A short history lesson:

The Chase et al. (1993) paper used sequences of rbcL (a chloroplast gene coding for the large subunit of RUBiSCO) and revealed that Ceratophyllum was the most basal angiosperm. The monocots appeared with the paleoherbs and Magnoliids. This group has been redefined as "monosulcates" (because their pollen has one aperature or hole where the pollen tube emerges)

Soltis et al. (1997) used 18S ribosomal DNA sequences and found that Ceratophyllum does not appear at the base! The Magnoliids do. Unlike Magnolias of today, these plants have smaller and simplier flowers, for example:

Illicium sp. (photo by Doug Soltis)

Austrobaileya scandens (photo by Peter Endress)

Current work analyzes many different genes including: atpB (another chloroplast gene) + rbcL + 18S (Soltis et al. 1999). With advances in computer technology, molecular systematists are able to analyze even more data so combined gene data sets are becoming the norm for phylogeny reconstruction. This results of this work implicate the simplier magnoliids as the basal angiosperms, not Ceratophyllum.

Current thinking places the base of the angiosperms with plants that have moderate sized flowers with a moderate number of parts. Amborella has now been implicated in most studies as the most primitve living angiosperm! Click here to see where Amborella is.

Amborella trichopoda in flower
(photo by Sandra Floyd)

So, the most primitive angiosperm has moderate sized, perfect flowers, where the parts are spirally arranged and have a moderate number of parts. This indicates that floral evolution moves towards unisexual flowers, larger AND smaller flower sizes, other perianth arrangements, and larger AND smaller numbers of flower parts.