Cytological Investigations in Mammillaria and Some Associated Genera

Cytological studies were made of 142 members of the Cactaceae. Chromosome numbers of 129 species and varieties of Mammillaria were counted, including species of different geographic distribution and different morphologic form. Chromosome counts were made in 13 species representing 8 associated genera; counts for Bartschella, Echinocactus, and Phellosperma are reported for the first time. Somatic chromosome counts were made from root tips obtained from roots of specially cultivated plants. Mitosis was found to be markedly periodic, most activity occurring between 7:00 and 8:00 A.M. 2. According to this investigation, the basic chromosome number in the Cactaceae is 11. Evidence indicates that previous reports of basic chromosome numbers of 9 and 12 are probably erroneous. 3. Mammillaria was found to have a polyploid series, including tetraploid, hexaploid, octoploid, and twenty-four ploid species. Autopolyploidy was indicated by evidence of duplication of chromosome complements in mitosis and by multivalents, lagging chromosomes in meiosis, and irregular microsporogenesis, irregularities which result in abnormal formation of seeds. Polyploid species showed wider geographic distribution and more adaptability to unfavorable environmental conditions than did diploid species. Observation did not indicate over-all size or morphological differences between polyploids and diploids. Size of cells and nuclei were positively correlated with chromosome number, but size of chromosomes was negatively correlated with chromosome number. In polyploid species, where meiotic counts could not be obtained to substantiate mitotic counts, comparative epidermal studies of stomatal size and frequency were substituted. Stomatal size was positively correlated, and stomatal frequency was negatively correlated, with chromosome number. 4. The reported evidence suggests that the following be considered distinct species: diploid M. angularis and tetraploid M. compressa; diploid M. dealbata and diploid M. elegans as distinct from tetraploid and octoploid M. parkinsonii; hexaploid M. prolifera (Cuba) as distinct from tetraploid M. prolifera (Haiti).