Floral structure and systematics in four orders of rosids, including a broad survey of floral mucilage cells

Abstract

Phylogenetic studies have greatly impacted upon the circumscription of taxa within the rosid clade, resulting in novel relationships at all systematic levels. In many cases the floral structure of these taxa has never been compared, and in some families, even studies of their floral structure are lacking. Over the past five years we have compared floral structure in both new and novel orders of rosids. Four orders have been investigated including Celastrales, Oxalidales, Cucurbitales and Crossosomatales, and in this paper we attempt to summarize the salient results from these studies. The clades best supported by floral structure are: in Celastrales, the enlarged Celastraceae and the sister relationship between Celastraceae and Parnassiaceae; in Oxalidales, the sister relationship between Oxalidaceae and Connaraceae, and Tremandraceae embedded in Elaeocarpaceae; in Cucurbitales, the sister relationship between Corynocarpaceae plus Coriariaceae, and the grouping of the core Cucurbitales (Cucurbitaceae, Begoniaceae, Tetramelaceae, Datiscaceae); in Crossosomatales, the sister relationship between Ixerbaceae plus Strasburgeriaceae, and between this clade and Geissolomataceae. The core Crossosomatales (Crossosomataceae, Stachyuraceae, Staphyleaceae) and Celastrales as an order are not strongly supported by floral structure. In addition, a new floral feature of potential systematic interest is assessed. Specifically the presence of special cells in flowers with a thickened mucilaginous inner cell wall and a distinct, remaining cytoplasm is surveyed in 88 families and 321 genera (349 species) of basal angiosperms and eudicots. These cells were found to be most common in rosids, particulary fabids (Malpighiales, Oxalidales, Fabales, Rosales, Fagales, Cucurbitales), but were also found in some malvids (Malvales). They are notably absent or rare in asterids (present in campanulids: Aquifoliales, Stemonuraceae) and do not appear to occur in other eudicot clades or in basal angiosperms. Within the flower they are primarily found in the abaxial epidermis of sepals.