A number of plant species are known to contain supernumerary B chromosomes in addition to the standard complement of A chromosomes (cf. Darlington and Wylie, 1955; Miintzing, 1958). The following numbers are recorded as the highest in the respective species: 2n = 14 + l OB in Secale cereale (Miintzing, 1954), 2n = 24 + llB in Lilium medeoloides (Samejima, 1958), 2n = 14 + 19B in Festuca pratensis (Bosemark, 1954), 2n = 20 + 22B in Centaurea scabiosa (Frost, 1957), and 2n = 20 + 34B in Zea mays (Randolph, 1941). During the past ten years, the structure and dynamics of the populations have been studied intensively, use being made of the frequencies of B chromosomes, e.g., in Secale cereale (Muntzing, 1954, 1958), in Festuca pratensis and Phleum phleoides (Bosemark, 1956a and b), and in Centaurea scabiosa (Frost, 1958a). According to the extensive observations of these workers, B chromosomes are known to be distributed over a wide geographical range; they are contained in wild populations with various frequencies. The frequencies of plants with B's range from nearly 100 to 0 in percentage, according to species and/or to populations of one and the same species. As the procedure in these investigations, the mechanisms of the maintenance of B's in the populations have been analyzed cytogenetically. In many grass species, such as Secale and Festuca, directed non-disjunction takes place at the pollen mitosis, which effects increase in the number of B's in the offspring. On the other hand, as a rule, plants with high number of B's are