Pollination of Cnidoscolus texanus (Euphorbiaceae) in South-Central Oklahoma

Abstract

The pollination phase in the reproduction of the monoecious Cnidoscolus texanus is primarily nocturnal: pollen and nectar production, peak fragrance emission, and most intense insect visitation to the blossoms all commence with dusk and conclude at dawn. The two principal visitors, the night foraging hawkmoths Hyles lineata and Manduca quinquemaculata, visit both the carpellate and staminate flowers for nectar and carry copious quantities of pollen on their proboscises. Although the plant species is self-compatible, the pattern of flowering for each plant essentially precludes self-fertilization. Therefore the primary mode of reproduction in Cnidoscolus is sphingophilous outcrossing. Bull nettle, Cnidoscolus texanus (Muel. Arg.) Small, is a common plant of sandy wastelands in south-central USA and north-central Mexico. Its floral biology was investigated for three populations of abandoned fields near The University of Oklahoma Biological Station, 2 mi E of Willis, Marshall County, Oklahoma over the summer of 1972. Cnidoscolus texanus, is monoecious with the flowers borne in numerous, terminal cymes. Each cyme includes one, very rarely zero or three, carpellate flowers subtended by 6-12, typically 9, staminate ones. The perianth is 5-lobed and corollalike, and in the staminate flowers, united to form a trumpet blossom (Faegri and van der Pijl 1971) with a tube 14-20 mm deep and a limb of five free lobes 10-16 mm long. The 10 stamens typically are arranged in two numerically equal but morphologically dissimilar and distinct series. The filaments of the inner whorl are basally coalescent and their anthers extend only to the mid-level of the perianth tube. Stamens of the outer whorl are free with the anthers slightly exserted. Occasional plants differ with respect to the numerical distribution of anthers between the series; a few are monadelphous; and finally in some, the central anthers are exserted and the outer series shorter.