Chromosome Numbers in Vernonia: Section Lepidoploa: Subsection Paniculatae Verae

Abstract

Chromosome counts of n = 17 for 7 species and 1 hybrid are re- ported for the first time. These include the following: Vernonia texana, V. greggii V. lindheimeri, V. guadalupensis, V. marginata, V. interior, V. angustifolia and V. acaulis. Previously reported chromosome counts are summarized and speculations are made as to the consistency of chromosome numbers in other subsections of Lepidoploa. Both aneuploidy and amphidiploidy are recognized as possible origins of the number n - 17. Vernonia is a world-wide genus with over 600 species (Gleason, 1906) ranging from predominantly annual and perennial herbs to arborescent forms. In spite of the fact that species reports for only 5 of Hoffmann's (1894) 16 sections are available (Table I), interesting variations in chromosome numbers are already apparent. The most frequent number reported for sections Stengelia, Strobocalyx and Tephrodes is n = 9. Section Decaneurum has a base number of x = 10, and Lepidoploa has had reports of n = 16 (Covas and Hunziker, 1954), n = ca. 17 (Turner et al., 1962) and n = 20 (Cabrera, 1944). Lepidoploa, with more than 200 species restricted to the New World, is by far the largest of Hoffmann's sections. Gleason (1906) recognized 7 subsections in the North American Lepidoploa, based on the dif- ferentiation of the pappus and on differences in the imbrication of the involucral scales. Counts are reported in this paper for 6 of Gleason's