Abstract
Data on chromosome numbers in eleven species and three forms ofCirsium are presented. The species treated areC. nivale (H.B.K.) Schultz Bip.,C. jorullense (H.B.K.) Spreng.,C. durangense (Greenman) Ownbey,C. ochrocentrum Gray,C. undulatum (Nutt.) Spreng.,C. velatum (Wats.) Petrak,C. mexicanum DC.,C. rhaphilepis (Hemsley) Petrak,C. occidentalis (Nutt.) Jepson subsp.acrolepis Petrak,C. conspicuum (Sweet) Schultz Bip., andC. ehrenbergii Schultz Bip. A case of autotriploidy is reported forC. durangense. Natural hybridization betweenC. durangense and a form ofC. mexicanum is reported. Photomicrographs of the mitotic or meiotic chromosomal configurations of nine species and two forms are reproduced.C. ochrocentrum Gray var.durangense Greenman is raised to species rank.
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