Dicots

The dicots (short for dicotyledons) have long been recognized as one of two major groups or classes (class Magnoliopsida) of flowering plants (di-vision Anthophyta or Magnoliophyta), the other major group being the monocots (monocotyledons; class Liliopsida). The dicots have traditionally been distinguished from monocots by a suite of morphological and anatomical features, all of which are subject to exception, however. For example, as the name of the group suggests, most dicots possess two seedling leaves, or cotyledons. In addition, dicots often possess netlike leaf venation, flower parts in fours or fives (or multiples thereof), vascular bundles in the stem arranged in a ring, with the potential for true secondary growth, and a root system of primary and adventitious roots. Monocots, in contrast, have one cotyledon, parallel leaf venation, flower parts in threes (or multiples thereof), scattered vascular bundles in the stem, lack true secondary growth, and have only an adventitious root system. As traditionally defined, the dicots comprise approximately 165,000 to 180,000 species; the monocots are the smaller of the two groups, consisting of about 60,000 species. The dicots include all the familiar angiosperm trees and shrubs (though not the conifers) and many herbaceous groups, including magnolias, roses, oaks, walnuts, legumes , sunflowers, snapdragons, mints, and mustards. Most recent classification schemes, such as those of Cronquist, Takhtajan, and Thorne, have divided the dicots into six subclasses: Magnoliidae, Hamamelidae, Caryophyllidae, Rosidae, Dilleniidae, and Asteridae.

Although the monocot-dicot division has been recognized since the late nineteenth century, recent phylogenetic studies demonstrate clearly that this split does not accurately reflect the evolutionary history of flowering plants. That is, phylogenetic trees depicting historical relationships have recently

been constructed for flowering plants (based on deoxyribonucleic acid [DNA] sequence data as well as morphological, anatomical, chemical, and other non-DNA characters). These diagrammatical trees indicate clearly that while the monocots form a clade , all of the dicots do not form a distinct group separate from the monocots. Instead, the monocots are imbedded in a clade of early branching lineages of flowering plants, usually referred to as magnoliids, all of which have the characteristics of the traditional dicots. These early branches of angiosperms, including the monocots, are characterized by pollen grains that have a single aperture (or line of weakness), or by pollen types that are derived from this single-aperture form.

The majority of angiosperms form a distinct clade and are referred to as the eudicots (or true dicots). Eudicots are characterized by pollen grains that typically possess three apertures; no other morphological or anatomical structures that mark this group have been identified, although the grouping of the eudicots is strongly supported by analyses based on DNA sequence data.

Thus, there is no monocot-dicot split in the angiosperms. Whereas monocot remains a useful term, dicot does not represent a natural group of flowering plants and should be abandoned. It is more useful to refer to eudicots, which represent a well-marked clade of flowering plants, and to specific groups of ancient dicotyledonous angiosperms (basal angiosperms). In many ways this conclusion is not surprising. Botanists long theorized that the monocots were derived from an ancient group of dicots during the early diversification of the angiosperms. Phylogenetic trees of relationship derived from molecular data confirm this longstanding hypothesis and pinpoint the possible close relatives of the monocots.

The eudicots, containing approximately 75 percent of all angiosperm species, comprise several distinct lineages. The earliest branches of eudicots are the Ranunculales, which include the Ranunculaceae (buttercup family) and Papaveraceae (poppy family), as well as the Buxaceae (boxwood family) and Platanaceae (sycamore family). Most eudicots form a large clade, composed of three main branches and several smaller ones. The main branches of eudicots are the eurosids (made up of members of the traditional subclasses Rosidae, Dilleniidae, and Asteridae), the asterids (containing members of subclasses Asteridae, Dilleniidae, and Rosidae), and the Caryophyllales; there is no clade that corresponds to subclass Dilleniidae.

The first angiosperms that appear in the fossil record possess those characteristics typically assigned to the dicots, and both the monocots and eudicots evolved later. The eudicots can be identified in the fossil record by their three-grooved pollen as early as 110 million years ago. Following the origin of this group, it diversified rapidly, and by 90 to 80 million years ago many of today's prominent families of angiosperms were established and are clearly recognizable in the fossil record.

see also Angiosperms; Evolution of Plants; Monocots; Systematics, Plant.

Doug Soltis

Pam Soltis

Bibliography

Cronquist, A. An Integrated System of Classification of Flowering Plants. New York: Columbia University Press, 1981.

Soltis, P. S., and D. E. Soltis. "Angiosperm Phylogeny Inferred from Multiple Genes as a Tool for Comparative Biology." Nature 402 (1999): 402-04.