“…This provided a diagnostic aid for the determination of the hybrid frequency in B. alba populations. Extensive investigations of natural populations, however, have shown that individuals with triploid and aneuploid 2n-numbers are very rare (JOHNssON, 1941;WETTSTEIN and PROPACH, 1939). Furthermore, it has proved that the few spontaneous, probable F 1 individuals are absolutely female-sterile (HELMS and J0RGENSEN, 1927;JOHNSSON, 1944).…”