| Résumé: | Chromosome numbers are reported for 18 taxa of Astragalus (Fabaceae) from northern Mexico and the southwestern United States. Twelve are reported for the first time and are as follows: COUNTS FROM BUDS: (n=11) A. cynaeus, A. eremiticus, A. insulares var. harwoodii, A. proximus; (n=14) A. hartwegii, (n=15) A. goldmanii, A. zacatecanus; CONTEOS PROVENIENTES DE SEMILLAS: (2n=22) A. cottamii, A. holmgreniorum, A. humillimus, A. wittmanii; (2n=28) A. vaccarum. Six confirm earlier reports and are: COUNTS FROM BUDS: (n=11) A. cobrensis var. maguirei, A. fucatus, A. lentiginosus var. variabilis, A. longissimus, A. mollissimus var.? (n=15) A. diphacus. Second and third records for the species, and a new record for Durando, are reported for A. zacatecanus, a species formerly known only from the type collection. An emendation of the description is made based on the morphology of the new collections. A brief discussion regarding the significance of the chromosome numbers is presented, noting that higher chromosome numbers are characteristic for certain Mexican groups of Astragalus.
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