EN
In a study of Plagiomnium undulatum from Poland, variability in karyotype formula and chromosome set length contrasted with high uniformity of chromosome number. Haploid chromosome number n = 6 was counted in 215 of 216 metaphases of plants originating from five populations, and n = 7 (6 + lm) in one cell. The chromosomes were examined and ranked by total and relative length and arm ratio. The haploid set of P. undulatum consists of three chromosome types: metacentric (m), submetacentric (sm) and subtelocentric (st). The frequencies of chromosome types differ between plates. In one population three types of karyotype were distinguished: 5m + Ism, 3m + 2sm + 1st, and 2m + 3sm + 1st. The arm ratio of the longest chromosome is stable, and the chromosome represents a mainly metacentric type.