Monocots comprise one-quarter of all flowering plant species, most of these are orchids, grasses, sedges, palms, and aroids. The group appears early in the fossil record of angiosperms, and seems to have radiated into most of its major clades before the end of the Cretaceous.
The monocots diverged form their dicot relatives very early in the evolution of flowering plants. This has made it difficult to identify their closest relatives, though because of molecular sequence analysis we now have several good candidates. These closely related dicots are the Piperales, from which we get black pepper, the Aristolochiales, which includes Dutchman's pipe, and the Nymphaeales, or water-lilies. All three groups share certain features with the monocots, and it is not clear which is most closely related to them, or whether they form a group sister to the monocots.
Monocots have a number of distinguishing features which are synapomorphic for the group. Characters useful within the group include leaf morphology, inflorescence structure, epidermal chemistry, and modification or loss of floral parts.
For current information on the evolutionary relationships among the monocots, visit their page on the Tree of Life.
R. M. T. Dahlgren and H. T. Clifford. 1982. The Monocotyledons: A comparative Study, 378 pp. Academic Press, London and New York.
M. R. Duvall, et al., 1993. Phylogenetic hypotheses for the monocotyledons constructed from rbcL sequence data. Ann. Missouri Bot. Gard. 80: 607-619.