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Ancestors of flowering plants diverged from gymnosperms around 245–202 million years ago; the first flowering plants known to exist are from 140 million years ago. They diversified enormously during the Lower Cretaceous and became widespread around 100 million years ago. They replaced conifers as the dominant plants by 60 million years ago.
Traditionally, angiosperms are divided into two groups, which in the Cronquist system are the classes Magnoliopsida (or Magnoliatae) and Liliopsida (or Liliatae). The English term dicotyledon(s) (abbreviated dicot[s]) is frequently used to refer to Magnoliopsida, while monocotyledon(s) (abbreviated monocots), refers to Liliopsida. These names derive from the observation that the dicots most often have two cotyledons, or embryonic leaves, within each seed, whereas monocots usually have only one. However, the rule is not absolute either way, so the terms dicot and monocot do not correspond perfectly with the family classifications.
Dicot species comprise the majority: roughly 75% of angiosperm species are eudicots, the group of dicots that belong to a monophyletic clade, that is, a group that includes all descendants of a common ancestor. Dicots are more diverse in habitat, and over half are woody plants, including many tree species, which have branched trunks. The monocots are generally herbaceous—fewer than 10% are woody--and almost all of these are in Arecaceae, the palm family, are characterized by a single unbranched trunk topped by a crown of large leaves.
The most diverse families of flowering plants in order of number of species, are the following: Asteraceae or Compositae (daisy); Orchidaceae (orchid); Fabaceae or Leguminosae (pea); Rubiaceae (madder); Poaceae or Graminae (grass); Lamiaceae or Labiatae (mint); Euphorbiaceae (spurge); Melastomataceae (melastome); Myrtaceae (myrtle); and Apocynaceae (dogbane). All are eudicots except Orchidaceae and Poaceae, which are monocots.
Agriculture depends almost entirely on angiosperms, either directly or indirectly through livestock feed. Of all plant families, Poaceae, the grass family, is most important, providing the bulk of all cereals and grains (rice, corn [maize], wheat, barley, rye, oats, millet, sugar cane, sorghum). Fabaceae (legumes), are second to Poaceae in providing human and livestock food. Also important are Solanaceae (nightshade family, including potatoes, tomatoes, and peppers); Cucurbitaceae (gourd family, including pumpkins, squashes, and melons); Brassicaceae (mustard family, including canola, cabbage, broccoli, and cauliflower); and Apiaceae (parsley family, including carrots). Many fruits come from Rosaceae (rose family, including apples, pears, cherries, apricots, plums) and Rutaceae (rue family, including oranges, lemons, and grapefruits).
Angiosperms also provide vital economic resources in the form of wood (although surpassed by gymnosperms in total timber production), paper, fiber (cotton, flax, and hemp, among others), sweeteners (sugar, maple syrup, agave), medicines (digitalis, camphor, and numerous others), decorative and landscaping plants.
(Cronquist 1988, Stevens/APG 2001, Takhtajan 2009, Wikipedia 2011)