Rosids are a very large and heterogeneous group of eudicots that exhibit great diversity in both reproductive and vegetative morphology. The group is recognised based on molecular data, but support is currently weak and there are no unique morphological or anatomical features that define the rosids as a group. Among the shared characters noted by Soltis et al. (2005) is the presence of a reticulate pollen exine, but this is a common feature in many other groups of angiosperms. Other potential defining characters, such as obdiplostemony, mucilage cells in flowers and/or leaves, simple perforations of vessel end-walls, and alternating inter-vessel pitting, are also found occasionally in other groups and their value as synapomorphies of rosids remains to be determined.
Classification of rosids
Core rosids include Vitales, which are sister to two large clades: fabids sensu Judd and Olmstead (2004; Eurosids I sensu APGII, 2003) and malvids sensu Judd and Olmstead (2004; Eurosids II sensu APGII, 2003) (Figure 14.1). Fabids include Zygophyllales, which are sister to the nitrogen-fixing clade, comprising Cucurbitales, Fabales, Fagales and Rosales, and the COM clade, comprising Celastrales, Oxalidales and Malpighiales. Molecular support for these groupings is not strong (Endress and Matthews, 2006a). Malvids include Geraniales + Myrtales, which are sister to the remaining orders (Crossosomatales, Picramniales, Sapindales, Huerteales, Brassicales, Malvales) (APGIII, 2009).