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The Hypochaeris sessiliflora complex (Asteraceae, Cichorieae) consists of nine species of the genus from South America (all in section Achyrophorus Scop.) that have sessile or nearly sessile flowering heads surrounded by a rosette of leaves. They occur at 1430–5100 m in elevation along the Andean chain from Venezuela to Chile and Argentina. Two species, H. sessiliflora Kunth and H. meyeniana (Walp.) Benth. & Hook. f. ex Griseb., are extremely polymorphic, and they vary conspicuously in the shape of the external phyllaries and presence or absence of different types of trichomes. They have the widest distributions (Venezuela to central Peru, and Peru to northern Chile and northwestern Argentina, respectively), they flower throughout the year, and they also are primarily associated with dry and sunny habitats. Hypochaeris meyeniana is characterized by retrorsely pinnatifid leaves (rarely lobate) and slightly narrower cypselar apices. Hypochaeris hohenackeri (Sch. Bip.) Domke and H. taraxacoides Ball...

Eighty-nine new chromosome counts are reported from 85 populations of 15 species of Hypochaeris (Asteraceae, Lactuceae) from South America, including first reports from nine taxa (H. apargioides, H. clarionoides, H. palustris, H. scorzonerae, H. setosa, H. sonchoides, H. spathulata, H. tenuifolia, H. thrincioides). Most are 2n = 8 with bimodal and asymmetric karyolype. Tetraploidy (2n = 16) is documented for the first time in some populations of H. meyeniana, H. scorzonerae, and H. tenuifolia. A summary of previous and new counts reveals that only two uniformly tetraploid species are known (H. incana and H. stenocephala). Five taxa also show infraspecific polyploidy: H. chondrilloides, H. meyeniana, H. scorzonerae, H. sessiliflora, and H. tenuifolia. Polyploidy, therefore, occurs in about 10% of the South American taxa. The South American representatives of the genus are all based on x = 4 in contrast to more conspicuous karyotypic differences among European taxa (2n = 6, 8, 10, 12). This pattern of greater cytological uniformity among many species of the New World (ca. 50 known, 32 now counted chromosomally) suggests that the group was derived from European ancestors rather than the reverse. It appears that Hypochaeris in South America radiated explosively into many habitats from sea level to over 3000 m, but that this radiation has not often been accompanied by conspicuous chromosomal change. The absence of aneuploidy and relatively low frequency of polyploidy, plus few known cases of present interspecific hybridization, suggest a weakened role for hybridization as an evolutionary mechanism in the group. Data so far suggest diploid recombinational speciation as the dominant mode accompanied by genetic rather than dramatic cytological alterations, plus occasional polyploid derivatives.

Chromosome counts are reported for 60 collections of Peruvian Compositae representing 59 species in 31 genera. Most of these are first reports for a taxon concerned, including the genera Ascidiogyne (n - 10), Diplostephium (n - 9), Oriotrophium (n - 9), Achyrocline (n = ca 14), Gynoxys (n = 40), Loricaria (n = ca 14), Monopholis (n - ca 30) and Pappobolus (n - 17). Supernumerary or "B" chromosomes have been found in Baccharis and Monopholis, and an unusual aneuploid series has been detected in Liabum (x = 9, 12, 14, and ca 18). Chromosome counts for 60 collections of Peruvian Compositae are presented in Table 1. Bud material was collected in the field by Dr. John Wurdack and airmailed to the senior author in Austin, Texas, where the counts were made by the usual squash techniques (Turner & Johnston, 1959). Since most of these collections were from remote areas, it was often several weeks before the fixed material could be mailed. These delays and the high temperatures to which the meiotic material was exposed probably account for the numerous approximate counts reported in this paper. (Label-data on the voucher specimens-deposited TEX, US-give approximate counts, plus or minus 1 or 2 chromosomes; in Table 1 we have listed all such counts as approximations only.) Identifications were made by J. Cuatrecasas except for those of Bidens and Coreopsis (by the late E. E. Sherff), Hypochaeris (by A. L. Cabrera) and Liabum rugosum (by R. Ferreyra); chromosome counts were made by Powell and Turner, either singly or in combination.