). Magnoliopsida is the larger of the two classes, containing approximately 175,000 species, although this number increases as exploration of the biodiversity within the tropics continues. The similarities between the dicots and the monocots are far greater than their differences. In fact, there is no single character that separates all monocots from all dicots. The most consistent character is the number of cotyledons. Nonetheless, some dicots, such as the lotus (Nelumbo of the family Nelumbonaceae), have only a single cotyledon as the result of the fusion of the cotyledons. Other dicots, such as those of the mustard family Brassicaceae, may have only a single cotyledon after one of the two original cotyledons aborts. Since all other characteristics of the families mentioned above are dicotyledonous, however, they are so classified. The dicots have net-veined leaves, while monocots generally have leaves with parallel veins. Some monocots have net-veined leaves, but this pattern is derived from cross-connections between parallel veins just as the parallel veins of some dicots are derived from net-veined patterns that have been reduced as the plant matures. Other characteristics of dicots are a taproot and/or adventitious root systems and tricolpate or tricolpate-derived pollen. Also characteristic of the dicots is the arrangement of the vascular tissue within the primary and secondary plant body. In dicots the vascular tissue is segregated into discrete bundles of xylem and phloem, which are usually arranged in a ring around a central pith, whereas in monocots the vascular bundles (also containing bundles of xylem and phloem) are scattered throughout the parenchyma and a pith does not characteristically develop. The most obvious feature of dicots, however, is the growth pattern of secondary tissue, often called wood, which distinguishes the generally woody dicots from the generally herbaceous monocots. The dicots usually develop a vascular cambium between the primary xylem and phloem, from which secondary vascular tissues (wood) develops. In monocots, however, secondary growth, when it occurs, usually results from thickenings of the meristematic tissues rather than from the primary vascular tissues. Exceptions to these basic characteristics are usually just thatexceptions to one character in a particular species, genus, or family. Most exceptions that do occur are monocot characteristics showing up in members of the dicots. The reason for this is that the monocots are related to a common ancestor within the dicots, probably within the order Nymphaeales. Dennis William Stevenson Additional reading Discussions of aspects of the dicotyledons are contained in C.R. Metcalfe and L. Chalk, Anatomy of the Dicotyledons, 2nd ed., 3 vol. (197987); Aaron Goldberg, Classification, Evolution, and Phylogeny of the Families of Dicotyledons (1986); Gertrude Dahlgren, The Last Dahlgrenogram: System of Classification of the Dicotyledons, in Kit Tan (ed.), The Davis & Hedge Festschrift . . . : Plant Taxonomy, Phytogeography, and Related Subjects (1989), pp. 249260; and J.W. Purseglove, Tropical Crops: Dicotyledons, 2 vol. (1968, reissued in 1 vol., 1974).