Artificial tetraploids of the Linum perenne group closely resemble their diploid progenitors in macroscopic characters, but differ markedly in their pollen morphology. The pollen of the diploids invariably has three furrows, that of the natural tetraploids normally has six pores and that of the artificial tetraploids typically has six furrows. The pollen of the artificial tetraploids is particularly variable because of tendencies towards syncolpy and the asymmetrical arrangement of furrows. The artificial autotetraploids tend to have lower pollen fertilities and more meiotic irregularities than the artificial allotetraploids. None of the artificial tetraploids resembled the natural tetraploid L. perenne subsp. anglicum particularly closely, but its origin is discussed in the light of the available evidence.