Chromosome numbers of Cruciferae

Along with general taxonomie research on the Cruciferae of the Western Hemisphere and more intensive revisionary studies of particular genera, a continuous program of chromosome counting and breeding system testing has been underway for some years. In many cases, seeds have been obtained from wild sources and plants were grown in the greenhouse to provide either root-tips or flowering buds for chromosome counting. Otherwise, fixations of bud material have been made in the field. The distinction between greenhouse collections and field collections is made where the data are presented. In all instances, voucher specimens were prepared and deposited in the Gray Herbarium. The most significant immediate problem in which chromosome number information is helping to establish a measure of understanding concerns certain taxonomically complex species groups of Arabis. The initial breakthrough came from the work of Bocher ( 1951 ) in which he established the presence of apomixis in the genus. In subsequent studies (Bocher 1954, 1969), evidence for widespread apomixis in Arabis was increased and the frequent association of asymmetrical chromosome numbers with apomixis was established. Facultative apomixis appears to be the rule. Previous studies by one of us (Rollins 1941, 1966) support the general assumption that several agamic complexes exist in the genus. The additional chromosome counts presented below further add to the overall evidence for this assumption. In the following lists, the somatic counts from root-tips, or occasionally from some other somatic cell, are given as 2n, and those from pollen mother cells or immature pollen grains (microsporocytes) as n.