No abstract

Accessory chromosomes are reported in four mosses belonging to the family Brachytheciaceae. In Brachythecium glareosum the basic chromosome complement is n = 9. In some gatherings two and in others three accessory chromosomes were observed. In B. velutinum the basic number is n = 10, but n = 1 0 + 2 accessory chromosomes were observed in one population. Two species of Homalothecium, H . lutescens and H. sericeum, have the basic chromosome number n = 10 in common. In some gatherings of both species one accessory chromosome was observed and in one population of H . sericeum three accessory chromosomes were found. The properties of the accessory chromosomes in mitosis, as well as the terms accessory chromosomes and m-chromosomes and the interpretation of the interphase nucleus in mosses are discussed. References are made to the behavior of the accessory chromosomes in meiosis, and also previously published chromosome complements for some species of Brachythecium and Homalothecium are interpreted on the basis of the results reported in this paper.In the interpretation of the chromosome complements of mosses accessory chromosomes and m-chromosomes have caused many problems, due mainly to two reasons. First, the terms accessory chromosomes and m-chromosomes have been used in different ways, and secondly, very often only one population of a species has been cytologically studied so that the basic chromosome complement has been misinterpreted when accessory chromosomes were present. Thus, the basic number is often lower than reported. On the other hand in some species chromosomes reported as accessories are not, in fact, to be regarded as true accessory chromosomes, but as regular members of the chromosome complement. Thus, these species have a higher basic number than reported.It is necessary to discuss the term m-chromosome more thoroughly. The genus Sphagnum is not included in this discussion as the cytological conditions in this genus are rather unique. The reader is referred to BRYAN (1955) for further information about the behavior of the m-chromosomes in Sphagnum. The term m-chromosome has been used in three different ways.