The grasses, members of the family Gramineae or Poaceae, are represented by over 10,000 species (Kellogg and Birchler 1993). Three of the domesticated grasses, rice, wheat, and maize, account for about half of total world food production. Although the oldest known grass fossils have been found in paleocene-eocene deposits that are ∼50–60 million years old (Crepet and Feldman 1991), morphological and molecular clock data suggest that the grasses had a monophyletic (single) origin over 70 million years ago (Linder 1986; Clark et al. 1995). Grasses are morphologically distinct from other plant families but are also highly diverse in morphology and growth habit. Grass species differ greatly in chromosome number and genome size. The genome of rice, for instance, is >11-fold smaller than the genome of barley (Arumuganathan and Earle 1991), despite their equivalent diploid states and apparently similar morphological and physiological complexity.