Ruellia is a large genus of ca. 300 species. Mexico and Brazil are centers of diversity, but the genus is common elsewhere throughout the Neotropics as well as in Madagascar and mainland Africa. A few species also occur in southeast Asia, Australia, and temperate regions of North America and South America.
Molecular work indicates Ruellia is monophyletic (Tripp 2007, Systematic Botany). Old World species form a basal grade and New World taxa are monophyletic and nested within this grade. The New World clade is composed of several lineages, three of them quite large in size (Euruellia, Physiruellia, and Ebracteolate), and six somewhat smaller in size (the Ruellia inundata, R. inflata, R. jaliscana, R. humilis, R. harveyana and Blechum clades). Floral morphology is diverse within Ruellia. Pollination syndromes (e. g. bee, hummingbird, bat, hawkmoth, butterfly) have evolved multiple times suggesting that pollinators have played an important role in the overall diversification of the genus. Pollen morphology (3-porate, spheroidal, coarsely reticulate) represents the best-known synapomorphy for the genus. A haploid chromosome count of x = 17 has been found for nearly all Ruellia species (over 50 spp. surveyed; Daniel & Chuang 1998).
Species of Ruellia can be found in a wide variety of habitats including understory rainforest, desert, swamp, tropical montane, and temperate grassland environments. Most are herbaceous but some are shrubs, small trees, or lianas. A majority of species of Ruellia produce cleistogamous flowers in addition to open (chasmogamous) flowers. The seeds of Ruellia, like other Ruellieae, are covered by hygroscopic trichomes that become mucilaginous when wet (Tripp et al. 2013, IJPS). These traits aid in successful reproduction and dispersal, and may help explain the widespread distribution of the genus. Several species of Ruellia are ornamentals, and others have been noted for their local, medicinal uses (e. g., for the treatment of headaches, dizziness, used to make teas, eye drops, and leaves smoked to induce hallucinations; see Daniel 2004 and Wasshausen 2005).
Daniel, T. F. and T. I. Chuang. 1998. Chromosome number of cultivated Acanthaceae and systematic implications. Pp. 309-330 In: Mathew P., Sivadasan M. (eds.) Diversity and taxonomy of tropical flowering plants. Mentor Books. Calicut, India.
Daniel, T. F. 2004. Acanthaceae of Sonora: Taxonomy and Phytogeography. Proceedings of the California Academy of Sciences 55: 690-805.
Grant, W. F. 1955. A cytogenetic study of Acanthaceae of Thailand. Brittonia. 8: 121-149.
Tripp, E. A. 2007. Evolutionary relationships within the species-rich genus Ruellia (Acanthaceae). Systematic Botany 32: 628-649.
Tripp, E. A., T. F. Daniel, S. Fatimah, and L. A. McDade. 2013. Phylogenetic relationships within Ruellieae (Acanthaceae) and a revised classification. International Journal of Plant Science 174: 97-137.
Wasshausen, D. C. 2005. Chapter 5.6 Acanthus: Family Acanthaceae. Pp. 112-114. In: G. A. Krupnick and W. J. Kress (eds.). Plant conservation: A natural history approach. University of Chicago Press, Chicago.