The paper summarizes the present state of cytogenetic knowledge of Polypodium in Europe and some parts of America, and discusses the evidence regarding the phyletie relations of the various taxa. New observations on wild European hybrids include (a) descriptions and analysis of chromosome pairing in two tetraploids believed to represent crosses between diploid and hexaploid, and (6) further observations on chromosome pairing in triploids. All possible types of hybrid between the three European taxa have been synthesized and chromosome pairing in them seen. In every case there is close agreement with the corresponding observations on wild plants. It is concluded that the hexaploid is in origin an amphidiploid hybrid between the European tetraploid and the European diploid as previously postulated, and that the two parental types are not otherwise closely related. Specific recognition for the three European cytotypes is recommended and names for them have been found in accordance with the International Rules of Nomenclature. A parallel study of Polypodium virginianum from eastern America has shown that diploid P. virginianum is able to pair chromosomes with one of the genomes present in European tetraploids and hexaploids. This genome is also present in tetraploid P. virginianum from eastern America as half the nuclear content. It is therefore concluded that not more than four different genomes are present, separately or combined, among the various taxa discussed, two of which have been identified singly in existing diploid species on opposite sides of the Atlantic. Preliminary observations on a diploid from western America (P. glycyrrhiza) have shown that this also is related to at least one of the European polyploids although the exact nature of the relationship is still under study.
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M. G. Shivas (1961) studied this question.