Figure 6.1 gives one recent phylogeny of monocots based on Chase et al. (2005). There is much uncertainty about the closest sister group of the monocots, which lies within the basal Angiosperms. Options, such as Ceratophyllaceae and Chloranthaceae, were discussed in Soltis et al. (2005) but are not resolved. Moore et al. (1997) suggested a sister group relationship of monocots and eudicots, including Ceratophyllaceae, but this was questioned by Endress and Doyle (2009), who associated monocots with magnoliids. The trimerous monocot floral formula (P3+3A3+3G3) is found in a number of basal angiosperms, but there is no certaintly of any morphological links and basal monocots have a more variable floral Bauplan.

Monocots consist of three major units: a basal Acorales–Alismatales grade, a lilioid grade and a higher commelinid clade.

The basal monocots: Acorales and Alismatales

The Alismatales contains 14 families and is the most diverse order of the monocots in organ number and floral morphs. While the formula P3+3A3+3G3 fits well with most other monocots, the number and development of organs is much more variable in the Alismatales. The link of Alismatales with Acorus is strong and the separation of an order Acorales is arbitrary, as some molecular studies place Acorus within Alismatales (see Buzgo et al., 2006).

A bract tends to be present (sometimes included in the flower: Acorus) or is obviously reduced or absent (Araceae, Aponogetonaceae).