Comparative stem anatomy of Mirabilis (Nyctaginaceae)

Mirabilis, a primarily American genus of 50-60 species almost restricted to the New World, is the most diverse within Nyctaginaceae. It not only has the greatest number of species, but also many life forms, with annual herbaceous, suffrutescent and shrubby species and with prostrate, decumbent to erect stems that are sometimes clambering. Stem anatomy has been studied only for M. jalapa, and its characteristics extrapolated to the entire genus. In this study we evaluated the taxonomic significance of stem anatomical characters from 24 species of Mirabilis, as well as their potential relation to habit evolution. Qualitative and quantitative characters of transverse and longitudinal sections were evaluated using light and scanning electron microscopy. Stem anatomy varies in several features. The phloem is arranged in short tangential spreading bands or in large tangential bands forming semi-complete rings; the conjunctive tissue is fibrous, with thin-walled sclereids and fibres, or parenchymatous; the vessels are solitary or grouped in radial multiples or clusters; the xylem fibres are very thin-walled or thin- to thick-walled with simple to minutely bordered pits; most species are raylessness; the pith has parenchyma or thin-walled sclereids or brachysclereids. The distribution of anatomical characters in Mirabilis does not correspond with the current infrageneric classification. We suggest that some stem anatomical characters are correlated with habit and that the vascular cylinder and pith characters are related to an increase in mechanical strength. This study provides new information and novel characters about the stem anatomy not only of Mirabilis, but of the family.