The eudicots are the largest group of flowering plants (angiosperms). The term eudicots derives from the term "dicotyledons." Historically, dicots were the group of flowering plants characterized by having two seeds leaves upon germination, presence of woody or secondary growth, tap root system, reticulate (netlike) venation in the leaves, and flower parts in groups of four or five. Recent studies based on molecular phylogenetic evidence suggest that the dicotyledons are an evolutionarily natural, or monophyletic , group. However, a smaller, monophyletic, well-supported lineage termed the eudicots or "true dicots" contains the majority, but not all, of the former dicots.
There are approximately 319 families of plants within the eudicots, and these include about three-quarters of all flowering plant species. One of the most important defining morphological features of the eudicots is the presence of pollen grains having three, long, grooved apertures or openings, and in recognition of that fact, the eudicots are also known as the tricolpates. Many eudicots also exhibit a specific cell ultrastructure: The sieve elements (sap-conducting cells) contain organelles called plastids, which contain starch grains.
The eudicots are believed to represent one of the early radiations, or evolutionary expansions, of flowering plants, but the relationship of the eudicots to other flowering plant lineages is not well known. The eudicots include many familiar flowering plants. The earliest diverging lineage includes a group called the Ranunculales (including Ranunculus, the buttercups; Podophyllum, the mayapples; and Papaver, the poppies). Other groups within the eudicots include Proteas (grown as an ornamental) and related plants, the sycamore or plane tree family (Platanaceae), and the so-called "core eudicots." The core eudicots are the largest group of eudicots and include a number of diverse plant families, such as the following groups and their relatives: the carnations, sandalwoods, saxifrages, geraniums, roses, and asters. The eudicots include many economically important plants, such as Nicotiana tabacum (tobacco), Brassica oleracea (cabbage, kale, and broccoli), the legumes (which include beans, peas, lentils, chickpeas, peanuts, and soybeans, among other crops), and Solanum tuberosum (potato).
The eudicots are one of the most diverse groups of flowering plants in terms of floral and vegetative shape, growth form, habitat, and association with animals for pollination, seed dispersal, or nutrition. Some eudicots, for example many woody trees (oaks, maples, hickories, and birches in the Northern Hemisphere), are wind-pollinated and have very reduced petals and sepals , while producing copious amounts of pollen. Others have evolved highly specialized associations with animal pollinators including many species of insects (including bees, butterflies, and moths), birds (especially hummingbirds) and even bats (some species of cactus are pollinated by bats).
Floral modifications characteristic of some eudicots include fusion of sepal and petal parts, and zygomorphic (asymmetical) shape (for example, Antirrhium, the snapdragon and its relatives, and many flowers of the mint family). This trait often provides a "landing pad" projecting from the flower, and is often associated with insect pollination. The aster or composite family (for example, the common daisy, dandelion, and sunflower) has a unique type of inflorescence that is composed of many tiny flowers, some of which form the rays and look like petals, and others which form the disk flowers in the center of the composite head.
Judd, W. S., C. S. Campbell, E. A. Zimmer, and P. F. Stevens. Plant Systematics: A Phylogenetic Approach. Sunderland, MA: Sinauer Associates, 1999.
Magallon, Susana, Peter R. Crane, and Patrick S. Herendeen. "Phylogenetic Pattern, Diversity, and Diversification of Eudicots." In Annals of the Missouri Botanical Garden 86 (1999): 297–372.