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The anatomy and pollination of subterranean cleistogamous flowers of Benghal dayflower (Commelina benghalensis) is described as a contribution to understanding its reproductive biology. Subterranean stems bear one spathe per node, each enclosing a single cleistogamous flower. Only the three anterior stamens produce functional pollen; the posterior three stamens are staminodes. Tapetum is amoeboid and endothecium is present. The three-carpellate superior ovary bears five dimorphic orthotropous ovules. Nearly mature flowers have straight to somewhat curved styles; at maturity, styles elongate and coil. Our observations indicate that coiled style growth causes rupture of anthers and brings pollen into contact with stigmas. Pollen tubes were observed in styles that had previously undergone coiling, located within mucilaginous secretions of the mature stylar canal. The subterranean cleistogamous flowers of Benghal dayflower and their apparently unique mode of pollination, viewed together with reproductive capacity of its aerial chasmogamous flowers, underscore the complexity and flexibility of the reproductive biology of this noxious weed species.
Nomenclature: Benghal dayflower, Commelina benghalensis L., COMBE.
Key words: Tropical spiderwort, Commelinaceae.
Benghal dayflower is an Old World species characterized as one of the world's worst weeds (Holm et al. 1977). This weed is distinguished by production of chasmogamous flowers borne near aerial stem tips and cleistogamous flowers borne on basal, often subterranean, branches (Faden 1993). Range expansion of this federally listed noxious weed in agricultural settings of the southeast United States has been documented by Krings et al. (2002) and Webster et al. (2005); Benghal dayflower is also known from southern California and Hawaii (Faden 1993). A small population of Benghal dayflower was discovered among landscape plants on the University of Richmond campus, in Richmond, VA, in October 2012 (Hayden 2013)-underscoring the warning articulated by Sermons et al. (2008) about the potential north-ward spread of this aggressive weed. The senior author collected three flowering plants and a dozen seedlings from this only known Virginia colony (now believed to be extirpated), simultaneously preserving specimens for the anatomical studies reported here.
Standard floristic and systematic works (e.g., Faden 2000a,b) present overviews of the morphology of the aerial chasmogamous flowers of Benghal dayflower, and Maheshwari and Maheshwari (1955) offer baseline morphological data on its cleistogamous flowers. No previous studies describe the microscopic anatomical structure of cleistogamous flowers of Benghal...